Alternating current (AC) induction motors are divided into two electrical categories based on their power source – single phase and polyphase (three phase). As with any motor, complications can arise that cause the motor to underperform. Below outlines common issues found in ac motors with identified likely causes and solutions.
Note: The suggestions outlined are intended to narrow down the source of the problem, not serve as a repair guide for customers. Please reach out to a qualified technician for all repairs.
Besides checking load, also inspect drive belt tension to ensure it’s not too tight may be too high. An unbalanced load will also cause the bearings to fail.
High ambient temperature.
If the motor is used in a high ambient, a different type of bearing grease may be required. You may need to consult the factory or a bearing distributor.
Stator is shorted or went to ground. Motor will make a humming noise and the circuit breaker or fuse will trip.
Disassemble motor and inspect windings and internal connections. A blown stator will show a burn mark. Motor must be replaced or the stator rewound.
Motor overloaded or load jammed.
Inspect to see that the load is free. Verify amp draw of motor versus nameplate rating.
Capacitor (on single phase motor) may have failed.
First discharge capacitor. To check capacitor, set volt-ohm meter to RX100 scale and touch its probes to capacitor terminals. If capacitor is OK, needle will jump to zero ohms, and drift back to high. Steady zero ohms indicates a short circuit; steady high ohms indicates an open circuit.
Starting switch has failed.
Disassemble motor and inspect both the centrifugal and stationary switches. The weights of the centrifugal switch should move in and out freely. Make sure that the switch is not loose on the shaft. Inspect contacts and connections on the stationary switch. Replace switch if the contacts are burned or pitted.
Verify that the load is not jammed. If motor is a replacement, verify that the rating is the same as the old motor. If previous motor was a special design, a stock motor may not be able to duplicate the performance. Remove the load from the motor and inspect the amp draw of the motor unloaded. It should be less than the full load rating stamped on the nameplate.
Ambient temperature too high.
Verify that the motor is getting enough air for proper cooling. Most motors are designed to run in an ambient temperature of less than 40°C. (Note: A properly operating motor may be hot to the touch.
Protector may be defective.
Replace the motor’s protector with a new one of the same rating.
Winding shorted or grounded.
Inspect stator for defects, or loose or cut wires that may cause it to go to ground.
If voltage is less than 90% of the motor’s rating contact power company or check if some other equipment is taking power away from the motor. If motor is run using an extension cord, verify that this extension cord is properly sized for motor's current draw.
Load increased.
Verify the load has not changed. Verify equipment hasn’t got tighter. If fan application verify the air flow hasn’t changed.
Inspect switch contacts and connections. Verify that switch reeds have some spring in them.
Bad bearings.
Noisy or rough feeling bearings should be replaced.
Voltage too low.
Make sure that the voltage is within 10% of the motor’s nameplate rating. If not, contact power company or check if some other equipment is taking power away from the motor.
Remove motor from load and inspect motor by itself. Verify that motor shaft is not bent. Rule of thumb is .001" runout per every inch of shaft length.
Motor bearings defective.
Test motor by itself. If bearings are bad, you will hear noise or feel roughness. Replace bearings. Add oil if a sleeve of bearing. Add grease if bearings have grease fittings.
Rotor out of balance.
Inspect motor by itself with no load attached. If it feels rough and vibrates but the bearings are good, it may be that the rotor was improperly balanced at the factory. Rotor must be replaced or rebalanced.
Motor may have too much endplay.
With the motor disconnected from power turned shaft. It should move but with some resistance. If the shaft moves in and out too freely, this may indicate a preload problem and the bearings may need additional shimming.
Winding may be defective.
Test winding for shorted or open circuits. The amps may also be high. Replace motor or have stator rewound.
The motor is not coming up to speed quickly enough.
Motor may not be sized properly. Verify how long the motor takes to come up to speed. Most single phase capacitor start motors should come up to speed within three seconds. Otherwise the capacitors may fail.
The motor is being cycled too frequently.
Verify duty cycle. Capacitor manufacturers recommend no more than 20, three-second starts per hour. Install capacitor with higher voltage rating, or add bleed resistor to the capacitor.
Voltage to motor is too low.
Verify that voltage to the motor is within 10% of the nameplate value. If the motor is rated 208-230V, the deviation must be calculated from 230V.
Starting switch may be defective, preventing the motor from coming out of start winding.
Ensure that motor was not damaged in shipment. Frame damage may not be repairable. If you cannot see physical damage, inspect the motor’s rotor and stator for strike marks. If signs of rubbing are present, the motor should be replaced. Sometimes simply disassembling and reassembling motor eliminates rubbing. End bells are also sometimes knocked out of alignment during transportation.
DC Motors
Another commonly used motor in industrial applications is the direct current motor. DC Motors are often used in applications where adjustable speed control is required. Below outlines common issues found in dc motors with identified likely causes and solutions.
Note: The suggestions outlined are intended to narrow down the source of the problem, not serve as a repair guide for customers. Please reach out to a qualified technician for all repairs.
Armature is shorted or went to ground. Motor may make a humming noise and the circuit breaker or fuse will trip.
Disassemble motor and inspect the armature for a burnt coil. Inspect the commutator for burnt bars. If this condition exists, the motor needs to be replaced. To test, set your OHM meter to the RX1 scale, touch probes to bars 180 degrees apart all around the commutator. The reading should be equal.
The brushes may be worn down too far and no longer make contract with the commutator.
Inspect the brushes to make sure that they are still making contact with the commutator. Refer to manufacturer’s recommended brush length chart.
Verify the load has not changed. Measure the amp draw of motor against the full load amp rating of the motor. If the amp draw is higher then rating, motor is undersized for application.
Motor controller not properly set.
Check controller manual for adjustments. The torque and/or IR compensation settings may need adjustment.
Motor may have an open connection.
Inspect the armature for an open connection.
Brushes may not be seated properly or worn beyond their useful length.
Verify that the brushes are properly seated and measure their length against the recommended brush length chart.
The accel trim pot of the controller should be adjusted.
Brushes are worn.
Verify brush length.
Bearings may be defective.
Inspect bearings for proper service. Noisy or rough bearings should be replaced.
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