Troubleshooting Three Phase Induction Motors

Troubles with the induction motors are generally divided into two categories. One is the electrical faults, such as the various faults of switches, buttons, fuses, electric brushes, stator winding, rotor, starting equipment and so on. The other is the mechanical problems, such as the problems of the bearing, fan blade, motor frame, coupling, end bell, bearing cover, spindle and so on.
Once the induction motor has any problem, there might be some abnormal phenomena, for instance, the rising temperature, over current, vibration and abnormal noise and so on. Therefore, learn about these phenomena, find out the cause, and then troubleshoot it. The following is some common faults and troubleshooting method of three-phase induction motors for your reference.

1. Induction motor doesn’t revolve, why and how to do?

• The power source is not connected.
  If the power source is not connected or bad contact, the motor will not revolve. At this moment, the electrician must inspect the switch, fuse, various contacts and connection wires. Thus, the fault will be gradually troubleshooting for maintenance.
• The fuse is blown out when starting.
  Find the reason for the fuse blown, troubleshoot the fault and then fit the proper fuse for the motor.
• The setting current of the over-current relay is too low.
  For this problem, just regulate higher properly.
• The load is too large or the transmission mechanism blocks.
  In this case, choose an induction motor with bigger capacity or relieve the load, and also check the condition of the transmission mechanism.
• The stator or rotor winding is in open circuit.
  Open the terminal box and check if the motor winding is broken (leads breakage) with the Ohm gear of the multi-meter. If any open circuit occurs, the resistance value might be abnormal. It is required to open the motor to make further inspection and connect the breakage point.
• Short circuit of the stator winding.
  Use a multimeter to check the winding and find out the two phases with short circuit. Afterwards, please disassemble the motor and disconnect the connecting wire of one phase, inspect the short circuit point in groups, find the problems and insulate once again.
• Short circuit of the stator winding to grounding
  Use the megger or multimeter for inspection to find the earthing winding. If the insulation breaks, insulate it once again. If it is serious, exchange the winding. If it gets wet, dry it and paint it with insulation varnish once again.
• Wrong wiring of the stator winding
  Open the motor, find out the problem, and connect the wires again.
• The slip ring of the wound-rotor induction motor has bad contact.
  Modify the surface of the slip ring and regulate the pressure of the carbon brush.

2. Why the revolving speed of the induction motor is lower than the rated value?

• The voltage of the power supply is too low.
  If the voltage is too low, the motor power might be insufficient. Therefore, the motor may run with the load at a speed lower than the rated value. Well, the input voltage of the motor can be measured with the voltmeter or multimeter.
• The load is too large.
  In this case, it is better to choose a induction motor with a great capacity or reduce the load.
• The bar of the squirrel-cage rotor is broken.
  Troubleshooting the problems 1, 2 as mentioned above, open the motor to find out the breakage and weld it.
• One-phase of the wound-type rotor has open circuit.
  Find the breakage with the multimeter and reconnect it.
• Bad contact of the starting resistance for the wound-rotor motor.
  Repair the contacts of the rheostat, or exchange the rheostat.
• For the wound-rotor motor, there is bad contact between the slip ring and the brush.
  Regulate the pressure of the carbon brush; improve the contact surface between the slip ring and the electric brush.

3. The induction motor frame is electrified.
The reason for the motor frame with electricity may be that the motor shell is not connected with the ground or has been damaged. If the grounding equipment is damaged, the motor winding is affected with the damp and the insulation is aged. As a result, the current may leak out due to poor insulation. Or the outgoing line has been connected with the motor enclosure.
For this problem, first of all, check if the motor grounding equipment is perfect and reconnect it with the ground. If the motor get humid, it needs to be dried. The winding with seriously aged insulation should be exchanged.

4. The induction motor sound is abnormal.

• In two-phase operation, there is a buzz sound.
  Restart the motor after shutting down. If the motor doesn’t rotate in two-phase operation, find the reason for phase loss and eliminate it.
• The three-phase current is unbalanced.
  Check the reason for the unbalanced three-phase power supply, if it is caused by the power voltage or the motor itself.
• The stator and rotor rubs with each other.
  Rectify the rotor spindle, file the extruded part of the stator or rotor core, or change the bearing.
• Some screws or components are loose
  Tighten the screws or bolts.
• Vibration
  Check if the foundation is stable, tighten the motor foot screws and inspect the rotor if it is balanced.
• Fan blade touches the enclosure.
  Rectify the blades and tighten the screws.
• The bearing seriously lacks of oil.
  Clean the bearing and fuel new lubricating oil with the capacity of no more than 70% of the volume.

5. The motor bearing is overheating.

• The match between bearing and shaft is too loose or too tight
  This situation will result in redundant friction, thus leading to the temperature rise of the motor bearing. The loose spindle can be inlayed with the bushing. If it is too tight, it is required to reprocess the spindle to the standard size.
• The bearing and end bell coordination is too loose or too tight.
  If it is too loose, the end bell can be inlayed with the bushing; if it is too tight, it is required to reprocess into the standard size.
• The lubricating oil of the bearing is too much or too little, or the oil quality is not good.
  Place certain lubricating oil or exchange the lubricating oil.
• The bearing has been damaged.
  Exchange the bearing.

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