Interpreting a Fault Symptom in a Motor Circuit
A three-phase induction motor driving a workshop fan trips its overload relay about 90 seconds after start-up every time. The motor runs normally when uncoupled from the fan. Line current measured on all three phases with a clamp meter is 6.8 A, 6.9 A and 7.0 A against a nameplate full-load current of 5.2 A. The supply voltage is balanced at 400 V. The fan turns freely by hand when the motor is isolated, but the belt is very tight and the fan bearings feel warm. The overload relay is set to 6.0 A.
1. Using the current readings, decide whether the fault lies in the motor, the supply, or the driven load, and justify your answer.
Read the worked answer
The three line currents are closely balanced (6.8, 6.9 and 7.0 A), within about 3% of each other. A motor winding fault or an unbalanced supply would normally produce a marked phase imbalance and often a single high phase, so the motor and supply are unlikely to be the cause.
The current is about 31% above the 5.2 A nameplate value, and the motor runs normally when uncoupled. That pattern points to the driven load: excessive belt tension and warm fan bearings mean the fan is harder to turn than it should be, so the motor draws more current to deliver the required torque. The overload relay is correctly tripping at 6.0 A, which is above nameplate but below the observed 6.8–7.0 A.
2. Recommend corrective actions in priority order and state how the overload setting should be treated.
Read the worked answer
First, isolate and lock off the motor circuit. Then release the belt tension to the manufacturer's recommended deflection and inspect or replace the fan bearings; warm bearings indicate excess friction and possible lubrication failure. Check that the fan rotates freely and that nothing obstructs it. Re-tension the belt correctly and test the fan by hand before re-energising.
After correction, run the motor and re-measure the line currents. They should fall toward the nameplate value. The overload relay should be set to the nameplate full-load current, not raised to stop tripping; increasing the setting would allow the motor to overheat and would mask a genuine mechanical fault. Record the before-and-after readings in the maintenance log.