Note:
Note: The source of the technical material in this volume is the ProfessionalThe source of the technical material in this volume is the Professional Engineering Development Program (PEDP) of
Engineering Development Program (PEDP) of Engineering Services.Engineering Services. Warning:
Warning: The material contained in this document was developed for SaudiThe material contained in this document was developed for Saudi Aramco and is intended for the
Aramco and is intended for the exclusive use of Saudi Aramco’sexclusive use of Saudi Aramco’s employees.
employees. Any material contained in Any material contained in this document which this document which is notis not
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Selecting the the Appropriate Appropriate Types Types of of DC DC Motors Motors 5454 WORK AID:
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SADP-P-113 for Selecting the Appropriate Types of Three-Phase AC Motors
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Established Engineering Practices for Selecting the Appropriate Types of Single-Phase AC Motors Appropriate Types of Single-Phase AC Motors Work
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OPERATING CHARACTERISTICS AND APPLICATIONS OF THREE-PHASE AC OPERATING CHARACTERISTICS AND APPLICATIONS OF THREE-PHASE AC MOT
MOT ORS ANORS AND SD SINGLE-PHASE AC MOTOINGLE-PHASE AC MOTO RSRS
The operating characteristics of three-phase and single-phase alternating current (AC) motors The operating characteristics of three-phase and single-phase alternating current (AC) motors are very different fr
are very different from each other and will be separateom each other and will be separately discussed. ly discussed. Each type of AC motorEach type of AC motor also has
also has a typical a typical application that is application that is based on based on the motor's the motor's operating characteristics. operating characteristics. TheThe following topics will be discussed:
following topics will be discussed:
•
• Operating Characteristics of Three-Phase AC MotorsOperating Characteristics of Three-Phase AC Motors •
• Typical Applications of Three-Phase AC MotorsTypical Applications of Three-Phase AC Motors •
• Operating CharacteristOperating Characteristics of ics of Singe-Phase AC MotorsSinge-Phase AC Motors •
• Typical Applications of Single-Phase AC MotorsTypical Applications of Single-Phase AC Motors
Oper
Oper ating Charating Char acteristiacteristics cs of of ThrThr eeee-Phase A-Phase AC MC M otootorr ss All three-phas
All three-phase AC motors can be supplied froe AC motors can be supplied from the same power network. m the same power network. The differencThe difference ine in three-phase motors is in the characteristics that the motor displays when the motor is in three-phase motors is in the characteristics that the motor displays when the motor is in operation.
operation. The following The following types of types of three-phase AC three-phase AC motor operating motor operating characteristiccharacteristics s will bewill be discussed:
discussed:
•
• Squirrel-Cage Induction MotorsSquirrel-Cage Induction Motors •
• Wound Rotor Induction MotorsWound Rotor Induction Motors •
• Synchronous MotorsSynchronous Motors
Squirr
Squirr el-el-Cage Induction MotorsCage Induction Motors
The squirrel-cage induction motor is the simplest and most rugged of the three-phase AC The squirrel-cage induction motor is the simplest and most rugged of the three-phase AC motors.
motors. The squirrel-cThe squirrel-cage induction motor can be usage induction motor can be used in a variety of applicated in a variety of applications due to theions due to the motors design.
motors design. The following topics The following topics of squirrel-cage induction motors of squirrel-cage induction motors will be will be discussed indiscussed in this section:
this section:
•
• Uses and ClassificationsUses and Classifications •
Uses and Classifications - The standard squirrel-cage motor is a general-purpose motor. The squirrel-cage induction motor is for use in driving loads that require a variable torque at a relatively constant speed and a high full-load efficiency.
There are different types of squirrel-cage induction motors. The main difference between types of squirrel-cage induction motors is the construction of the rotor. A change in the construction of the rotor causes a change in the resistance characteristics of the rotor; a change in the resistance characteristics of the rotor causes a change in the torque and current characteristics of the motor.
The National Electrical Manufacturers Association (NEMA) classifies squirrel-cage motors in accordance with the motor's electrical characteristics. Squirrel-cage motors have the following classifications:
• NEMA Class A motors are the most popular motors. Class A motors have a
normal starting torque, a normal starting current, and a low slip.
• NEMA Class B motors are built to develop a normal starting torque with a
relatively low starting current.
• NEMA Class C motors have a high starting torque, a low starting current, and a
low slip.
• NEMA Class D motors are special purpose motors. Class D motors have a very
high starting torque, a high slip (15-20%), a low starting current, and a low efficiency.
Operating Char acteristics - Figure 1 shows the basic torque/speed characteristics of an induction motor. Figure 1 shows two curves: curve 1 represents the load torque and curve 2 represents the motor torque. The following specific torques that are associated with the operating characteristics of an AC induction motor are identified on Figure 1:
• Locked-rotor torque - The minimum torque that is developed by a motor at the
instant that rated power is supplied to the motor terminals. Locked-rotor torque also is called breakaway or starting torque. The motor must have enough locked-rotor torque to start turning the load. A motor cannot start a connected load when the connected load has a higher torque rating than the motor.
• Accelerating torque - The torque that a motor develops between zero speed and full
rated speed when the motor is supplied with its rated input power. Accelerating torque is the net difference between the motor torque and the load torque. Accelerating torque determines the rate at which the motor can accelerate a load to full rated speed.
• Full-load torque - The torque that a motor can develop when the motor is at rated
speed and the motor is supplied with its rated input power. The previous motor torques normally are expressed as a percent of the full-load torque value.
The torque of a motor is a rudimentary operating characteristic. Analysis of torque fluctuations during the stages of motor operation will provide an understanding of the types of conditions in which a particular type of motor can be utilized. The torque that a motor produces depends on a set of operational variables. The following proportion shows how the operational variables relate to motor torque. The operating characteristics of a squirrel-cage induction motor can be derived through an analysis of the operational variables.
where: _ = Motor torque
I = Motor current
V = Motor terminal voltage L = Load on the motor
N = Motor speed
Basic Torque/Speed Characteristics of an Induction Motor Figure 1
The best way to analyze how each of the variables effect the torque of the squirrel-cage induction motor is to look at the different phases of motor operation. There are three phases of motor operation to analyze:
_How the motor responds at starting.
_How the motor responds to changing loads. _How the motor responds to an overload.
A motor at standstill must produce enough starting torque to cause rotation of the motor and the connected load. The development of motor start torque can be seen through an analysis of the variables in the following torque relationships:
The relative values of the variables in the torque relationship at the moment a squirrel-cage induction motor start are as follows:
_I - Current - When the motor is energized, the starting current that is drawn will be high. The high starting current is due to the fact that no counter electromotive force (CEMF) is being produced in the motor.
_V - Voltage - The applied voltage will equal line voltage and will not fluctuate.
_L - Load - The load is constant at this point.
_N - Speed - The motor speed at the instant of start is zero. The speed of the motor at start is as low as possible, which causes torque to be high.
_R - Resistance of the Rotor - This value will not vary with a particular motor. The only way that the resistance will vary is to design the motor differently.
As the variables change, so does the motor torque. At the moment of start, the torque is high because current is high and speed is low. The starting torque that is developed by a motor must be larger than the torque that is required by the load. Starting torque that is equal to or less than load torque will not cause rotation of the motor and load. Figure 2 shows the minimum starting torque for a squirrel-cage induction motor as a percentage of full load torque for various numbers of motor poles. The minimum starting torques are established by the National Electrical Manufacturers Association (NEMA).
After the motor has been energized and the motor develops starting torque, the rotor will start to rotate. The rate at which the motor accelerates depends on the motor's developed torque and the torque that is required by the load. The difference in these torques is known as net accelerating torque. The change in motor torque from standstill to full rated speed can be analyzed through reference back to the torque proportion.
_I - Current - The motor current will continue to increase initially until there is sufficient rotation to produce CEMF that will limit the current flow. The increase in current will cause an increase in torque.
_N - Speed - The speed will continue to rise as the motor accelerates. The rise in speed will tend to lower torque, but until the motor reaches about 80-85% of full rated speed, the rise in current will greatly outweigh the rise in speed.
_V-L-R - Voltage, load, and resistance will all remain relatively constant during motor acceleration.
The net accelerating torque of a squirrel-cage induction motor will be large at the moment of starting. Net accelerating torque will continue to increase until the motor reaches about 80-85% of the motor's rated speed. After reaching 80-85% of rated speed, the net accelerating torque of the motor will start to decrease.
The next phase of motor operation is how the motor responds to a change in load. Analysis of how torque and the variables of torque vary during a change in load will explain the operating characteristics of a running squirrel-cage induction motor. The torque proportion for use in this analysis remains the same as during starting.
A motor that is running at rated speed will develop just enough torque to maintain the load rotation at a predetermined speed. As the variables of torque change during operation, the changing variable will cause other variables to change, which keeps the proportion balanced and the load running. Figure 3 shows a graphic representation of how the variables of torque change during operation.
Speed of a squirrel-cage induction motor will vary as load is added or subtracted from the motor. A squirrel-cage induction motor's operating range is from approximately 90% synchronous speed to 100% synchronous speed. A load (L) increase will cause motor speed (N) to decrease. The decrease in motor speed will cause the current (I) of the motor to increase. The resultant increase in current will cause torque of the motor to increase to a level that is high enough to support operation of the added load. This relationship between load, speed, and torque will continue until the point of breakdown torque is reached. The changes in speed on the curves of Figure 3. The bottom axis shows that as speed decreases, both current and torque will increase. The amount of motor slowdown for a load increase is a characteristic of a particular squirrel-cage induction motor design. Torque and current of a squirrel-cage induction motor will decrease when speed increases as load is removed.
Because the torque of a squirrel-cage induction motor also varies with the square of the terminal voltage that is applied to the motor, low terminal voltage will significantly reduce a squirrel-cage induction motor's torque.
The final phase of motor operation is how a squirrel-cage induction motor responds to an overload. All squirrel-cage induction motors are designed to operate under a certain amount of overload; however, the overload cannot exceed the breakdown torque of the motor. The breakdown torque is the point at which the torque that is required to run the load at overload exceeds the maximum torque that the motor can produce.
A squirrel-cage induction motor will react to the increase in load (overload) as previously discussed. Every time more load is added to the motor, the motor's speed will decrease and the motor's current will increase. The resultant change will be an increase in motor torque. Torque will continue to increase as load is added to the maximum value of torque that the motor can produce. A motor that operates at maximum torque will operate just to the right of the breakdown torque point on Figure 3. Any more load that is added to the motor will cause the motor's speed to drop more and will cause current to increase. The torque that is produced by the motor will not be large enough to continue operation of the motor, and the motor will stop.
Typical Relationship Between Current, Torque, and Speed in a Squirrel-Cage Induction Motor
Figure 4 shows the comparative torque speed characteristics of the different classifications of squirrel-cage induction motors. Typically, starting torque is 150% to 250% of the full load torque. All of the NEMA classes of motors will respond to operational changes in the same manner. The difference in the four types of cage induction motor classes is the construction of the rotor. A change in the squirrel-cage induction motor's rotor construction will change the resistance of the motor's rotor circuit. A change in the rotor circuit's resistance will cause the motor's torque characteristics to change.
Comparative Torque-Speed Characteristics of
Different Classifications of Squirrel-Cage Induction Motors Figure 4
In addition to torque, the following operating characteristics are important to an understanding of the operation of a squirrel-cage induction motor:
_Slip
_Power factor _Efficiency
For a given motor, slip is the difference between synchronous speed and motor speed. Slip is expressed as a percentage of the synchronous speed. The amount of slip of the motor depends on the amount of load. The slip of the motor will increase and the motor will run slower when the load is increased. At full load, the motor only slows slightly, which amounts to one to four percent of synchronous speed. Because of the small changes in speed from no load to full load, a squirrel-cage induction motor is considered to be a constant speed motor. The actual speed of the motor rotor will never reach the motor's synchronous speed. A difference between the speed of an induction motor and synchronous speed is necessary because of the way the rotor field is developed in an induction motor.
The most common method for calculation of slip in induction motors is through use of the following formula:
The synchronous speed of a motor is found through use of the following formula:
where: Ns = Synchronous speed
The following is an example of how to determine the slip of an induction motor. A three-phase, squirrel-cage induction motor with four poles is operating on a 60 Hz, AC power circuit at a motor speed of 1,728 rpm. The slip of this squirrel-cage induction motor can be determined through substitution of the following values into the previous formulas:
Because of the natural slip characteristic of squirrel-cage induction motors, the conclusion can be made that the squirrel-cage induction motor is not suitable in industrial applications where a great amount of speed regulation is required. The reason for non-selection of a squirrel-cage induction motor is that the speed only can be controlled by a change in frequency, the number of poles of the rotor, or the motor slip. Speed of a motor is seldom changed through change of the frequency. The number of poles can be changed either through use of two or more distinct windings or through reconnection of the same winding to establish a different number of poles. Slip is an inherent characteristic of the motor's design.
A squirrel-cage induction motor will operate most efficiently when the power factor range is maintained in the design range of the motor. A squirrel-cage induction motor's power factor will vary as the load on the motor changes. The power factor of the squirrel-cage induction motor will be lowest at no load and will increase to the highest value at rated full load of the motor. Load that is added to the motor beyond full load will cause the power factor to start to decrease.
The power factor of a squirrel-cage induction motor also is a factor of the motor's design speed. The power factor of a slow-speed squirrel-cage induction motor will be lower than the power factor of a squirrel-cage induction motor that operates at a higher rated speed. The change in power factor over the range of motor speed is due to the high leakage reactance of the squirrel-cage induction motor at lower speeds.
The efficiency of a squirrel-cage induction motor is the last characteristic that must be discussed. Efficiency is the ratio between the input and the output of a motor. The efficiency of a motor can be described by the following equation:
This equation can be restated as:
The losses of a squirrel-cage induction motor will vary with the exact construction and application of the motor. Some examples of the losses that are experienced by a squirrel-cage induction motor are:
_I2R _Winding
_Bearing friction _Hysteresis _Eddy currents
The efficiency of a squirrel-cage induction motor also will vary with the load on the motor. A lightly-loaded squirrel-cage induction motor will have a lower efficiency than an identical squirrel-cage induction motor that is supplied with rated load. Because a motor is more expensive to operate when the efficiency is lower, each motor that is installed should be selected so that the actual load and the rated load of the motor are as close as possible.
Comparison of AC Induction Motor Curves Figure 5
Wound Rotor Induction M otors
The wound rotor induction motor is another form of the three-phase induction motor. The wound rotor induction motor has operating characteristics that are similar to the squirrel-cage induction motor. The only real difference in the operating characteristics of the two types of induction motors is that some of the operating characteristics of the wound rotor induction motor can be varied. The operating characteristic that can be varied are torque, current, speed, and efficiency. These characteristics are varied through a change in the amount of external resistance that is connected in series with the wound rotor windings.
Figure 6 shows typical torque, current, and speed relationships of wound rotor induction motor with different amounts of external resistance added. Curve 1 shows the torque speed characteristics of the wound rotor motor with no external resistance added to the rotor. Curve 2 is the torque speed characteristics of the wound rotor motor with 10% external resistance added to the rotor. The external resistance is given as a percentage of the external resistance value required to give full load torque at standstill.
The starting torque of the wound rotor induction motor with no external resistance adds is approximately 90% of full load torque. Through addition of 10% external resistance to the rotor circuit, the starting torque produced by the wound rotor induction motor can be raised to approximately 200% of full load torque. The starting torque required by the load can be achieved through change in the amount of external resistance that is added to the circuit. Also, the addition of the resistance in the rotor circuit will cause the starting current of the motor to drop.
During operation, the wound rotor induction motor will produce the necessary running torque that is required to support the operation of the load. The variations in running torque of the wound rotor induction motors shown in curve 1 and curve 2 are the rate of change in running torque as compared to speed. A wound rotor induction motor with no external resistance added to the rotor circuit will develop more running torque for a given drop in speed than a wound rotor induction motor with 10% external resistance added. The difference in the rate of development of running torque is due to the current change between the motor in curve 1 and the motor in curve 2. The resistance added to the rotor circuit of the motor in curve 2 will limit the rise in current as motor speed decreases. The lower increase in current will cause less running torque to be produced.
As mentioned above, the breakdown torque of a motor is the maximum torque that a motor can produce when the motor is supplied with its rated input power. Through change of the amount of external resistance added to the rotor circuit of a wound rotor induction motor, the value of breakdown torque can be varied and the speed at which the motor reaches breakdown torque can be varied. The breakdown torque of the wound rotor induction motor with no external resistance shown by curve 1 has a breakdown torque of approximately 250% of full load torque; the breakdown torque of the motor is reached at approximately 83% of synchronous speed. Addition of 10% external resistance the rotor circuit will cause the motor's breakdown torque to change as shown in curve 2. The value of the breakdown torque will only slightly vary by a few percentage points of full load value. The biggest change is the speed of the motor when breakdown torque is reached. The motor in curve 1 reached breakdown torque at approximately 83% of synchronous speed, but, if 10% external resistance is added to the rotor circuit, the motor will not reach breakdown torque until the motor slows to approximately 50% of synchronous speed.
Typical Torque, Current, and Speed Relationship of Wound Rotor Induction Motors With Different Amounts of External Resistance
The speed and efficiency of the wound rotor induction motor are dependent upon each other. The speed of the wound rotor motor can be varied by about 50 to 75 percent. To change the speed of the wound rotor induction motor under a constant load condition, resistance is added or removed from the rotor circuit. The speed of the motor is decreased through the addition of resistance to the rotor circuit. The resistance will cause the current flow to drop in the rotor; the torque produced will be reduced; and the speed of the motor will slow. Conversely, the speed of the wound rotor induction motor is increased through a removal of resistance from the rotor circuit. The wound rotor motor is not designed to run at speeds that are slower than rated speed for extended periods of time. The addition of resistance to the rotor circuit to lower speed will generally only be done for short duration duties.
A consequence of the addition of resistance to the wound rotor induction motor is the change in the motor's efficiency. The addition of resistance to the motor rotor circuit to lower the motors speed will cause the efficiency of the motor to drop. Operation of a wound rotor induction motor with external resistance added for extended periods of time will significantly add to the operating cost of the motor due to the drop in efficiency of the motor. With all the external resistance removed from the motor's rotor circuit, the wound rotor induction motor's overall efficiency will be about 2 to 3% less than the overall efficiency of a comparable squirrel-cage induction motor because of a difference in the motor's construction.
The power factor of a wound rotor induction motor is a factor of the motor's design. The power factor of the motor will vary over the load of the motor just as the power factor varied on the squirrel-cage induction motor.
Synchr onous Motors
Synchronous motors have many of the same relationships and characteristics as induction motors; however, there are differences. Figure 7 shows the relationship between the speed, torque, and current in a synchronous motor. Note the location of the torque points on Figure 7 and the high starting current and low running current at different percents of synchronous speed. The high starting current and low running current at different percents of synchronous speed are typical of the following torques in a synchronous motor:
_Starting Torque is the torque that is developed when full voltage is applied to the armature windings and when there is no motion of the motor rotor. Because the synchronous motor itself has very little starting torque, an alternate starting method must be used to develop a large enough starting torque.
_Pull-In Torque is the torque that is developed during the transition from slip speed to synchronous speed. Pull-in torque is the maximum constant torque with which the motor will pull its connected load into synchronism, with rated power input, when field excitation is applied.
_Pull-Out Torque is the value of the torque when the rotor will fall out of synchronism with the rotating stator field. With increases in the motor load, the rotor will fall behind the rotating stator field but not out of synchronism. If the load is increased beyond the pull-out torque point, the motor will "slip a pole" or pull-out of synchronism. The mechanical pull-out point of a synchronous motor is approximately half of the distance between adjacent poles.
Synchronous Motor Speed/Torque/Current Curves Figure 7
The speed of a synchronous motor is determined through the frequency of the power supply and the number of poles of the motor. The operating speed of a synchronous motor will be constant for a given frequency and the number of poles. The following formula is for use in the determination of synchronous motor speed in revolutions per minute (RPM).
Because synchronous motor speed is controlled by the number of poles in the motor, a synchronous motor can be designed for a specific speed application. Figure 8 shows synchronous motor speeds in rpm for motors that are designed with different numbers of poles for different supply frequencies (Hertz).
The speed of a synchronous motor must always remain constant no matter how the load changes. The angle between the rotation of the field and the rotation of the rotor will increase as load is increased on a synchronous motor. This increase in angle will cause torque to increase, but the speed of the synchronous motor will remain constant. Load can be added to the synchronous motor until the developed torque of the motor reaches pull-out torque. The addition of any more load to a motor that is operating at pull-out torque will cause the motor to lose synchronism and stall.
Power factor and power factor correction are important aspects of a synchronous motor's operation. Power factor is defined as the ratio of real power to apparent power and is usually expressed as a percent leading when the current in the circuit leads the voltage in the circuit, or as a percent lagging when the current in the circuit lags the voltage in the circuit. Power factor is a measure of the efficiency of a circuit. Power factor takes into account inductive and capacitive reactance that dissipates power that is not available to do real work. The capacitive reactance in a capacitive circuit causes the current in the circuit to lead the voltage in the circuit; therefore, capacitive circuits have leading power factors. The inductive reactance in an inductive circuit causes the current in the circuit to lag the voltage in the circuit; therefore, inductive circuits have lagging power factors.
Power factor is expressed as a unitless fraction. Power factor equals one for a purely resistive circuit (no inductance or capacitance) and is less than one for circuits with inductive or capacitive reactance. Power factor is essentially a ratio of the pure resistance of a circuit to the circuit's total impedance. The power factor of a synchronous motor is controlled by the amount of field excitation that is supplied to the motor.
In a synchronous motor that is pulling a constant load, a variation of the stator current is accomplished through variation of the field current. Figure 9 shows synchronous motor "V" curves for no load, 1/2 load, 3/4 load, and full load conditions. The V curves describe the relationship between stator current and field current. The curves are called V-curves because of their shape. For any given load and any given motor, there is a single value of field current that will give a unity power factor at the motor terminals. An increase in the field current above the point for unity power factor (moving right) will cause a corresponding increase in stator current that will cause the power factor to become increasingly leading. A decrease of the field current from the unity point will cause the stator current to increase, and the power factor will become more lagging (moving left).
Synchronous Motor "V" Curves Figure 9
The motor field current is set at the value that is stamped on the nameplate and is kept at this point for all loads during operation. Maximum pull-out torque is maintained through sustenance of rated field current. Sustenance of rated field current provides the maximum level of power factor correction. In a motor that is operated at reduced load for a long period of time, reduction in the field current may be desired. Such a reduction of the field current would increase the motor efficiency. For a motor that operates at part load with a unity power factor, the field current can be adjusted until the stator current is at a minimum value.
The following equation is for use in the determination of the required stator current for a given pf:
A motor that operates at other than unity power factor will supply the system with either leading or lagging kVA. The amount of kVA that is supplied to the system can be determined, but first the correct stator current to achieve a desired power factor must be determined. The amount of kVA that is supplied to a system by a synchronous motor must be known to allow protective devices and operating mechanisms to be set. A synchronous motor that operates at full load and rated excitation delivers to the power system a leading kVA equal to:
where:
hp rating = The horsepower of the synchronous motor Eff = The efficiency of the synchronous motor
cos _ = Power factor
Electrical Engineers should note that more leading kVA is supplied at partial loads and rated excitation. The curves in Figure 10 show the reactive kVA for synchronous motors at four different power-factor ratings and at varying load conditions. These curves are based on maintenance of full-load rated field current at all loads. For example, a 100 hp (74.6 kW) 80% power factor synchronous motor operated at 75% load supplies a leading reactive kVA equal to approximately 66 percent of the motor's horsepower rating, or 66% reactive kVA. The unity power factor synchronous motor (100% pf motor), whose curve is shown in Figure 10, only supplies a leading reactive kVA when the load is less than 100%. The unity power factor synchronous motor, although providing no leading reactive kVA at full load, still improves the system power factor through addition of kilowatt load without increase to the system reactive-kVA load. A synchronous motor that operates at 90, 80, or 70% of power factor will provide a leading reactive kVA at all loads.
Reactive kVA for Synchronous Motors Figure 10
The efficiency of a motor is a ratio of the input power to the output power of the motor. Because a synchronous motor has no slip as load is added, the synchronous motor will have a higher efficiency than a corresponding induction motor. The full load efficiency of a synchronous motor is generally one to three percent higher than that of an induction motor. Typical Applications of Thr ee-Ph ase AC M otors
The squirrel-cage motor is one of the most widely used machines because the squirrel-cage motor can be built with electrical characteristics to suit almost any industrial requirement. Another reason the squirrel-cage induction motor is widely used is the motor's simplicity of construction. Squirrel-cage motors are not suitable in situations where a high starting torque is required, but, when the starting-torque requirements are of a medium or low value, the squirrel-cage induction motor is very suitable.
Typical applications of the squirrel-cage induction motor include blowers, centrifugal pumps, and fans. Because of the absence of any exposed electrical connections, the squirrel-cage induction motor is suitable for use in areas with hazardous environments.
The wound rotor induction motor is very similar to the squirrel-cage induction motor in application, but the wound rotor induction motor has the ability to start extremely heavy loads. The following are specific applications of the wound rotor induction motor:
_To drive various types of machinery that require development of considerable starting torque to overcome friction.
_To accelerate extremely heavy loads that have a flywheel or inertial effect. _To overcome back pressures set up by fluids and gases in the case of reciprocating pumps and compressors.
_When motors must be started frequently without overheating the motor. The advantages of a wound rotor motor over a squirrel-cage induction motor are:
_High starting torque. _Moderate starting current.
The main disadvantage is that both initial and maintenance costs of a wound rotor motor are greater than those costs of the squirrel-cage rotor motor. Also, the efficiency of the wound rotor induction motor is lower than the efficiency of a squirrel-cage induction motor.
A synchronous motor can be used for almost any application for which a squirrel-cage induction could be used. The main applications of synchronous motors fall into three areas:
_Power-factor correction
_Constant-speed, constant-load drives _Voltage regulation
Synchronous motors have two advantages over AC induction motors: _A constant speed with no variation due to changes in load.
_An ability to improve power factor when operated with high DC excitation. Another factor that must be taken into account in the decision between an induction or synchronous motor is cost. The cost of the higher-speed, low-horsepower, squirrel-cage induction motor and control is lower than the cost of the corresponding synchronous motor. The motor costs are reversed for higher horsepower and lower speeds; the synchronous machines are less costly.
Running cost also must be considered in selection between a synchronous motor and an induction motor. The full-load efficiency of an induction motor is generally one percent to three percent lower than that of a synchronous motor of the same horsepower and speed rating. The greater efficiency of the synchronous motor over the induction motor can pay cost dividends over the life of the motor operation.
The synchronous motor should not be used where fluctuations in torque are violent. As a general rule, synchronous motors also are not used in small sizes (under 50hp) because they require DC excitation and are more difficult to start than induction motors. Synchronous motors also fall out of step quite readily when system disturbances occur.
Oper ating Ch ar acteristics of Single-Phase AC Motors
Single-phase motors were one of the first types of motors developed for use on AC circuits. Single-phase motors have been perfected over the years from the original repulsion type into many improved types. The following are the types of single-phase motors that will be covered:
_Split-phase motor
_Repulsion induction motor _Capacitive start motor _Universal motor
Split-Phase Motor
The phase induction motor is the most popular of all the single-phase motors. The split-phase motor consists of a squirrel-cage rotor and two stator windings, a main winding, and a starting winding. Current that is applied to the motor will cause both windings to produce a magnetic field. The magnetic fields that are produced by the main winding and the starting winding will be mechanically and electrically displaced. The mechanical displacement is produced through position of the windings in the stator. The electrical displacement is produced through the use of windings with different electrical properties.
The main winding is produced to have a low resistance and a high inductance. The starting winding will have a high resistance and a low inductance. The different characteristics of the two windings produce a weak rotating electric field. The interaction of the two fields that are produced by the windings produce the motor's starting torque. In a split-phase motor, the starting torque is 150 to 200 percent of the full-load torque, and the starting current is six to eight times of the full-load current.
Figure 11 shows the speed torque characteristics of a split-phase induction motor. Upon energization of the motor, the combined windings produce the rotating magnetic field that will produce the necessary torque to start the motor. The motor accelerates to 75 to 80 percent of synchronous speed. At this speed, a starting switch (usually centrifugally operated) opens to disconnect the starting winding, and the motor operates with the main winding only. The function of the starting switch is to prevent the motor from drawing excessive current from the line and to protect the starting winding from damage due to overheating.
Speed-Torque Characteristics of a Split-Phase Induction Motor
Repulsion In duction Motors
The repulsion induction motor has a combination of a squirrel-cage and a repulsion winding on the rotor. Because of the combination windings, occasionally the motor is referred to as a squirrel-cage repulsion motor.
A repulsion induction motor can be designed to have either a constant speed or a variable speed characteristic. In the repulsion induction motor, the desirable starting characteristics of the repulsion motor (such as high starting torque) and the constant speed characteristics of the induction motor are obtained. Unfortunately, the two types of motors are impossible to combine and obtain only the desirable characteristics of each. Because the combination of both windings will cause the running torque of the repulsion induction motor to be less than a comparative split phase induction motor, a larger repulsion induction motor would be necessary for the same load rating.
Figure 12 shows the torque-speed characteristics of a typical repulsion induction motor. The rotating magnetic field of the repulsion induction motor is produced in the same way as in the split phase induction motor. The construction of the repulsion induction motor was discussed in Module EEX 203.01. The torque-speed curve of the repulsion induction motor is very similar to that of a repulsion motor. The repulsion induction motor has a high starting torque (approximately 300-350% full load torque) and can operate at relatively high speeds under light loads. The similarity between the repulsion induction motor curve and the curve of a repulsion motor is due to the dominance of the commuted repulsion winding when the repulsion induction motor is started.
Torque/Speed Characteristic of a Typical Repulsion-Induction Motor Figure 12
Brush position of the repulsion induction motor is very important in determination of the motor's operating characteristics. Figure 13 shows the characteristic curves of a repulsion induction motor that illustrates the effects of adjustment of brush position. Through adjustment of the brushes, the direction of rotation of the motor can be changed from clockwise to counterclockwise or vice versa. The other main effect of a shift in the brush position is the effect on motor starting torque. When the brushes are at the 0 brush position, the repulsion induction motor will produce zero starting torque. Starting torque can be maximized through shift of the motor brushes to 25 degrees off center. The torque graph is a quantative analysis of how the motor's torque will change. The line of zero torque shows the relative amount of motor starting torque as compared to other brush positions. The shift in brush position also will lower the motor's current.
Capacitive Start Motors
The capacitive-start motor is another form of split-phase induction motor that has a capacitor that is connected in series with the auxiliary winding. The auxiliary circuit of a capacitive start motor is opened when the motor has attained a predetermined speed. The net effect of the capacitor in the auxiliary circuit is to give its motor a starting torque of about four times the motor's rated torque. Once the capacitive start motor has come up to speed and the starting winding has been disconnected, the motor will have the same running characteristics as the split-phase motor.
The rotating magnetic field is produced identically to the way in which this field is produced in the split-phase motor. The larger starting torque comes from the addition of a capacitor in series with the starting winding. The addition of the capacitor will cause the electrical displacement of the two fields to increase. This increase in the displacement of the electrical fields produces the larger torque.
Figure 14 shows a comparison of the torque slip curves for a capacitor start and a split-phase motor. Curves are shown for both types of motors to show the comparison. Various starting capacitor values (200 _F, 300 _F, 400 _F, and 500 _F) also are shown for comparison. Through change of the value of the starting capacitor, the starting current will be greatly effected. An increase in the capacitor size will cause an increase in the motor's starting torque. The capacitor start type of motor has certain advantages over the other single-phase AC motors in that the motor has a considerably higher starting torque that is accompanied by a high power factor. Notice how the torque of the capacitive start motor drops at the point where the centrifugal switch opens. The capacitive start motor operates in the same speed range as a split-phase induction motor.
Torque-Speed Curves for Capacitor-Start and
Torque-Speed Curves for Capacitor-Start and Split-Phase MotorsSplit-Phase Motors Figure 14
Univ
Universal Motorersal Motor ss
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Universal motors are very susceptible to changes in speed and these changes in speed and these changes in speed must bemust be considered whenev
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_A change in load. _A change in load.
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15 shows the torque-speed characteriseed characteristics tics for a typical univerfor a typical universal motor with a change in thesal motor with a change in the frequency of th
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Adjustment of the speed of a universal motor is very ead of a universal motor is very easily accomplishsily accomplished. ed. The speed of theThe speed of the universal motor can be adjusted through adjustment of the input voltage to the motor. universal motor can be adjusted through adjustment of the input voltage to the motor. Adjustment of the input voltage to the motor is accomplished through use of a variable Adjustment of the input voltage to the motor is accomplished through use of a variable resistor.
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Torque/Speed Characteristics of a Typical Universal Motor Torque/Speed Characteristics of a Typical Universal Motor
Figure 15 Figure 15
Typical Applications of Single-Pha se AC Motor s
The split-phase induction motor is the most popular of the fractional-horsepower motor types. The split-phase motor is most commonly used in sizes that range from 1/30 hp (24.9 W) to 1/2 hp (373 W) for applications such as fans, business machines, automatic musical instruments, and buffing machines.
The split-phase motor has the advantage of a very low initial cost. A disadvantage of the split-phase motor is that the motor has a relatively low starting torque.
The capacitive-start motor is made in sizes from 1/4 hp (150W) to 10 hp (7.5 KW). The starting capacitor is a dry-type electrolytic cell made for AC use. Typical values of the capacitors are from 200 to 600_F. The major advantage of the capacitor start motor is the increase in starting torque. The starting torque of a capacitive start motor can be about four times the rated torque of the motor. This increase in starting torque makes the capacitive start motor very useful. The disadvantage in the capacitive start motor is the increased cost over the split-phase motor. Typical applications of the capacitive start motor would be a compressor or a pump drive because of the large starting torque that is developed by the capacitive start motor.
The repulsion induction motor is especially suitable to drive frequently started devices such as compressors, air pumps, and water systems. The two advantages to the repulsion induction motor are its low starting current and its constant speed. The low starting current of the repulsion induction motor is what makes this motor so suitable for applications that require frequent starting. The motor's constant speed characteristics add to the motor's efficiency. The only disadvantage to the repulsion induction motor is the increased cost of the motor over the split-phase induction motor. The repulsion induction motor will generally cost about twice as much as a split-phase induction motor.
The universal motor is often preferred because of this motor's ability to operate on direct current (DC) or on alternating current (AC). In areas where both AC and DC are available, use of a universal motor increases the flexibility of the motor's application. Most universal motors are used in high speed applications (such as portable tools) because of the difficulty in obtaining similar performance from AC and DC power supplies at low speeds.
The ability to adjust the speed of a universal motor at will by adjustment of the resistance is an advantage in uses where speed must be adjusted over a large range.
The disadvantage of a universal motor is the increased cost. The increased cost is due to the increased winding insulation requirements. The winding insulation requirements increase over a comparable series wound DC motor because of the peak voltage to which insulation is
OPERATING CHARACTERISTICS AND TYPICAL APPLICATIONS OF DC MOTORS
Each type of DC motor, although it contains the same basic parts as discussed in EEX 203.01, has very different operating characteristics. The operating characteristics of the different types of DC motors will be determined by how the windings of the motors are employed. The following topics will be covered in this section:
_Operating Characteristics of DC Motors _Typical Applications of DC Motors Oper ating Char acteristics of DC Motors
Different types of DC motors have different operating characteristics. Because of these differences, the proper type of DC motor should be selected when the load to be driven is known. The following types of DC motors are discussed below:
_Series Motors _Shunt Motors
_Cumulative Compound Motors _Differential Compound Motors.
Series Motor s
The series motor has the highest starting torque of all DC motors and is ideal for applications (such as hoists, cranes, and locomotives), that require high torque and slow speeds. The speed of a DC series motor is controlled by the size of its load.
Figure 16 shows the torque/speed characteristics of a DC series motor. Note that the motor speed varies greatly with respect to the torque. At point 1, there is no load on the motor, and the motor will overspeed and destroy itself from excessive speed. At point 2, 50% of the load has been applied to the motor. The torque increases, and speed will be about 150% of full load speed. At point 3, 100% load has been applied to the motor; the torque has increased to 100% full load torque; and speed is at 100% of full load speed. As load is increased past 100% full rated load, the speed of the series motor will drop rapidly. A heavy load must always be applied to a series DC motor otherwise, these motors will speed out of control and
Operating Characteristics of A DC Series Motor Figure 16
Shunt Motors
The shunt motor, in comparison to the series motor, has a very low starting torque that requires the shaft load to be relatively small. A DC shunt motor has a no load speed point and can be operated without a connected load. Operation of a DC shunt motor without load will not cause the motor to speed out of control.
Figure 17 shows the torque/speed characteristics of a DC shunt motor. Figure 17 shows that this motor will run at nearly the same speed at any load within the motor's capacity and that the motor will not slow very much even when it is greatly overloaded. There is only a slight drop in speed from no load (point 1) to full load (point 2). The slight difference in speed is called the droop of the motor.
Figure 17 also shows the development of linear torque through addition of load to the motor. The linear addition of torque allows for very smooth operation of the motor over a varying load.
Operating Characteristics of a DC Shunt Motor Figure 17
The shunt motor's speed can be varied through variance of the amount of current that is supplied to the shunt field. Control of the current to the shunt field allows the rpm to be changed by 10 to 20 percent when the motor is at full rpm. A shunt motor's speed control usually is accomplished through placement of a rheostat in series with the shunt field. Change in the position of this rheostat will increase or decrease the voltage that is applied to the field. This change in the voltage that is applied to the field results in a corresponding change in field current and strength. When the field current is decreased, the motor speed will increase. Motor speed will increase because of the following chain of events:
_When motor field decreases, CEMF decreases (FC _ N ¢f ).
_When CEMF decreases and applied EMF stays the same, armature current increases.
_When armature current increases in a shunt motor, torque increases (_ a _ _f Ia).
_When torque increases with constant load, speed increases (N _ _). In the above explained sequence:
Fc = Force of the CEMF
N = Speed of rotor
_f = Flux of the field
Ia = Armature current
Ea = Armature voltage
Ec = CEMF voltage
Ra = Armature resistance
_= Torque
From this sequence, the net effect of a decrease in the shunt motor field current is an increase in the shunt motor's speed. The opposite is true when shunt motor field current is increased. The shunt motor's rpm also can be controlled through regulation of the voltage that is applied to the motor armature. If such regulation is applied, and if the motor is operated on less voltage than is shown on its nameplate, the motor will run at less than full rpm. The shunt motor's efficiency will drastically drop off when the shunt motor is operated below the motor's rated voltage. The drop in motor efficiency is caused by an increase in heat loss in the motor windings. Because the motor will tend to overheat when operated below full voltage, motor ventilation must be provided. The motor's torque also is reduced when the motor is operated below the full voltage level.
Cumulative-Compound Motors
In a cumulative compound motor, the series and shunt windings are connected so that the flux that is produced by the windings aid each other. The DC cumulative compound motor will have a combination of the operating characteristics of a series DC motor and a shunt DC motor. The cumulative compound DC motor will have more starting torque than a
shunt DC motor but not as much starting torque as a series DC motor. The cumulative motor will have larger speed droop than a shunt DC motor but not as much speed droop as a series DC motor. The characteristics of the cumulative motor will be determined by the amount of turns in the series field. The more turns there are in the series winding, the more closely the operating characteristics will emulate those of a series DC motor. When a cumulative compound DC motor has few turns in the series field DC motor, the motor will more closely resemble the operating characteristics of a DC shunt motor.
Figure 18 shows the torque/speed characteristics of a cumulative compound DC motor. Notice how the speed droops as the load is increased. Although the droop is greater than that of a shunt DC motor, the cumulative compound motor does have a no load speed and will not runaway. The torque development of a cumulative DC motor is relatively linear. Because speed is not easily controlled in a cumulative compound DC motor, the cumulative compound DC motor is not suitable for applications that require adjustable speed.
Torque/Speed Characteristics of a Cumulative Compound Motor Figure 18
Differential Compound M otors
A differential compound DC motor is of a very similar design to a cumulative compound DC motor. The only difference in the two types of compound motors is that in the differential compound DC motor the series and shunt windings will be connected so that their individual flux will be in opposition to each other. A change in the connections of the fields in the differential compound DC motor will cause the field fluxes to oppose each other. The differential compound DC motor will have a lower starting torque but a more constant speed characteristic than the cumulative compound motor.
Figure 19 shows the torque/speed characteristics of a differential compound DC motor. Notice that the torque will raise in an approximately linear manner as in the cumulative compound DC motor, but the speed will not droop as much as it will in the cumulative compound DC motor. Figure 19 also shows that the differential compound DC motor will have a rising speed characteristic when load is added beyond the full load of the motor.
Torque/Speed Characteristics of a Differential Compound Motor Figure 19
Typical Applicat ions of DC Motor s
A series DC motor is used in applications where a high starting torque is required but where running speed regulation is of little concern. The advantages of the series DC motor are the motor's high starting torque and relatively low initial cost. One disadvantage of the series motor is that the speed will decrease steadily as more load is applied to the motor. Another disadvantage of the series DC motor is that the motor must be hard-connected to a load because the series DC motor has no "no load" speed. Loss of load on a DC series motor will cause the motor to speed out of control and destroy itself.
A shunt DC motor, as compared to the series DC motor, will have a lower starting torque but much better speed control. Because of the motor's speed control, the shunt DC motor is very useful in applications where speed accuracy is required but where a large starting torque is not required. A good example of where speed control would be necessary is on machine tools or lathes. Because a shunt DC motor also has a no load speed, runaway is not a concern of the shunt DC motor. A no load speed makes the shunt motor very useful in running belt drive equipment such as a conveyor belt. The main advantage of a shunt DC motor is speed control. The main disadvantage of a shunt DC motor is the low starting torque.
The use of the cumulative and differential compound DC motors are very similar. Because both the cumulative and differential compound DC motors are the same except for electrical connections, cost is not an issue in selection of the motor type. Both of the compound DC motors cost more than the series or shunt motors. The cumulative compound DC motor would be used where a higher starting torque is required but where speed control is not a vital issue; examples of this application would be in some hoisting and conveying machinery. The differential compound DC motor would be used in situations where a high starting torque is not required but where speed regulation is more important. Examples of typical applications of the differential compound DC motor would be in pumps or paper cutting machines. The main advantage of a compound DC motor is that these motors can be designed to combine the desired characteristics of the shunt DC motor and the series DC motor. The main disadvantage of compound DC motors is that these motors cost more than the shunt DC motor or the series DC motor.
Figure 20 shows an overall composite of the speed, torque, and current (load) characteristics for compound, shunt, and series DC motors of equal size. This figure provides a comparison of the torque and speed characteristics that each DC motor type will exhibit. The cumulative and differential compound motors are shown as one line that is called compound because the
Figure 20 also shows that the series motor speed droop is much greater than that of the compound or shunt motors. If speed control is the most important consideration, the shunt motor is the best selection. The DC compound motors are good examples of a compromise in both torque and speed characteristics. The selection of this compromise will cause costs to increase due to the complexity of the motor.
Speed-Torque and Current Characteristic Curves for Compound, Shunt, and Series DC Motors of Equal Size
SELECTING THE APPROP RIATE TYPES OF THREE-PHASE AC MOTORS
Selection of the appropriate type of three-phase motor for a Saudi Aramco application, requires that all the motor selection factors to be considered. Early failure may occur in a three-phase AC motor that does not completely fit the intended application. The selection of a type of 3_ AC motor basically involves a choice between an induction and a synchronous motor. The subselection of a squirrel-cage induction versus a wound rotor motor is based on torque requirements and cost. The selection of a motor also will take into account the area into which the motor will be installed.
Motor Selection Factors
A designer must weigh all of the factors that bear on the selection of the type of 3_ AC motor for a particular application. The following factors must be considered:
_Preferred Voltage and Horsepower Ranges _Load Characteristics
_Motor Starting Characteristics _Speed
_Power Output Required
_Limitations of the Supply Network _Cost
Pr eferr ed Voltage and H orsepower Ra nges
Three-phase AC motors come in a variety of standard voltage and horsepower ratings. A special order for a specific rating that is not in a standard rating would cause the cost of the motor to increase. Saudi Aramco allows a limited choice of voltage and horsepower ranges. The actual table of the allowed voltage and horsepower ranges for use in Saudi Aramco installations is in Work Aid 1.
The second table in Work Aid 1 shows the preferred voltage and horsepower ranges of Saudi Aramco installations. Column 1 of the table in Work Aid 1 shows the nominal system voltages. Column 2 of the table in Work Aid 1 shows the nameplate voltage or utilized voltage. The European practice is to quote the nameplate voltage, but the American practice is to quote nominal system voltage. To avoid confusion of voltage, all motors for use in Saudi Aramco will be specified by the nameplate voltage only. Column 3 of the table in Work Aid 1 gives the number of phases in the motor. Column 4 of the table in Work Aid 1
The table shows that synchronous motors are only used on high horsepower applications unless the operating speed is required to be less than or equal to 1200 rpm.
Load Char acteristics
Load characteristics refer to the following: _Load horsepower
_Required load starting torque
_Speed at which the load must operate _Steadiness or unsteadiness of a load
Load horsepower will determine the size of the motor that is required for the application. In actuality, any type of motor can be designed for a specific horsepower requirement. Column 4 of the second table in Work Aid 1 gives the approved Saudi Aramco horsepower ranges for motors.
Required Load Starting Torque of a load will vary with the type of load. For loads that obey a square-law characteristic, a motor's required load starting torque should be at least 60 percent of the full-load torque for liquid pumps, and 40 percent of full-load torque for gas-handling pumps. Pumping requirements over 11,000 kW (15,000 Hp) should be referred to Consulting Services Department.
A constant speed motor must be applied for loads that must operate at constant speed. The synchronous motor by design must always run at a constant speed. The speed of a synchronous motor was designed by its construction and the supply frequency. Saudi Aramco uses 60 Hz input frequency for all motors. The synchronous motor would be the best choice for a load that must be operated at a constant speed.
The steadiness of a load also plays a large role in the selection of a type of 3_ AC motor. Induction motors account for varying loads through adjustment of the amount of slip. A synchronous motor does not compensate for a varying load. Where the variations in load are excessive, such as a compressor application, a synchronous motor may slip out of synchronism and stall. The pole slippage could be for a short duration such as one pole or a complete stoppage of the motor. Slippage or stoppage depends on the amount of load variation.
Figure 21 shows typical load speed/torque curves for Saudi Aramco equipment. The figure shows that the profile of the curve for the centrifugal pump can be changed through variance of the load (valve shut or valve open). Reduced load starting (valve shut) should be done only when absolutely necessary because of excess heat build up in the motor. The axial compressor load torque drops initially after the load commences to move because of the inertia of the load. The axial compressor will develop a smooth torque build up.
Typical Load Speed/Torque Curves Figure 21
