The thermal capacity of the stator windings greatly limits the length of starting time for a part winding motor. 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 (Figure 40 – 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 Figure 40 – 4, when the Start button is pressed, motor 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 en- ergizes 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 Figure 40 – 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 aux- iliary 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 Figure 40 – 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 Figure 40 – 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.
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