A technician observes that a DC motor runs slower than its rated speed and draws higher than normal current. The power supply voltage is confirmed to be correct. What is the MOST likely electrical issue causing this behavior?
- AAn open circuit in the motor field winding.
- BA high resistance connection in the armature circuit.
- CA short circuit in the motor windings.
- DExcessive load on the motor shaft.
Show answer & explanationAnswer & explanation
Correct answer: B. A high resistance connection in the armature circuit.
A DC motor's speed is inversely proportional to its armature current (due to back EMF) and directly proportional to the applied voltage. A high resistance in the armature circuit would reduce the current through the armature for a given back EMF, thus reducing the torque produced. To compensate for the load, the motor would try to draw more current (if it can) but would ultimately slow down due to the increased voltage drop across the internal resistance and the added series resistance, leading to a higher overall current from the source to maintain some torque. If the motor slows down, back EMF reduces, allowing more current to flow (I_a = (V_t - E_b) / R_a). An external resistance in the armature path would increase R_a, causing the motor to draw more current to try and maintain torque, but still run slower.
Why the other options are wrong
- A. An open circuit in the field winding of a shunt DC motor would cause it to overspeed or 'run away' (as field flux drops, back EMF drops, and speed increases rapidly), not run slower.
- C. A short circuit in the motor windings would typically cause extremely high current and immediate OCPD tripping, or severe damage, not just slower speed and higher current.
- D. While excessive mechanical load would cause the motor to run slower and draw higher current, the question asks for the MOST likely *electrical* issue. If the load is truly 'excessive' beyond design, it's a mechanical issue. If it's the *electrical* issue causing it to act like it has excessive load, then it's an internal electrical problem.
DC Motor Armature Resistance Effect
In a DC motor, increased resistance in the armature circuit reduces the back electromotive force (EMF) for a given speed, leading to higher armature current but reduced speed under load.
- Motor speed is proportional to (V - I_a*R_a) and inversely proportional to field flux.
- Back EMF (E_b) opposes the applied voltage (V).
- If R_a increases, the voltage drop across R_a increases.
- To maintain torque, current increases, but speed decreases due to reduced effective voltage across the armature.
Memory trick: Speed vs. Current: Back EMF, Resistance, Flux dictate the dance.