Note: The source of the technical material in this volume is the Professional
Engineering Development Program (PEDP) of Engineering Services.
Warning: The material contained in this document was developed for Saudi
Aramco and is intended for the exclusive use of Saudi Aramco’s employees. Any material contained in this document which is not
already in the public domain may not be copied, reproduced, sold, given, or disclosed to third parties, or otherwise used in whole, or in part, without the written permission of the Vice President, Engineering
Specifying Motor Enclosure Requirements 28
Specifying Motor Starting Methods 43
Specifying Motor Protection Requirements 61
WORK AID
Work Aid 1: Motor Design Requirements for Saudi Aramco 111 Installations Compiled from SADP-P-113, NEMA
MG-1 and Established Engineering Practices
Work Aid 2: Motor Enclosure Requirements for Saudi Aramco 119 Installations Compiled from SADP-P-113, NEMA
MG-1 and Established Engineering Practices
Work Aid 3: Conditions Under Which the Various Types 121
of Motor Starters Should be Specified for Use at Saudi Aramco Installations, Compiled from
SADP-P-113, NEMA MG-1, and Established Engineering Practices
Work Aid 4: Conditions Under Which the Various Types of Motor 126 Protection Should be Specified for Use at Saudi Aramco
Installations, Compiled from SADP-P-113, NEMA MG-1, and Established Engineering Practices
SPECIFYING MOTOR DESIGN REQUIREMENTS
This section will provide information on the following topics that are pertinent to specifying motor design requirements:
_Stator _Rotor _Bearings
_Vibration Monitoring _Mechanical Noise
_Shaft Circulating Currents _Stator Windings and RTD's _Rotor Windings
_Mounting Details _Cooling System
_Control and Supply Leads _Nameplates
_Space Heaters
_Testing Requirements _Painting and Coating _Packing
Stator
A stator is defined as the stationary part of a machine that houses the windings. The stator is the integral unit that consists of the outerhousing and the baseplate. The only real design requirement for the stator frame and baseplate is that they be constructed of fabricated steel that is strong enough to withstand all of the stresses to which the stator will be exposed during shipping and operation.
For motors that are above 150 kW (200 Hp), mechanical alignment devices must be installed in the baseplate to provide for accurate horizontal alignment. Examples of motor alignment devices are placement pins or foot pegs. The length of these devices can be varied to raise or lower one area of the motor. Stator mechanical alignment devices should not be the sole means of support for the stator. Shim material also should be provided with the motor to allow for accurate motor alignment and support prior to initial motor operation.
Rotor
A rotor is defined as the rotating component of a machine that has a shaft. The rotor of a motor must support the field winding. The following types of rotors are for use in Saudi Aramco motors:
_Cylindrical Rotors _Salient Pole Rotors
Cylindrical Rotors
Squirrel-cage induction cylindrical rotors are the only approved rotors that are used in induction motors for Saudi Aramco installations. The squirrel-cage induction rotor is a simple, sturdy design that allows the rotor to withstand arduous conditions. The construction of the squirrel-cage induction rotor begins with a simple shaft. Laminated supports are connected to the shaft, and, together with the shaft, they form the iron core of the rotor. The iron core increases the permeability of the rotor. The laminated supports are insulated from the rotor and from each other. Rotor bars, which are the material into which a voltage is induced, are attached to the outside of the laminated supports. An ending or shorting ring is attached at each end of the squirrel-cage induction rotor to electrically connect all of the rotor bars to complete the electrical circuit.
Synchronous motors also can be designed with a cylindrical rotor, which is sometimes called a turbo rotor. The cylindrical rotor for the synchronous motor is constructed through the embedding of the windings in slots that are machined into the iron core. The embedding of the windings limits centrifugal force on the rotor and allows the cylindrical rotor to be operated at higher speeds without damage. The synchronous cylindrical rotor is used on
motors that run at 3600 rpm or faster. Synchronous motors that have cylindrical rotors are very rarely applied in Saudi Aramco applications.
Salient Pole Rotors
Salient pole rotors, which are used in synchronous motors, are available in the following types:
_Laminated, salient pole rotor with a cage damper winding in each pole for starting.
_Solid pole rotor with solid, bolted pole pieces.
Either form of salient pole rotor is acceptable for use in Saudi Aramco applications, but most Saudi Aramco synchronous motor applications use the solid pole design. The solid pole design is preferred because of the very simple heavy duty construction and the rotor's high thermal capacity. The basic advantage that the solid pole rotor has over the laminated salient pole rotor is the absence of damper bars and end rings, and this absence ensures that there are fewer failure points on the solid pole rotor.
Solid pole salient rotors for Saudi Aramco applications can be constructed through use of the following designs:
_Solid, forged rotor shaft and pole body with solid pole shoes.
_Cast steel body and hub with forged steel stub shafts and solid bolted shoes. When a rotor is specified for use in Saudi Aramco applications, the critical speed of the rotor must be examined. A rotor that operates at or near the critical speed of the motor will cause excessive vibration of the motor. The running speed of any motor must be different than the critical speed of the motor in order to prevent vibrational damage to the motor. The following two types of salient pole rotor shafts are acceptable in the analysis of critical speeds:
_Rigid shaft rotors in which the first critical speed for vibration exceeds the running speed of the motor.
_Flexible shaft rotors in which the first critical speed for vibration is less than the running speed of the motor.
The first critical speed for ridged shaft rotors will be at least 115% of rated rotor speed. The first critical speed for flexible shaft rotors will be between 65% and 85% of rated motor speed.
The second critical speed for both rigid and flexible shaft rotors must not be within plus/minus 10% of the second harmonic, which occurs at two times the rotor speed.
Bearings
Bearings are involved in a majority of motor failures. Because many motor failures are related to bearings, much attention should be paid to them as both the possible cause of a problem and a symptom of a problem. This discussion of bearings will include the following topics:
_Bearing Types and Applications _Bearing Lubrication
_Bearing Housing and Protection _Bearing Life
Bearing Types and Applications
The following types of bearings are used in Saudi Aramco motors: _Antifriction
_Sleeve
Antifriction bearings are classified according to the type of rolling mechanism in the bearing. The rolling mechanism of an antifriction bearing can be a ball-type or a roller-type mechanism. The ball-type bearing that is shown in Figure 1A contains small balls, and the roller-type bearing that is shown in Figure 1B contains small rollers.
The following is a comparison of the load capacity and the misalignment capabilities of ball-type and roller-ball-type antifriction bearings:
Radial Load Thrust Load Misalignment
Bearing Type Capacity Capacity Capability
Ball Type Good Fair Fair
Roller Type Excellent Poor Fair
Each antifriction bearing application will have an equivalent ball-type and roller-type bearing that can be used. The type (ball or roller) of antifriction bearing that is selected should be based on the speed and load characteristics of the installation. Ball-type antifriction bearings have a small area of contact between the ball and the race. The small area of contact allows the ball-type bearing to operate at higher speeds, but the ability to carry load is reduced. Roller-type antifriction bearings have a much larger area of contact between the roller and the race. The larger area of contact allows the roller-type antifriction bearing to carry a higher load, but the speed capability of this bearing is reduced.
Antifriction Bearings
The sleeve bearing will be of the journal type. Sleeve bearings on horizontal machines are split to facilitate installation and maintenance. The two halves of horizontal motor sleeve bearings must be mechanically interchangeable.
The applications for each type of bearing are based on the speed factor (Dn) of the bearing. The speed factor (Dn) is the product of the internal diameter of the bearing in millimeters (mm) and the motor speed. The application of bearing types for any given speed factor is based on Saudi Aramco experience, and it is presented in Saudi Aramco Design Practice SADP-P-113. The following list shows the type of bearing that should be applied for different speed factors:
Speed Factor Bearing Type
up to 250,000 Antifriction, grease or oil lubricated
up to 300,000 Antifriction, oil lubricated
above 300,000 Sleeve
For an example, the speed factor (Dn) for a motor that has a shaft diameter of 127mm and that operates at 3600 RPM can be calculated as follows:
Dn = (Internal Bearing Diameter) (Motor Speed) Dn = 127mm x 3600 RPM
Dn = 547,200
This calculation shows that the motor should have sleeve bearings because the speed factor exceeds 300,000.
Bearing Lubrication
The lubrication of bearings can be accomplished with a variety of oils and greases that are applied through use of several methods. The type of lubricant and the method of lubricant application that best suits the installation will be determined by the lubricant's characteristics. This section will cover the following topics that are pertinent to bearing lubrication:
_Bearing Lubricants: Types and Applications _Methods of Bearing Lubrication
Bearing Lubricants: Types and Applications - Bearings can be lubricated through the use of oil or grease. The selection of the correct type of lubricant, either oil or grease, depends on the properties of the lubricant and the specifications of the installation.
The first type of bearing lubrication for use in Saudi Aramco applications is oil. Lubricating oils can be manufactured from mineral oil or from synthetic oil. Lubricating oil for use in Saudi Aramco installations must be manufactured from highly-refined turbine oil stocks, and it must be blended with additives to produce balanced oil stocks. Two of the main properties to consider in the selection of oil as a lubricant are the oil's viscosity and the oil's viscosity index.
The viscosity of an oil is the oil's resistance to flow. An oil with a viscosity that is too high or too low can lead to the early failure of the motor. Saudi Aramco applications require that lubricating oils have a viscosity of 61.2 - 74.8 centistokes (cSt) at 40oC. Oils with a viscosity in this range are designated as ISO viscosity grade 68. The equivalent U.S. viscosity range is 317 - 389 saybolt universal seconds (SUS).
The viscosity index is an empirical measurement of how the viscosity of an oil changes with temperature. An oil must be utilized that will meet the viscosity needs of the installation over the entire range of operating temperatures. The temperatures at which an oil can successfully perform its function will vary with the oil that is selected. Oils that have large viscosity indexes have the least change in viscosity for a given change in temperature.
Greases, which are semisolid lubricants, are the other type of bearing lubrication that can be selected. Greases are used when the lubricant must stay in one place or must stick to a part. Most greases are made from mineral oil, but other materials such as waxes can be utilized. The lubricating properties of greases are determined by the following components from which greases are made:
_Fluid base _Thickener _Additives _Fillers
Mineral oil will be the fluid base for most greases. The fluid base will determine the viscosity of the grease. Greases that are designed for high temperature, low speed service are produced through the use of high viscosity oils, and greases that are designed for low temperature, high speed service are produced through the use of low viscosity oils. The minimum viscosity of the oil that is used as the fluid base for the grease must be 100 cSt at 40oC.
The thickener is added to the fluid base to stiffen the grease. The most common type of thickener is soap. Soap is made from the combination of a fatty material and an alkali. Greases are generally named for the type of thickener that is used.
Additives are the chemical compounds that are added to grease to change or add to the properties of grease. The additives in grease will increase the temperature range at which the grease can be utilized or will change the breakdown temperature of the grease. Additives can also increase the life span of the grease.
Fillers are added to the grease to make the grease more solid and stable. Graphite is the most commonly used type of filler. Lubricating greases will have a variety of different properties that change dependent on the materials that are used during the production of the grease. To select the appropriate type of grease for an installation, the properties of the grease must be matched to the requirements of the installation. Lubricating greases for use in Saudi Aramco installations are required to perform under continuous temperatures of up to 120oC.
The type of lubricant that should be specified for antifriction and sleeve bearings that are used in Saudi Aramco applications are as follows:
Antifriction bearings oil or grease
Sleeve bearings oil
Methods of Bearing Lubrication - The method of bearing lubrication for use in a motor must account for startup and rundown lubrication of the bearings. The method of bearing lubrication should be designed so that the bearing will be lubricated during startups that follow periods of extended shutdown, and it should permit the uncoupled motor to run down to standstill without damage to the bearings.
The lubrication of antifriction bearings should be accomplished through the use of tapped holes in the bearing housing. Relief holes or drain plugs shall be located 180o from the grease point to provide for removal of old or excess lubrication.
The lubrication of sleeve bearings can be accomplished in two ways. The method that is used depends on the velocity of the shaft journal as follows:
Shaft Journal Velocity (Meters/Seconds) Lubrication Method
Below 11 Uncooled ring or disc oil
lubrication
Above 11 Circulated feed oil lubrication
The following formula is used to determine shaft journal velocity in meters per second from RPM:
For example, the shaft journal velocity of a motor that has a shaft diameter of 100mm and that operates at 1800 RPM can be calculated as follows:
The calculation shows that the bearings should be lubricated through use of the uncooled ring or disc method.
The lubrication of sleeve bearings by an uncooled ring or disc involves the use of a loose ring that hangs on the motor shaft or a fixed disc that dips into an oil reservoir that is below the shaft. The ring or the disk also rotates as the shaft rotates, and this rotation transfers oil from the reservoir to the bearing surface. Heat is removed from the oil through use of natural heat transfer through the bearing housing to the ambient. The lubrication of sleeve bearings through the use of the circulation of feed oil requires an entire external system to support the bearing. When a circulating feed oil lubrication system is used, two separate pump units must be provided. Each of the two oil pumps must be able to supply 100% of the total operating oil requirements of the bearing. The circulated feed oil lubrication option for lubricating sleeve bearings is only chosen when it is required by the manufacturer.
Bearing Housing and Protection
The bearing housings will contain the bearing and the lubrication that are necessary for the proper operation of the motor. The bearing housing should be designed to prevent physical damage to the bearing from external sources. All horizontal motors that are 3730 kW or above (5000 Hp and larger) must be equipped with pedestal bearings that are supported from the motor's baseplate.
Bearing housings also must be designed to protect the bearing and the lubricant from contamination by external foreign matter. This contamination protection will also protect the bearing against the transfer of lubricant out of the bearing housing and into the surrounding atmosphere.
Bearing Life
Because of the dispersion in life of identical bearings that are operated under identical conditions, a statistical result must be obtained for bearing life. Bearing life is expressed as the number of operational hours that 90% of a group of identical bearings will achieve or exceed under a given set of conditions, and it is referred to as the L10 life.
There are multiple variables that are taken into account for a bearing life calculation. Because of the numerous variables, this section only discusses the basic bearing life equation.
The following is the basic bearing life equation for an antifriction bearing: where:
L10 = Fatigue life for a 90% reliability
N = Operating speed
C = Dynamic load rating
P = Equivalent radial load in newtons or pounds
k = Constant that is equal to 3 for ball bearings and 10/3 for roller bearings
The dynamic load rating (C) is determined by the type of bearing that is used and by the number of active bearings that are mounted adjacent to one another.
The equivalent radial load (P) is determined by the following factors: _Applied thrust load
_Thrust load factors
_Number of adjacent bearings _Basic static load
There also are three life adjustment factors that could be placed into the basic bearing life equation. In most instances, the life adjustment factors can be assumed to be one, which will cancel out of the equation. The life adjustment factors that could be included are as follows:
_Reliability _Bearing material _Application conditions
The bearing life calculation will generally only be done by the manufacturer during the design of a new installation. The manufacturer should include the bearing life value with the bearing information.
Bearing life calculations, although they are not routinely performed by the field Electrical Engineers, can be used for performance data. Maintenance of bearing life records can be used to evaluate the actual life of bearings against the calculated life expectancy, and they can be utilized to identify bearing application problems.
Vibration Monitoring
The vibration of motors to some degree is expected as a result of the motor's rotation. Vibration monitoring is employed to detect the occurrence of excessive vibration and to avoid damage to the motor or to adjacent equipment. There following types of vibration monitoring equipment are available for use in Saudi Aramco applications:
_Seismic _Proximity
Seismic-type vibration monitoring equipment is physically mounted so that the detector is connected to the bearing housing and moves with the motor. The movement of the motor causes a slug within the seismic detector to move back and forth, which changes the electrical coupling in the detector. The motor's vibration is proportional to the vibration in the electrical coupling of the detector. The advantages and disadvantages of the seismic probe result from the method of probe mounting. The advantages of the seismic probe are its rugged design and its ease of mounting. The seismic probe directly mounts to the bearing housing. The disadvantage of the seismic probe is that the failure rate of the seismic probes increases due to the extra moving parts that are used to physically mount seismic probes.
Proximity probes are not connected to the bearing housing, and they will not move with the motor. The proximity probe measures the distance between the probe tip and the bearing casing. The proximity probe establishes a small magnetic field of the tip of the probe and, as the bearing casing vibrates in the magnetic field, the magnetic field will be distorted. The amount of distortion in the magnetic field is proportional to the amount of motor vibration. The advantages and disadvantages of the proximity probe also result from the method of mounting the probe. The advantages of the proximity probe are a much more accurate indication and a much lower failure rate. The accurate indication and lower failure rate are result from to the fact that the probe does not directly connect to the motor; therefore, the probe is not susceptible to damage and faults that result from the vibration of the motor. The disadvantage of the proximity probe is the elaborate mounting assembly that must be constructed. Because the proximity probe does not connect directly to the bearing housing, the extra mounting is necessary. Another disadvantage that results from the extra mounting assembly is the need to accurately align the probe with the motor bearing housing. Any misalignment between the probe and the motor bearing housing will result in an erroneous indication.
Proximity probes can be used for frequency ranges of 1 to 1500 Hz, and seismic probes can be used for frequency ranges of 1 to 20,000 Hz. The actual requirements for determining when each type of probe should be used are contained in Work Aid 1.
In general, any motor that operates at greater than 185 kW (250 Hp) will be supplied with vibration monitoring. The method and amount of vibration monitoring depends on the size of the motor and how the motor is mounted. Horizontal motors that operate from 750 kW (1000 Hp) to 3000 kW (4600 Hp) will have one seismic detector mounted on each bearing. Horizontal motors that operate above 3000 kW (4000 Hp) will have two proximity type detectors that are mounted 90o apart on each bearing. For vertical motors that operate at greater than 185 kW (250 Hp), two seismic detectors that are mounted 90o apart around the circumference of the top bearing housing are required. Proximity probes are never used with vertical motors.
Mechanical Noise
Mechanical noise will always be generated in a motor during operation. Different motor designs and motor mounting techniques will increase or decrease the mechanical noise that is produced by an operating motor. The following are the terms that are used to discuss mechanical noise:
_Sound Power Level _Sound Intensity _Sound Pressure Level _Sound Level
The Saudi Aramco noise limits are based on the sound level of the motor installation, and an understanding of the previously mentioned terms is necessary to facilitate this discussion.
Sound Power Level
Sound power level (SWL) is a machine-related property that is independent of environmental conditions or distance from the machine. SWL is defined through use of the following equation:
SWL = 10 log10 (P/Po) in decibels
where: P = Measured sound
Po = Reference level of 10-12 watt (1 picowatt)
Because of environmental conditions, SWL cannot be directly measured. Surrounding equipment would add to any measured sound power level; therefore, another means of directly measuring SWL is necessary.
derived through integration of the sound intensity over an enclosed, hypothetical surface of measurement.
Sound Pressure Level (SPL)
Sound Pressure Level (SPL) is the level of pressure in the sound conducting medium that results from the sound intensity at the concerned point. SPL can be expressed as follows:
SPL = 20 log10 (P1/P2) dB
where: P2 = Reference pressure that is equal to 20
micropascals (2 x 10-5n/sqm)
P1 = The sound pressure
Sound Level
Sound level is a weighted measure of the amount of noise that is produced by a machine at a given point. Note that sound intensity and SPL at a point are a function of both the combined surroundings and the source of the noise.
The following equation is for use in the direct calculation of SWL from measured free field sound:
SWL = SPL + 20 log10r + 8 dB
where: SWL = Sound power level referred to 10-12 watts
SPL = Average sound pressure level that is referenced to 20 micropascals
r = Radius of hemisphere in meters
Saudi Aramco limits the sound level to a maximum of 90 dB when the sound level is referenced to a base of 20 micropascals for an eight-hour exposure period per day. Areas in which the SWL exceeds the 90 dB maximum must have the exposure time shortened to prevent injury to the personnel.
Typical sound power levels from a motor will depend not only on the motor, but also on the type of enclosure of the motor. Certain types of enclosures such as dripoff, total-enclosed fan-cooled (TEFC), and weather protected type II (WPII) will tend to shield a portion of the sound. Figure 2 shows typical sound power levels for various motor horsepower and kilowatt ratings at various speeds and with different enclosure types. The sound levels are given in decibels.
Typical Sound Power Levels
Shaft Circulating Currents
Shaft circulating currents are caused by stray voltages that are induced in the rotor during operation. The stray voltages that are induced in the rotor can form a closed loop for current flow. To complete the closed loop for current flow, circulating currents must bridge the oil film on the bearing surfaces. Because of the high resistance of the bearing supports to ground, the induced voltages cannot be shunted away. When the oil film on the bearing surfaces is bridged, a closed loop for current flow will exist from the rotor through the bearing housing, through the stator, through the other bearing housing, and back to the rotor. The circulating currents, if allowed to exist, will cause a problem in the form of damage to the bearing and shaft surfaces. The damage will occur in the form of pitting at the point where the current passes through the shaft/bearing connection.
To prevent damage that results from shaft circulating current, a method of prevention must be obtained. On horizontal motors that are rated 375 kW (500 Hp) and above, both of the bearings must be electrically insulated from the motor frame. Vertical motors that are rated above 185 kW (250 Hp) only require insulation on the top bearing. The insulation resistance of the bearing must be greater than one megohm.
Stator Windings and RTD's
Stator windings are required to be designed to withstand environmental conditions that are common at Saudi Aramco installations. The stator windings must be treated to withstand the tropical conditions and the corrosive effects of industrial sulfurous atmospheres. The varnish impregnation should be a resin-rich type or a vacuum/pressure impregnation type process for form wound windings. The windings of weather-protected type motors should be provided with an additional protective coating to inhibit insulation abrasion by sand and salt that is entrained in the cooling air.
Stator windings also need to be braced against excessive vibration to prevent damage to the stator insulation. Stator leads that require bracing within the motor enclosure should be provided with removable insulated supports to facilitate maintenance.
Stator windings must be supplied with type F insulation systems that are designed so that the insulation will not exceed the class B temperature rise as measured by an RTD that is imbedded in the stator. The maximum temperature rise is based on a maximum ambient temperature of 50oC.
To reduce the need for surge suppressors on all motors, the stator windings must be designed to withstand the surges that are caused by normal switching actions or lightning. In older
2V + 1 kV
where: V is the phase-to-phase rms voltage of the motor.
Recently, research has shown that in short time surges, most of the voltage will fall across the first turn of the stator winding. When existing motors are evaluated, an interturn basic insulation level (BIL) of only 25% of the coil to frame insulation level can be assumed; therefore, the allowable peak surge voltage can be determined by the following equation: New motors are specified with stator winding interturn insulation requirements that exceed 25% of the coil to frame insulation level to minimize the need for surge suppressors; however, Saudi Aramco still requires both high BIL level and surge suppression for 13.8 kV motors, regardless of the stator winding interturn insulation level.
Rotor Windings
The rotor windings of induction machines should be of the cage-type, they should be formed of copper, copper alloy, or aluminum bar, and they should be treated to withstand tropical conditions. End-ring connections on cage-type rotors should be of high mechanical strength. Filler metals that are part of the cage-type rotor should be resistant to attack by corrosive sulfurous gases. Copper alloy rotor construction should conform to American Welding Society (AWS) A5.8, and it should contain a minimum 40% silver. Copper-phosphorous, bronze-type fillers are unacceptable.
The rotor body of synchronous machines should be of the salient pole type with windings of insulated copper wire or strip that also are treated to withstand tropical conditions.
The insulation of rotor windings for both NEMA frame integral motors and form-wound motors will be class F. The temperature rise above 50oC must not exceed those values that are acceptable for class B insulation. Form-wound motor insulation systems should consist of low-hygroscopic materials.
Mounting Details
Motors that are manufactured to IEC and NEMA standards use "dimension letter" codes to define machine dimensions. To facilitate the replacement of IEC motors by NEMA motors, and vice versa, a comparison of dimensional code details is needed. Figures 3A and 3B show the dimensional measurements that are necessary for motor replacement and the IEC and NEMA dimension code letters that correspond to the measurements. The NEMA code letters are shown in parentheses.
Motor Dimensions with NEMA and IEC Dimensions
Motor Dimensions with NEMA and IEC Dimensions
Vertical
Figure 4 is a list of the most common vertical motor measurements and their associated NEMA and IEC dimension code letters. This list can be used to determine the name of actual vertical motor measurements that were shown in Figures 3A and 3B.
Horizontal
Figure 5 is a list of the most common horizontal motor measurements and their associated NEMA and IEC dimension code letters. This list can be used to determine the name of the actual horizontal motor measurements that were shown in Figures 3A and 3B.
Cooling System
The IEC defines cooling as the means by which the heat that results from losses that occur in a machine is given up first to a primary coolant by means of an increase in coolant temperature. The heated primary coolant can be replaced by a new coolant at a lower temperature, or can be cooled by a secondary coolant in some form of heat exchanger.
Because of the increased heat that is produced in larger motors, the importance of motor cooling increases as the size of the motor increases . The environmental conditions to which a motor is exposed will also dictate the amount of cooling that is required. The maximum temperature rise of Saudi Aramco motors cannot exceed the temperature rise that is approved for class B insulation. The maximum temperature rise that is allowed for Class B insulation is 80oC.
Where totally-enclosed machines utilize heat exchangers, closed, air-circuit, air-cooled (CACA) heat exchangers should be mounted on the motor. Top-mounted heat exchanger assemblies should have flanges that extend downward to overlap the motor enclosure on all sides by a minimum of 10 mm (0.4 in).
High-voltage motors with integral air-to-air heat exchangers should be provided with removable sections or doors to allow easy access to the motor and the cooling fan balance planes without dismantling the motor or rotor assembly.
When air-to-air heat exchangers require auxiliary fan cooling, a shaft-mounted cooling fan or fans should be provided. Auxiliary motor-driven fans should not be specified. Internal and external cooling fans should be constructed of steel, bronze, or copper-free aluminum that is suitably treated to resist corrosion. Synthetic materials such as plastic are acceptable only for fractional kilowatt motors.
Internal and external fans that are designed for dual rotation are preferred. When uni-directional fans are necessary to meet the motor performance specifications, preference will be given to fans of a reversible design that will facilitate future reversal of motor rotation.
Control and Supply Leads
The control and supply leads for Saudi Aramco motors must be designed to be moisture- and heat-resistant. The conductors of the control and supply leads should be made of copper, and they should be designed for operation at a maximum ambient temperature of 50oC. Control and supply leads must have a minimum conductor size of stranded 2.5 mm sq (14 AWG), and each lead should be clearly and permanently marked with a PVC sleeve wire marker.
Resistance temperature detectors (RTD) are used for temperature monitoring. The RTDs should be of the platinum, three lead type, that are calibrated to a resistance of 100 ohms at 0oC (32oF). The RTDs should be located in the slot portion of stator winding coils as follows:
_Motors that are rated above 150 kW (200 Hp) and below 1300 kW (1750 Hp) should have one RTD per phase. Motors that are rated 1300 kW (1750 Hp) through 7500 kW (10,000 Hp) should have two RTDs per phase. Motors that are rated above 7500 kW (10,000 Hp) should have three RTDs per phase. The hottest reading RTD should be identified by the vendor during factory testing. _Motors that are rated up to 1 kW (1.34 Hp) should be provided with a built-in thermal protective device that will open the motor supply circuit. RTDs should not be used for these motor ratings.
Nameplates
The nameplates of Saudi Aramco motors should include all the information that is required by NEMA MG1 and IEC 34-1 and the additional information that is required by SAES-P-113. The following is a list of the information that NEMA MG-1 requires on motor nameplates.
_Manufacturer's name, serial number or date code, and suitable identification. _Horsepower output or kilowatt.
_Time rating. _Temperature rise. _RPM at rated load. _Frequency. _Number of phases. _Voltage. _Rated-load amperes.
The following additional data is required by SAES-P-113 and can be supplied on a separate nameplate(s):
_Buyer's Purchase Order number. _Year of manufacture.
_Manufacturer's location.
_Manufacturer's order reference number.
_Anti-friction bearing number and manufacturer.
_Class, Group, and Division (explosion-proof motors, only). _Maximum ambient temperature.
_Insulation system designation. _Rotor weight.
_Total weight of motor.
Where two or more identical motors are supplied on one Purchase Order, the nameplates for all motors must show the temperature data and locked rotor current from the tested motor. Saudi Aramco also requires that a separate nameplate be supplied to show the direction of motor rotation. The direction of rotation should be indicated by an arrow and the nameplate should be located on the non-drive end of the motor.
The nameplate(s) and rotation arrows must be made from 300 series stainless steel or monel, be securely fastened to the motor by pins of similar material, and be located for easy visibility. The entries on the nameplates must be marked by etching, engraving, or other permanent method of marking.
Space Heaters
The insulation of machines that are out of service for prolonged periods can absorb enough moisture to reduce the insulation resistance to a value that is below the allowable limit. Maintenance of the winding temperature 5oC above the surrounding ambient temperature will prevent moisture absorption of the insulation. Space heaters are used to maintain the winding temperature at 5oC above ambient.
Electric strip heaters are the most common source of heat, and these heaters are convenient, easy to control, and inexpensive. The only inspection that is suggested for space heaters is an occasional measurement of heater circuit current to detect burned out units or loose connections. The space heaters should have no exposed elements.
The amount of heat that is required to raise the winding temperature of a given enclosed horizontal motor approximately 5oC above ambient temperature, where the machine is closed except for a small vent at the top and bottom for circulation, is given by the following formula:
H = 0.28 DL
where: H = Heat in kilowatts
D = Machine end-bell diameter in meters
L = Machine stator length between end-bell centers in meters
For example, the heat (kW) that is required to raise the temperature of a horizontal motor with an end-bell diameter of one meter and a length of three meters to the specified 5oC above ambient can be calculated as follows:
H = .28 DL
H = (.28) (1M) (3M)
H = .84 kW
The space heaters normally should be specified to operate on 120 Volt power supplies because such supplies are normally available in all locations. In some existing Saudi Aramco installations, it may be necessary to connect heaters in series to supply them from an existing 480V system.
To ensure long operating life for a motor space heater, the heater nameplate voltage, as specified in 17-SAMSS-502, must be twice the supply voltage that is indicated in the data schedule. The following methods are available to control space heaters:
_Manual Switching.
_Thermostats that automatically energize and deenergize the heaters based on the temperature inside of the enclosure.
_Auxiliary contacts that automatically energize the heaters when the motor is deenergized and that deenergize the heaters when the motor is energized.
Saudi Aramco requires that auxiliary contacts in the switchgear be used for heater control whenever heaters are installed; therefore, manual switching or thermostats are unacceptable
Heaters should be included with all motors that are supplied with motor operated valves and with all motors that are rated 2.3 kV or higher. Other motors that are installed outdoors and that are used only as standby equipment can also require heaters.
The surface temperature of space heaters for motors that are installed in classified areas should not exceed the listed maximum allowable temperature for the area. The following are the maximum allowable surface temperatures for classified areas of Saudi Aramco installations:
Area Classification Maximum Allowable
Surface Temperature
Class I, Group C 160oC (320oF)
Class I, Group D 215oC (419oF)
Class II, Group E 200oC (392oF)
Class II, Group G 120oC (248oF)
Testing Requirements
The following tests should be made on machines that are completely assembled in the factory and that are furnished with a shaft and a complete set of bearings:
_Measurement of winding resistance
The motor winding resistance test is performed to ensure that the correct winding configuration and electrical connections have been made.
_No-load measurements of current, power, and nominal speed at rated voltage and frequency.
These measurements are performed to ensure that the motor operates within the no-load nameplate data.
_High-potential test
The high-potential test is performed to ensure that the motor's insulation system is adequate.
_Vibration test
The motor vibration test is performed during the no-load run test to ensure that the motor is balanced during operation. The vibration test also sets a baseline level of vibration for future comparison.
_Measurement of bearing insulation resistance
The bearing insulation resistance must be measured to ensure adequate insulation for the protection of personnel and equipment. Performance of the bearing insulation resistance test after all of the auxiliaries have been installed to the bearing housing will ensure that no breach in the bearing resistance system has been made.
_Bearing/lube oil temperature measurement
The bearing/lube oil temperature measurement is performed during the no-load run test to verify that the bearings operate within the established limits for the installation and for the lubricant.
The following tests should be performed when specified in the motor installation description data sheet.
_Performance Determination
The performance determination test are performed to verify, after installation, that the motor is performing its design function within the limits that are established by the installation.
_Temperature Tests
The temperature test on the stator is performed to ensure that, under load, the insulation temperature does not exceed the maximum allowable temperatures. _Miscellaneous Tests
Miscellaneous tests are any performance tests that the Engineer believes to be necessary for the installation. The exact tests that are performed and the acceptance criteria are established by the Cognizant Design Engineer.
_Surge Tests
The surge test is performed to ensure that the motor winding insulation is sufficient to protect the motor windings from harm during any expected surges. A detailed description of each of these test is included in Module EEX 203.04.
Painting and Coating
All steel surfaces must have the Vendor's standard finish with a minimum of 0.127 mm (5 mil) dry thickness. The purpose of painting and coating all steel surfaces is to protect the motor from the environment.
Packing
The packing of equipment should be suitable for shipment by sea and by vehicular transportation over unpaved, desert roads. Packing should be in accordance with Buyer's Packing Specification No. 1 and 1.1 of Vendor's standard export packing. Vendor's standard export packing should be subject to the approval of Saudi Aramco.
SPECIFYING MOTOR ENCLOSURE REQUIREMENTS
The motor enclosure requirements will vary with the type of motor that is installed and the area in which the motor is installed. There are many different degrees of protection that are afforded by the design of motor enclosures. The discussion here will concentrate only on those motor enclosures for use in Saudi Aramco applications, and it will cover the following topics:
_Motor Enclosure Functions
_General Motor Enclosure Requirements _Saudi Aramco Motor Enclosure Requirements _Motor Enclosures for Classified Areas
_Enclosures for Motor Auxiliary Equipment _Connection Boxes
_Conduit Boxes _Grounding
Motor Enclosure Functions
All motor enclosures must provide the following functions:
_To protect personnel from the motor's energized and rotating parts.
_To protect the motor from the injurious effects of the environment, such as sand, dust, rain, and water from cleaning operations (e.g., splashing).
_To afford a reasonable degree of mechanical protection against external damage to the motor.
_To protect a hazardous environment from a possible source of ignition.
General Motor Enclosure Requirements
All motor enclosures must protect against environmental and mechanical damage. Protection from the environment is afforded through use of an enclosed air circulation system to prevent fumes or gases from damaging the motor. Mechanical damage to the motor is prevented through use of screens and tight fittings that do not allow foreign material to enter the motor.
All enclosures for use in Saudi Aramco installations must meet a level of cooling and protection as defined in International Electrotechnical Commission 34-5 (IEC 34-5) and in International Electrotechnical Commission 34-6 (IEC 34-6). IEC 34-5 designates the degree of protection that the enclosure must provide for the motor. IEC 34-6 designates the degree and method of cooling a motor.
IEC 34-5 has different protection codes that can be applied to motor enclosures. Figure 6 shows the degree of protection that is indicated by the first characteristic numeral of an IEC code. The first column of Figure 6 shows the possible first characteristic numerals (0-5). The second column of Figure 6 gives a brief description of the objects against which that particular enclosure will protect. The description of the object is based on the size of the object. The third column of Figure 6 gives a definition that further describes the degree of protection.
Figure 7 shows the degree of protection that is indicated by the second characteristic numeral of the IEC code. The second characteristic numeral indicates the degree of protection the enclosure provides from water ingress. The first column of Figure 7 shows the possible second characteristic numerals (0-8). The second column of Figure 7 gives a brief description of the type of water protection that is indicated through use of the second characteristic numeral. The third column of Figure 7 gives a definition of the type of protection that is indicated by each second characteristic numeral that further describes the
All enclosures for Saudi Aramco installations must be designated as IP44. IEC 34-5 defines the protection code IP44 as follows:
IP = Ingress protection.
4 = First characteristic numeral indicates the degree of protection that is provided by the enclosure with respect to persons and to parts of the machine that are within the enclosure.
4 = Second characteristic numeral indicates the degree of protection that is provided by the enclosure with respect to the harmful effects of the ingress of water.
Saudi Aramco enclosures also must be designed for cooling in accordance with IEC 34-6. The degree and method of cooling is also designated by an IEC code. The IEC designation code for the method of cooling of a machine consists of the following:
_The letters IC that indicate an IEC designation.
_A group of one capital letter and two characteristic numerals for each motor cooling circuit (e.g., A01).
The capital letter designates the medium that is used as the coolant, the first characteristic numeral designates the circuit arrangement for circulating the coolant, and the second characteristic numeral designates the method that is used to supply power for circulating the coolant.
The following list indicates both the possible mediums that can be used as coolants and the IEC code letters that are associated with those mediums.
For other coolants that are not listed, the nature of the gas or liquid must be stated in full text. When the only coolant is air, the IEC code letter that designates the cooling medium can be omitted. Air is the only cooling medium that is approved for use in Saudi Aramco motors. Figure 8 is a complete list of the first characteristic numerals for IEC cooling method codes. The first characteristic numeral describes the physical arrangement of the coolant circulating system. The first column is a list of the possible first characteristic numerals (0-9). The second column is a short designation of the coolant system arrangement. The final column is a definition that further describes the short designation of the coolant circuit arrangement for each first characteristic numeral.
Figure 9 is a complete list of the second characteristic numeral for IEC cooling method codes. The second characteristic numeral describes the method of supplying power for circulating the coolant. The first column is a list of the possible first characteristic numerals (0-9). The second column is a short designation of the method of supplying power for circulating the coolant. The final column is a definition of each second characteristic numeral that further describes the short designation of each code number.
The following is an example of an IEC code that designates the degree and the method of cooling a motor:
ICA01
IC - Indicates that this designation is an IEC designation.
A - The coolant medium is air.
0 - The circuit arrangement is free circulation.
I - The method of supplying power to circulate the coolant is self-circulation.
When more than one cooling circuit is needed to cool a machine, the IEC designation consists of the following:
_The letters IC.
_A group of one letter and two numerals for the circuit on the user's side that is at the lower temperature (secondary cooling circuit).
_A group of one letter and two numerals for the circuit that is closer to the winding and that is at the higher temperature (primary cooling unit).
The IEC cooling codes are the same as in the single system. The following is an example of an IEC code that designates the degree and the method of cooling a motor that requires two cooling circuits:
ICA01A61
IC - Indicates that this designation is an IEC designation. A01 - Secondary cooling circuit (low temperature).
A - The coolant medium is air.
0 - The circuit arrangement is free circulation.
- The method of supplying power to circulate the coolant is self-circulation.
A - The coolant medium is air.
6 - The circuit arrangement is a machine-mounted heat
exchanger that uses the surrounding medium.
1 - The method of supplying power to circulate the coolant is self-circulation.
Saudi Aramco Motor Enclosure Requirements
NEMA MG-1 allows the use of numerous types of motor enclosures; however, only three types of enclosures are approved for use in Saudi Aramco applications. The following are motor enclosures that are allowed in Saudi Aramco applications:
_Totally-enclosed fan-cooled (TEFC)
_Environmental protection totally-enclosed air-to-air cooled (CACA) _Weather protect type II (WP-II).
TEFC
NEMA MG1 defines a TEFC enclosure as a totally-enclosed fan-cooled machine that is equipped for exterior cooling through use of a fan or fans that are integral with the machine, but that are external to the enclosing parts. The level of environmental protection that is provided by a TEFC enclosure will vary, and stringent environmental protection requirements cause this enclosure to be more complicated and more expensive. TEFC enclosures without heat exchangers are not permitted for motors that are rated above 11,000 kW (15,000 Hp). This requirement is due to the heat dissipation requirements of the motor.
TEFC enclosures for use in Saudi Aramco applications must have the following IEC designation codes for environmental protection and cooling:
_IP44 for protection _ICAO1A41 for cooling
CACA
CACA is a variation of the simpler TEFC machine, but it includes an air-to-air heat exchanger to provide more effective cooling on larger machines. CACA is commonly known as a closed air-circuit, air-cooled type of enclosure.
NEMA MG1 defines a CACA as a totally-enclosed air-to-air cooled machine that is cooled through circulation of the internal air through a heat exchanger that, in turn, is cooled through circulation of external air. The level of environmental protection that is provided by a CACA enclosure will vary, and stringent environmental protection requirements also cause this enclosure to be more complicated and expensive. A CACA enclosure is provided with an air-to-air heat exchanger for cooling the internal air, a fan or fans that are integral with the rotor shaft or separate for circulating the internal air, and a separate fan for circulating the external air.
CACA enclosures should be specified for induction motors and for salient pole synchronous motors that are rated up to 11,000 kW (15,000 Hp).
CACA enclosures for use in Saudi Aramco applications must have the following IEC designation codes for environmental protection and cooling:
_IP44 for protection _ICA01AG1 for cooling
WP-II
NEMA MG1 defines a WP-II as an open machine with ventilating passages that are so constructed as to minimize the entrance of rain, snow and air-borne particles to the electric
parts, and with ventilated openings that are so constructed as to prevent the passage of a cylindrical rod that is 0.75 inch in diameter.
The WPII ventilating passages at the intake and the discharge are so arranged that high-velocity air and air-borne particles that are blown into the machine by storms or high winds can be discharged without entering the internal ventilating passages that lead directly to the electric parts of the machine itself. The normal path of the ventilating air that enters the electric parts of the machine should be arranged through use of baffles or separate housings to provide at least three abrupt changes in direction, none of which can be less than 90o. In addition, an area of low velocity that does not exceed 3 m/s (600 ft/min) should be provided in the intake air path to minimize the possibility of moisture or dirt being carried into the electric parts of the machine.
The WP-II type of enclosure does not afford the same degree of protection as TEFC types, but it may be acceptable for synchronous motors with rated outputs that are above 11,000 kW (15,000 Hp) where the cost advantage over a TEFC type of enclosure is significant.
WPII enclosures for use in Saudi Aramco applications must have the following IEC designation codes for environmental protection and cooling:
_IP44 for protection _ICA01 for cooling
Motor Enclosures for Classified Areas
This section only covers the usual Class I, Division 1 and 2 locations with Group D hazards that are found in Saudi Aramco installations.
For a Division 1 area, the motor enclosure must be explosion proof (Exd). The totally-enclosed flameproof motor is preferred for motor sizes up to about 500 kW (700 Hp) because of the TEFC ruggedness and simplicity. For larger motor sizes, the normal practice is to avoid Division 1 locations because of the cost of the enclosures.
For a Division 2 area, the motor enclosure must be non-sparking (Exn). The type of protection "n" is such that during normal operation, the motor is not capable of causing ignition, and a fault that is capable of causing ignition is not likely to occur; therefore, any type of enclosure that prevents sparks can be utilized.
To verify that a motor that is installed in a hazardous area is permitted in that area, additional information must be included on the nameplate. All the information that is required by NEMA MG1 must be on the nameplate, plus the following additional information:
_Class, division, and/or group of hazardous atmosphere type for which the machine is approved.
_Type of protection that is provided.
_Temperature class for which the motor is approved. _Maximum exposed temperature of the machine.
Enclosures for Motor Auxiliary Equipment
Motor heaters are mounted within the motor enclosure; therefore, no special enclosures are required for motor heaters. The surface temperature limitations of the area apply to all surfaces that are in contact with the air that is inside and outside the motor. The motor heaters must not exceed the maximum allowable exposed temperature in the area.
Instruments that can be fitted into the motor enclosure also require no special enclosure. Where instruments are external to the motor enclosure, the instrument enclosure should be equal to, or better than, the motor enclosure. Instruments that are fitted to motors in Zone 1 and Zone 2 classified areas should be flameproof Exd, unless these instruments are certified or approved as part of the motor. Sparking devices must be housed in hermetically-sealed or Exd enclosures.
Connection Boxes
The terminal enclosure for termination of main windings, control/measurement circuitry, and auxiliary electrical supplies should meet or exceed the enclosure requirements for the main machine. At a minimum, the connection boxes must meet the requirements of NEMA 4. Terminal boxes and connectors must withstand the effect of faults within the enclosure as follows:
_To accommodate, without detriment, the maximum through fault current that is available at the terminals with a fault clearance time of 250 milliseconds. The prospective fault MVA's should be as follows:
System Voltage Fault MVA
480 2,400 4,160 6,900 13,800 40 250 350 500 750
_To contain, or relieve, the consequence of an internal terminal box fault without there being an external detrimental effect to personnel.
All motors that are rated 1 kW (1.5 Hp) and above should be provided with a main connection box that is located on the right hand side of the motor frame as viewed from the non-drive end of the motor. When it enters the box, the conduit should be parallel to the shaft, and it should enter the box as follows:
_The conduit should enter horizontal motors from the side that is opposite of the shaft extension.
_The conduit should enter vertical motors from the side of the shaft extension. The connection box can be mounted on the end of the motor that is opposite the shaft extension on motors with rated outputs below 1 kW (1.5 Hp).
Connections for auxiliaries are not permitted in the main connection box. Separate boxes, which should normally be mounted on the opposite side of the motor to the main terminal box, should be used for each type of instrument or auxiliary supply. A single box for all instruments or transducers that are of one type is preferred. No auxiliary wiring is to be taken through the main terminal box. Circuits that have different voltages are not permitted in the same box unless special precautions are taken and suitable warning labels are provided.
Connection Boxes (Cont'd)
High voltage motors that are rated 2.3 kV and above must be supplied with separable connectors. The premolded slip-on type cable terminators are suitable for the cable size as specified in the purchase order. A vented terminal enclosure should be specified to provide mechanical protection and to terminate the conduit: it is not necessary to equal the protection that is provided to the motor itself. The terminal enclosure does not need to meet the enclosure requirements of the motor because there must be a seal between the two enclosures. The terminal enclosure must be metallic and meet or exceed the motor enclosure specification. The terminal enclosure must withstand fault conditions as noted above, and it may require anti-condensation heaters and a condensate drain. A rotatable, diagonally-split box or enclosure is preferred.
Conduit Boxes
All cabling must be enclosed in rigid or flexible steel conduit unless protection is provided through the use of armored cables. Flexible connections only can be used where movement is to be expected in service.
The following conduit construction requirements must be met by all conduit installations: _All conduit connections to motor terminal boxes must be made through use of threaded conduit hubs that have tapered pipe threads of which at least five threads are fully engaged.
_The conduit assembly must form a weatherproof and dust-tight system that is highly resistant to mechanical damage.
Connection through the use of couplings can be installed, if necessary, when conduit boxes are installed in Class 1, Division 1, locations, and the conduit box must be sealed within 18 inches to complete the explosion-proof enclosure.
Grounding
Saudi Aramco motors must be grounded for the following reasons:
_To safeguard a person from electric shock by ensuring that, under fault conditions, all surfaces with which the person is in contact (including the surfaces of metallic equipment and the ground) remain at safe relative potentials.
_To reduce the possibility of static discharge and fire risk in hazardous areas. The main frames of motors that are rated up to 150 kW (200 Hp) should be provided with a corrosion resistant grounding stud for connection to the ground grid by means of 25 mm sq (No. 4 AWG) grounding cable. A grounding stud should be provided in the motor terminal box to ground the cable shield. Motors that are rated 150 kW (200 Hp) and above should be provided with flat corrosion resistant grounding pads that are drilled and tapped for NEMA two hole connectors and that are located on diagonally opposite corners of the non-removable portion of the motor main frame.
The ground connection must accommodate the following minimum size of ground cable:
Motor Rating Cable Size
kW (Hp) mm sq (AWG/MCM) 185 < 370 370 < 3360 3360 & above (250 < 500) (400 < 4500) (4500 & above) 70 120 185 (2/0) (4/0) (350)
The main connection box and all auxiliary connection boxes should be provided with an internal grounding clamp or bolt to provide cable grounding facilities. The dimensions of the grounding clamp or bolt should accommodate the grounding core of main or auxiliary cables, and they should provide a terminal point for the cable ground shield.
SPECIFYING MOTOR STARTING METHODS
When specifying a method of motor starting, the Electrical Engineer must evaluate each possible method of starting the motor and the different limitations of each method of starting a motor. This section will cover the following topics that are pertinent to specifying motor starting methods:
_Methods
_Factor Considered in Starting Method Selection
Methods
There are many methods that can be used to start a motor. The designer must compare each method in order to specify the correct method of starting a motor. The following starting methods are available:
_Full Voltage Starting _Autotransformer Starting _Primary Reactor Starting _Wye-Delta Starting
_Primary Resistance Starting _Part Winding Starting _Variable Frequency Starting
Methods (Cont'd)
Full Voltage Starting
Full voltage starting is the preferred method of starting Saudi Aramco motors. Figure 10 is a simplified diagram of a full voltage motor starter. Contacts 1, 2, and 3 are shut through use of a circuit breaker or a contactor to start a motor that uses a full voltage motor starter. When contacts 1, 2, and 3 are shut, power from the line will be applied to the motor stator at full rated voltage.
Full Voltage Motor Starter
Methods (Cont'd)
Figure 11 shows the typical full voltage starting torque, speed, and kVA characteristics. Full voltage starting provides the most starting torque of the possible starting methods. A byproduct of the large starting torque of the full voltage starter is that it draws both the largest starting current of any of the methods of motor starting and a high initial kVA. The starting current of a motor that uses full voltage starting will remain relatively high until the motor's speed reaches about 90% of synchronous speed. In a motor with a long run up time, the large amount of current becomes a concern because of the extra heating effect of the large kVA and current values. The high torque that is created will reduce the time that the motor requires to reach rated motor speed.
Full voltage starting is the least expensive and the simplest method of starting. Relative cost comparison tables for all types of motor starting are contained in Work Aid 3.
Full Voltage Starting Typical Torque, Speed, kVA Characteristics
Autotransformer Starting
Figure 12 is a simplified diagram of an autotransformer motor starter. An autotransformer is placed in each phase (A, B, and C) of the supply voltage that will supply a percentage of full rated voltage to the motor stator during starting. Contacts 2, 3, 4, 6, and 7 must be shut to start a motor that uses the autotransformer starter. Shutting contacts 2, 3, 4, 6, 7 will apply a portion of the rated voltage to the motor stator to start the motor. The percent of full rated voltage that is applied to the motor is determined by the position of the autotransformer line taps. When the motor is started and running, an operator or an automatic control circuit will transfer the motor from start to operate. The transfer causes contacts 2 and 7 to open, contacts 1, 5, and 8 to shut, and contacts 3, 4, and 6 to open. This sequence of contact operation applies full voltage to the motor stator for running the motor.
Autotransformer Motor Starter
The line current, motor starting torque, and motor maximum torque are proportional to the square of the motor's applied voltage. For this reason, the actual starting torque of a motor that uses an auto-transformer can be varied through a change in the position of the autotransformer line taps.
Figure 13 shows the typical autotransformer starter torque, speed, and current/kVA characteristics. The motor torque for the autotransformer is compared to what the motor torque would be if full voltage starting was employed for comparison. The large spike in motor kVA, current, and torque at about 75% synchronous speed is the point at which the motor is switched from autotransformer starting to full line voltage.
Autotransformer starting requires the least amount of starting kVA for an equal initial torque requirement as compared with other starting methods (part-winding excepted). Auto-transformer starting results in a higher initial torque than resistor or reactor starting for an equal supply (line) current.
The current that is drawn by the autotransformer starter is less than the current that is drawn in the full voltage starter method, but starting torque is proportionally lower.
An autotransformer starter costs approximately two to five times the price of a full voltage starter. A comparison of cost of the autotransformer starting method with the other starting methods is included in Work Aid 3.
Primary Reactor Starting
Figure 14 is a simplified drawing of a primary reactor starting circuit. The addition of the reactors to the motor circuit will lower the voltage that is applied to the motor. Such lowering of voltage will lower the starting current and torque. Contacts 1, 2, and 3 are closed to apply voltage from the line to the motor stator to start a motor with a primary reactor starter. The stator will be connected together through reactors that will limit the amount of current flow in the motor stator during starting. When the primary reactor starter is shifted from start to run, contacts 1, 2, and 3 will remain closed and contacts 4, 5, and 6 will close. Closing contacts 4, 5, and 6 will short out the reactors and lower the impedance of the stator. The lower impedance of the stator will allow full voltage to be applied to the motor for running operations.
Primary Reactor Starting Circuit
Methods (Cont'd)
Figure 15 shows typical motor torque, kVA, and speed curves for a motor with a primary reactor starter. The motor torque of a motor that is started with full-voltage is given for use as a comparison of different methods of starting. Notice the disturbance in KVA and torque during the transfer from start to run. A quick transfer from start to run will help provide a smooth run-up of the motor with primary reactor starting. Autotransformer motor starting will draw less line current for the same amount of initial torque as a primary reactor started motor; however, primary reactor or primary resistors give higher accelerating torque over the starting period for the same initial torque conditions because the voltage across the motor increases as the motor comes up to speed. With autotransformer starting, the motor's applied voltage is constant until the transition is made.
Primary reactor starting provides a smooth run-up speed with only a slight disturbance at the transition from "start" to "run." The use of a variable reactor can further improve the run-up characteristics. The line current at starting is proportional to the motor's applied voltage, and the starting torque is proportional to the square of the motor's applied voltage.
A primary reactor starter costs approximately 250% of the cost of a full voltage starter. A comparison of the primary reactor starting method with the other methods is contained in Work Aid 3.
Wye-Delta Starting
Figure 16 is a simplified diagram of a wye-delta motor starting diagram. The motor is started in a wye configuration and is switched to a delta configuration at, or just below, full speed. Contacts 1, 2, 3, 4, 5, 6 are shut to start a wye-delta starting motor. These contacts connect the motor stator winding in a wye configuration. Connection of the stator windings in a wye configuration will reduce the starting current that is necessary to develop the required starting torque by the. When the motor is just below full speed, the motor is switched to a delta configuration. Contacts 4, 5, and 6 will open and contacts 7, 8, and 9 will shut to switch the motor from a wye to a delta configuration. When contacts 7, 8, and 9 are shut, the stator windings will be connected in a delta configuration, and this configuration applies full line voltage to the motor stator windings for normal running operation.
Wye-Delta Motor Starting Diagram
Figure 17 shows the torque, speed, and kVA curves for a wye delta starting motor. Motor torque at full voltage is shown for comparison. Through connection of the motor in a wye configuration during starting, the starting kVA and starting torque are reduced to approximately one-third of their full-voltage values. Wye-delta starting can be used where low motor torques are required. The motor's current will follow the kVA requirement when the wye-delta motor starting method is used.
When the motor switches from a wye to a delta configuration, a large jump in motor kVA and torque will result. The jump in the motor kVA and torque can cause disturbances in the motor operation, and this jump in torque and kVA must be considered in the selection of this method because it can cause the motor speed run up to be rough. Some equipment cannot withstand the rough run-up when this starting method is used.
A wye-delta starting motor will cost between three and six times the cost of a full voltage starting motor. A comparison of the relative cost of a wye-delta starting motor and the other methods of starting is given in Work Aid 3.
Torque, Speed, and kVA Curves for a Wye Delta Starting Motor
