Replacing a Motor Starter with Incorrect Overload Setting
A maintenance technician is replacing a failed motor starter on a 5 HP, 230 V, 3-phase conveyor motor. The motor nameplate shows a service factor of 1.15 and a full-load current of 14 A. The existing starter had a thermal overload set at 14 A. The replacement starter comes with a new overload relay, but the technician sets it to 14 A because 'that's what the old one was.' The conveyor is in a dusty environment and the motor has a history of nuisance trips. The technician also notices the motor has a nameplate temperature rise of 40°C. The facility uses NEC Article 430.
1. What is the correct overload setting for this motor, and why might the old setting have caused nuisance trips?
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Per NEC 430.32(A)(1), for a motor with a service factor of 1.15, the overload should be sized at 125% of the nameplate full-load current if the motor is not marked with a temperature rise over 40°C. Here the temperature rise is 40°C, so the 125% rule applies. 125% of 14 A = 17.5 A. The old setting of 14 A was too low, which explains nuisance trips. The correct setting is 17.5 A. If the overload relay cannot be set exactly, use the next higher size per NEC 430.32(C).
2. The technician sets the overload to 17.5 A, but the motor still trips after 20 minutes under normal load. What should the technician check next, and what does this indicate?
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The technician should check:
- Actual current draw with a clamp meter under normal load. If it exceeds nameplate FLA, the motor may be overloaded mechanically or electrically.
- Voltage balance and phase loss.
- Motor winding resistance and insulation resistance.
- Ambient temperature at the motor; a dusty environment can reduce cooling.
- Overload relay class and ambient compensation. If current is within nameplate and trips persist, the overload may be defective or miswired, or the motor may have an internal fault. The technician should not simply increase the overload setting beyond code limits.