FAA AMT (Aviation Maintenance Technician) Airframe Practice Exam

A series wound dc electric motor will normally require more current at what speed?

1. At high RPM

2. At low RPM

3. At medium RPM

4. Irrelevant of the RPM

The correct answer is: At low RPM

A series wound dc electric motor will normally require more current at low RPM because at low speeds, the resistance of the circuit is higher, causing the current to increase in order to maintain the same torque. At high speeds, the back EMF (electromotive force) generated by the motor counteracts the current flow and reduces the overall current consumption. Similarly, at medium speeds, the back EMF and resistance are balanced, resulting in a stable current consumption. Therefore, low RPM is the correct answer as it requires the most current due to the higher resistance. Option A is incorrect because at high RPM, the back EMF reduces the current consumption. Option C is incorrect because at medium RPM, the back EMF and resistance are balanced, resulting in a stable current consumption. Option D is incorrect because the speed of the motor does affect the current consumption.