How to troubleshoot three-phase electric motors, like if unable to start or overheats?

02 Nov '18, 02:28

Nov. 2, 2018, 2:28 a.m.
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If a three-phase motor fails to start, the trouble may be:

  1. Burned-out fuse
  2. Open rotor bars
  3. Worn bearings
  4. Wrong internal connections
  5. Overload
  6. Frozen bearing
  7. Open phase
  8. Defective controller
  9. Shorted coil or group
  10. Grounded winding

If a three-phase motor does not run properly, the trouble may be:

  1. Burned-out fuse
  2. Open parallel connections
  3. Worn bearings
  4. Grounded winding
  5. Shorted coil
  6. Open rotor bars
  7. Reversed phase
  8. Incorrect voltage
  9. Open phase

If the motor runs slowly, the trouble may be:

  1. Shorted coil or group
  2. Overload
  3. Reversed coils or groups
  4. Wrong connection (reversed phase)
  5. Worn bearings
  6. Loose rotor bars

If the motor becomes excessively hot, the trouble may be

  1. Overload
  2. Motor running on single phase
  3. Worn bearings
  4. tight bearing
  5. Loose rotor bars
  6. Shorted coil or group

Burned-out Fuse.

Remove fuses and test with test lamp, if the lamp lights, the fuse is good. A burned-out fuse is indicated when the test lamp does not light. To test fuses without removing them from the holder, a voltmeter must be used. If a test light designed for 230 volts is mistakenly used on 460 volts, it will blow out and may trigger a severe electrical explosion. If the fuse is open, there will be a line voltage read across it.

Worn Bearings.

If a bearing is worn, the rotor will ride on the stator and cause noisy operation. When the bearings are so worn that the rotor rests firmly on the core of the stator, rotation is impossible. To check a small motor for this condition, try moving the shaft up and down. Motion in this manner indicates a worn bearing. Remove and inspect the rotor for smooth, worn spots. These indicate that the rotor has been rubbing on the stator. The only remedy is to replace the bearings.


To determine whether a three-phase motor is overloaded, remove the belt or load from the motor and turn the shaft of the load by hand a broken part or dirty mechanism will prevent the shaft from moving freely. Another method is to use an ammeter on each line wire. A higher current reading than on the nameplate may indicate an overload.

Open Phase.

If an open occurs while the motor is running, it will continue to run but will have less power. An open circuit may occur in a coil or group connection. The motor will continue to run if a phase opens while the motor is in operation but will not start if at a standstill. The conditions are similar to those of a blown fuse.

Shorted Coil of Group.

Shorted coils will cause noisy operation and also smoke. After locating such defective coils by means of the eye or balance test, the motor should then be rewound. When the insulation on the wire fails, the individual turns become shorted and cause the coil to become extremely hot and burn out. Other coils may then bum out, with the result that an entire group or phase will become defective.

Open Rotor bars

Open rotor bars will cause a motor to lose power. One sign of open bars is when a motor is connected to the right voltage at no load, it has a very low amp reading. A light load will pull down the speed, and at full load the motor will run below the nameplate speed.

Incorrect Voltage

Some T-frame motors are designed for a definite voltage. Thus a motor designed for 208 volts will overheat when operated on 250 volts, and a motor designed for 250 volts will not have enough power if operated on 208 volts. If the motor is rate 208-220-440 volts on the nameplate, it will operate well on a range of voltages.

Wrong Internal Connections

A good method of determining whether or not a polyphase motor is connected properly is to remove the rotor and place a large ball bearing in the stator. The switch is then closed to supply current to the winding. If the internal connections are correct, the ball bearing will rotate around the core of the stator, if the connections are incorrect, the ball bearing will remain stationary.

Frozen Bearing

If oil is not supplied to the part of the shaft that rotates in the bearing, the shaft will become so hot that it will expand sufficiently to prevent movement in the bearing. This is called a frozen bearing.

Defective Controller

If the contacts on the controller do not make good contact, the motor will fail to start.

Grounded Winding

This will produce a shock when the motor is touched. If the winding is grounded in more than one place, a short circuit will occur which will burn out the winding and perhaps blow a fuse. Test for a grounded winding with test lamp and repair by rewinding or by replacing the defective coil.

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02 Nov '18, 02:37

Nov. 2, 2018, 2:37 a.m.
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