To answer the following questions refer to Figure 12–10. When the thermostat contacts close the blower motor starts and the heating elements turn on normally. When the thermostat contacts open, the heating elements turn off and the blower motor stops running immediately. Which of the following could cause the problem? a. The thermostat contacts did not open. b. The flow switch contacts did not open. c. The fan thermostat contacts did not close. d. BMR coil is open. FIG. 12–10 Air flow must be present before heating elements can be energized.

Choose the correct option that causes the heating elements to turn off and the blower motor stops running immediately when the thermostat contacts open.

Explanation of Solution

Refer to Figure 12-10 in the textbook which shows a basic control circuit for an electric furnace. A double pole circuit breaker protects this circuit. This circuit contains three heating elements and two timed contacts. When HR contacts close, the power is supplied to first heating element. After a time delay, the timed contact T1 closes and the power is given to the second heating element. After another time delay, the timed contact T2 closes and supplies power to the third heating element.

To operate the circuit, the transformer steps down the voltage from 240 V to 24 V. When the thermostat contact closes, the CR relay coil energizes and the two CR contacts are closed. Due to the open flow switch, the HR coil is not energized. The contactor coil BMR is energized, which causes the two BMR contacts to close and connect the blower motor to the power line. When the blower motor starts, the airflow occurs and the flow switch is closed, the HR contactor coils are energized...

Electric Motor Control

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Electric Motor Control

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