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Part winding starting is another method of reducing the starting current of squirrel cage induction motors. Motors designed to be used for part winding starting contain two separate stator windings (FIG. 1). The stator windings may be wye or delta connected, de pending on the manufacturer. These two windings are designed to the connected in parallel with each other.
When the motor is started, only one of the windings is connected to the power line. Since only half the motor winding is used during starting, this method of starting is called part winding starting. Part winding starting reduces the normal locked rotor current to approximately 66% of the value if both windings are connected during starting and the torque is reduced to approximately 50%. It should be noted that neither of the two windings is individually capable of withstanding the starting current for more than a few seconds. The first winding will overheat rapidly if the second winding is not connected within a very short period of time. As a general rule, a time delay of two to three seconds is common before the second winding is connected in parallel with the first.
Part winding starting is accomplished by bringing out both sets of motor leads so that external connection is possible (FIG. 2). When the Start button is pressed, motor starter 1M energizes and connects the first motor windings to the line. The normally open 1M auxiliary contact closes and starts on-delay timer TR.
After a 2 second time delay, timed contact TR closes and energizes motor starter 2M. This causes the 2M load contacts to close and connect the second stator winding to the power line.
Note that two motor starters are used in the circuit, and each contains an overload relay. Each winding is individually protected by thermal overload heaters. The heaters for each overload relay should be sized at one half the motor nameplate current. The contacts of both overload relays are connected in series so that an overload on either relay will disconnect both motor windings. It should also be noted that since each starter carries only half the full load current of the motor, the starter size can generally be reduced from what would be required for a single starter. Another advantage of part winding starters is that they provide closed transition starting, since the motor is never disconnected from the power line during the starting time.
Dual Voltage Motors
Some, but not all, dual voltage motors may be used for part winding starting. The manufacturer should be contacted before an attempt is made to use a dual voltage motor in this application. Delta connected dual voltage motors are not acceptable for part winding starting.
When dual voltage motors are used, the motor must be operated on the low voltage setting of the motor. A 240/480 volt motor, for example, could only be operated on 240 volts. A dual voltage motor connection is shown in FIG. 3. Motor terminal leads T4, T5, and T6 are connected together forming a separate wye connection for the motor.
Part winding starting is typically used for motors that supply the moving force for centrifugal pumps, fans, and blowers. They are often found in air conditioning and refrigeration applications. They are not generally employed to start heavy inertia loads that require an excessive amount of starting time.
The thermal capacity of the stator windings greatly limits the length of starting time for a part winding mo tor. To help overcome this problem, it is possible to provide a third step in the starting process and further limit starting current. This is accomplished by connecting resistance in series with the stator winding during the starting period (FIG. 4). The resistors are generally sized to provide about 50% of the line voltage to the stator winding when the motor is first started. This provides approximately three equal increments of starting for the motor. In the circuit shown in FIG. 4, when the Start button is pressed, mo tor starter 1M energizes and connects one of the stator windings to the power line through the series resistors.
After a delay of 2 seconds, TR1 timed contact closes and energizes contactor S. The S load contacts close and shunt the resistors out of the line. One stator winding is now connected to full line voltage.
After another 2 second delay, motor starter 2M energizes and connects the second stator winding to the power line. The motor now has both stator windings connected to full voltage.
Part winding motors are very sensitive to the length of time that one winding can be connected before thermal damage occurs. If the second winding is not connected to the power line within a short period of time, the first winding can be severely damaged. To help prevent damaging the first winding, some circuits contain a timer that disconnects power to the motor if the second winding is not energized within a predetermined time.
This timer is often called a watchdog timer because its function is to watch for proper operation of the circuit each time the motor is started. A circuit with a watch dog timer is shown in FIG. 5. Watchdog timers are often set for twice the amount of time necessary for the second winding to energize. When the Start button is pushed, the watchdog timer begins its count. If the circuit operates properly, the normally closed 2M auxiliary contact will disconnect the timer before it times out and de-energizes control relay CR.
1. A dual voltage 240/480 volt motor is to be used for part winding starting. Which voltage must be used and why?
2. Are the stator windings of a motor designed for part winding starting connected in parallel or series?
3. The nameplate of a part winding motor indicates a full load current rating of 72 amperes. What current rating should be used when sizing the overload heaters?
4. What is a watchdog timer?
5. Refer to the circuit shown in FIG. 5. When the Start button is pressed, the motor does not start.
Which of the following could not cause this problem?
a. The control transformer fuse is blown.
b. Overload contact #2 is open.
c. TR1 timer coil is open.
d. Control relay coil CR is open.
6. Refer to the circuit shown in FIG. 5. When the Start button is pressed, the motor does not start.
After a 4 second time delay, control relay CR de-energizes. Which of the following could cause this problem?
a. TR1 timer coil is open.
b. 1M starter coil is open.
c. CR coil is open.
d. 2M starter coil is open.