Elevator Mechanic Practice Exam

You have an old DC motor and are adding an SCR drive, what do you do to the interpoles?

1. Disconnect the interpoles

2. Reconnect the interpoles in reverse

3. Leave the interpoles alone

4. Replace the interpoles

The correct answer is: Disconnect the interpoles

When converting an old DC motor to work with a Silicon Controlled Rectifier (SCR) drive, it's important to consider the function of the interpoles, which are used to improve commutation in a DC motor by generating an additional magnetic field. In the context of an SCR drive, the operation of the motor changes significantly due to the way SCRs control voltage and current. Disconnecting the interpoles is the appropriate action because, under SCR operation, the voltage and current waveforms become pulsating, which can alter the characteristics of the magnetic field generated by the interpoles. If left connected, the interpoles might interfere with the newly established operating characteristics of the motor, potentially leading to poor commutation or even damage. Choosing to reconnect the interpoles in reverse or replacing them may not necessarily accomplish the desired outcomes, and leaving them alone could result in improper operation due to incompatibility with the SCR control method. Disconnecting them, on the other hand, allows the motor to operate more effectively with the SCR drive's control strategy.