Motor starter rheostats for DC motors operated from a constant voltage supply must be equipped with automatic devices that will interrupt the power supply before the speed of the motor falls to a certain level. What is this speed?

• A. 1/2 its normal rate
• B. 1/3 its normal rate
• C. 3/4 its normal rate
• D. 2/3 its normal rate

Answer: (B)

When a DC motor is fed from a constant voltage, its speed under load is determined by the balance of applied voltage and back-EMF. As the motor slows, back-EMF drops, so the winding current tends to rise. That increased current causes more heating, which can damage the motor and the starter if allowed to continue. To prevent this, automatic devices are set to interrupt power before the speed falls too far. The chosen cutoff speed is about one-third of the motor’s normal running speed, which provides a safety margin: it halts operation before current becomes excessive, yet it isn’t so low that normal startup or brief load changes cause false trips. This threshold preserves protection without unnecessarily interrupting operation, keeping the motor from overheating at low speeds.