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
Primary Resistance Starting
The primary resistance starting method is essentially the same as a primary reactor starting method. The only difference between the primary reactors and primary resistance starters is that, in the primary resistance starting method, the reactors are replaced by resistors in the starting circuit. Figure 18 is a simplified diagram of a primary resistance motor starter. Notice that the diagram is the same as the diagram for a primary reactor starter, except that the reactors have been replaced by resistors. Contacts 1, 2, and 3 are closed to apply line voltage to the motor stator to start a motor with primary resistance starting. The resistors that are in series with the stator limit the current flow in the stator. When the motor accelerates to rated speed, contacts 4, 5, and 6 close to short out the resistors and to allow full current to flow.
Primary Resistance Motor Starter
Figure 19 shows the typical motor torque, speed, and kVA characteristics for a primary resistance motor starter. Motor torque at full voltage is shown for comparison. Notice that the primary resistance motor starter curves are identical to the starter curves of a primary reactor motor starter. The line current at starting is proportional to the motor's applied voltage; starting torque is proportional to the square of the motor's applied voltage. Motor starting kVA is high for the amount of starting torque that is developed by the motor. The motor speed run-up will be smooth until the motor is switched from start to run. The rapid jump in motor torque and kVA can cause a disturbance in the motor's speed that is not acceptable for certain loads.
A primary resistance motor starter costs approximately 520% of the cost of a full voltage starter. For economic reasons, reactors rather than resistors are used with all but the smaller sizes of motors. A comparison of the relative cost of a primary resistance motor starter and other motor starting methods is given in Work Aid 3.
Torque, Speed, kVA Curves for Primary Resistance Motor Starter
Part Winding Starting
Figure 20 is a simplified diagram of a part winding motor starter. The part winding motor starter has two sets of contacts to supply the motor. Part-winding motors are similar in construction to standard cage motors except that two parallel windings are provided in the stator and six leads must be included. Contacts 1, 2, 3 are closed to start a part winding motor. This closing will energize one set of windings in the motor. After the motor is almost at full speed, contacts 4, 5, 6 are closed. This closing will energize the other set of windings to supply full line voltage to the motor stator.
Part Winding Motor Starter
Figure 21 shows the typical torque, speed, and kVA curves for a part winding starting motor. Motor torque at full voltage starting is shown for comparison. Starting torque on the first starting point varies from 48% to 72% of full load torque. The actual percentage depends on motor design, size, and speed. Notice the spike in motor torque and kVA when the second set of windings is energized. The sudden change in motor torque and KVA can cause a large disturbance in motor speed and run-up. Starting current varies from 50% to 80% of the locked-rotor current with both windings. The actual percentage depends on motor design, size, and speed.
Part-winding starting can only be used with limited types of loads because of the small amount of starting torque that is generated on the first step of acceleration. Part-winding starters are used with motors that drive low inertia, low-torque starting loads such as air- conditioning compressors, refrigeration compressors, centrifugal pumps, fans and blowers. This method also is used where reduced starting torque is necessary.
A part-winding motor starter will cost approximately two to four times a full voltage starter. A cost comparison of part-winding motor starting with other motor starting methods is given in Work Aid 3.
Typical Torque, Speed, and kVA Curves of a Part Winding Starting Motor
Variable Frequency Starting
Figure 22 is a simplified diagram of a variable frequency motor starter. Variable frequency starting is essentially the same as full voltage starting except that the full voltage and frequency input to the motor stator can be converted to different values. The frequency and voltage converter may be a short-time rated motor-generator set or a solid-state unit.
Both the output voltage and frequency of a variable frequency motor starter must be adjusted proportionally to each other. The output voltage and frequency must vary proportionally to ensure that the motor does not draw excessive current as a result of the lower impedance that is present at low frequencies.
Contacts 1, 2, and 3 are shut to start a motor that uses a variable frequency starter. When these contacts shut, the voltage and frequency at the output of the voltage and frequency converter will be applied to the motor stator.
Variable Frequency Motor Starter
One frequency and voltage converter may be used for several motors on one bus if only one motor is to be started at any one time. A smooth motor speed run-up is possible through the steady increase of the supply frequency to the motor from the starting value to 60 Hz. Because of the rise in the frequency that is applied to the motor, no current surge is imposed on the supply system.
The torque, speed, kVA, and current curves of a variable frequency motor will follow the same shape as for full voltage starting. The values of torque, speed, kVA, and current will change proportionally with the applied value of voltage and frequency. Maximum motor torque up to the full voltage, full frequency, breakdown torque is possible throughout the run- up period.
Variable frequency drives are mainly used for speed control. The use of a variable frequency driver only as a starting device is generally cost prohibitive.
Electronically-Controlled Reduced Voltage Starting
Electronically-controlled reduced voltage starters employ back-to-back, phase-controlled thyristors in two or three of the lines to the motor as shown in Figure 23. The thyristors (G1 - G6) are controlled during the starting period to maintain the starting current at about 300% of the full load current through the gradual increase of the motor voltage from the initial value up to full line voltage.
Electronically Controlled Reduced Voltage Starter
The torque, speed, kVA, and current can be easily adjusted through change in the amount of time that the thyristor will conduct, and this change will change the applied voltage. The motor run-up will be smooth because the applied voltage will gradually increase as the motor speeds up and because there is no mechanical switching.
The electronically-controlled reduced voltage starter is applied where the line current is critical and where repetitive motor starting would limit the life of electromagnetic contactors. The cost of an electronically-controlled reduced voltage starter is prohibitively high in most instances.
Factor Considered in Starting Method Selection
Full voltage starting is used for the majority of Saudi Aramco induction and synchronous motor applications. Reduced voltage starting only should be considered when one of the following conditions exist:
_The calculation of the voltage drop that results from motor starting indicates that the applied voltage at the motor terminals will be less than 80% of the motor nameplate voltage.
_The load and/or the connection between the load and the motor may be damaged by the sudden application of full voltage starting torque.
_The motor will be started several times an hour or the motor draws excessive starting current.
In certain applications, the motor must be capable of starting under the worst case conditions. The characteristics of a motor that one chooses for a particular application must match an entire range of load torque characteristics; however, one must avoid the unnecessary expenditures that can result from over-specification. The worst case conditions are assumed because, if a motor will start under these conditions, the motor will start under any conditions. Worst case conditions are hypothetical conditions that assume that all possible detriments simultaneously occur. The worst case condition is when there is maximum load on the motor, while there is the lowest possible bus voltage, all other loads are running, and the largest motor starts. The resulting voltage drop can cause longer acceleration time for the largest motor and a heating that can result in deterioration and/or failure of the insulation. Also, the voltage drop can trip breakers and deenergize other loads. Many more possible conditions that depend on the conditions that are present at a given installation could exist; therefore, each installation must be individually evaluated.
Factor Considered in Starting Method Selection (Cont'd)
The following factors are considered in the selection of a starting method: _Permissible system voltage drop
_Required starting torque of the load/load connection _Load current (heating) limitations
_Comparative cost
Terminal voltage that drops below 80% of full voltage value can still result in successful starting of the motor; however, the drop in the terminal voltage will cause the necessary current draw to increase in proportion to the drop in terminal voltage. The problem of excessive voltage drops when motors are started is that other loads on the system can be effected. Checks should be made to ensure that the motor controllers for any running motors that are on the same bus or on any other bus that is affected by the voltage depression remain held-in, and that the running motors do not stall. Figure 37 in Work Aid 3 shows the approximate voltage drop that results from full voltage starting of a motor.
Motors and their respective loads must be connected by some means. The method of connection can be a hard permanent connection, a spider contact, or a simple belt. The method of motor/load connection must be considered in the selection of a motor starting