The three-phase induction motor is one of the most popular and most widely used electric motors. They are used in many applications due to their simple and rugged construction with almost maintenance-free operation.
It is an asynchronous motor that works on the principle of induction. But similar to other motors these motors are also vulnerable to faults, that may result in damage to the motor. Thus monitoring of the induction motor has to be done for continuous operation without any interruption.
There are many factors like heavy operating duty, poor working conditions, improper installation of motor and manufacturing factors of the motor, etc, that can cause failure to the motor operation. If the faults are not rectified beforehand, can result in large revenue loss to the industry as well as can pose threat to reliability and safety of operation.
Let us see how to identify the faults and rectification of faults in ac three-phase induction motors. The below are the possible causes of faults and their remedies.
Faults in Three Phase Induction Motor :
Motor Fails to Start :
| Causes | Remedies |
|---|
| Supply may be cut off. | Check the main supply with the test lamp. |
| The main switch may be in an off position. | Check the main switch and switch ON. |
| Fuses may be blown off. | Check them physically and replace them. |
| Overload control may be tripped. | Let the overload control be cooled and start again. |
| Connections of the control circuit may be wrong. | Check physically and correct them. |
| Connections may be loose at the terminals of the motor or starter | Check and tighten them. |
| Stator or rotor windings may be open-circuited. | Check the windings with a series test lamp or megger and connect the broken ends, if possible or get them rewound. |
| Stator winding may be short-circuited. | Check the winding coils with a series test lamp or megger and insulate the winding and if the winding is burnt then get the stator re-wound. |
| Earth fault may be in the winding. | Check the fault with the series test lamp and insulate the winding with cotton tape. |
| Bearing or grease may be too stiff. | Grease the bearings or replace them. Use special grease fit for the purpose. |
| The motor may be overloaded. | Check and reduce the load. |
Motor Giving Noise :
| Causes | Remedies |
|---|
| One of the three phases may be cut off and the motor is running on single-phase. | Check for the 'open' phase with the test lamp and if a fuse is found blown, replace it. |
| Misalignment of the motor. | Check and correct the alignment. |
| Rotor may be rubbing the stator cores. | Set the rotor shaft correctly. |
| The belt may be tight. | Loose the belt. |
| Any part of the motor may be loosely fitted. | Check and tighten it. |
| Shaft may be bent. | Check and straighten it. |
| The bearing may be squeezed. | Check and re-grease them. |
| The coupling may be loose. | Check and tighten it. |
Motor is Overheated :
| Causes | Remedies |
|---|
| The motor may be overloaded. | Check and reduce load. |
| Control may be defective. | Check the starter, fuses, main switch, etc, and correct them. |
| Ventilation may be poor. | Maintain the proper ventilation. |
| Supply voltage maybe two high. | Check with the voltmeter and stop the motor till the supply voltage falls to normal. |
| Drive troubles. | Check and correct. |
| Loose connections may be in the rotor winding. | Tighten them, if possible, or replace the rotor. |
| Stator winding may be short-circuited or earthed. | Check for short circuit and earth fault with a series test lamp or megger and remove the defects. |
Bearings Overheated :
| Causes | Remedies |
|---|
| Bearings may be squeezed. | Check and re-grease the bearings with proper grading lubricant. |
| Belt may be too tight. | Check and reduce the tension of the belt. |
| Excessive gear end thrust. | Reduce the gear pressure. |
| The rotor shaft may be bent. | Check and straighten it. |
| The bearing maybe not properly fitted. | Set the bearing correctly. |
| Using the wrong grade of lubricants. | Use the lubricant of proper grade. |
| Foreign material in the lubricant. | Replace the old lubricant with a new one according to I.S.I rule. |
Motor Starts and Stops :
| Causes | Remedies |
|---|
| Starting gear may be defective. | Check the contacts and correct them. |
| Connections of stator and rotor windings may be loose. | Check and tighten them. |
| Loose connections at the motor's terminals. | Check and tighten them. |
Motor Works With Reduced Speed :
| Causes | Remedies |
|---|
| Supply voltage may be reduced. | Check with the voltmeter and stop the motor till the voltage is increased up to normal. |
| The motor may be over-loaded. | Check and reduce the load. |
| Stator winding may be open-circuited. | Check with the series test lamp and join disconnected ends. |
Motor Works With High Speed :
| Causes | Remedies |
|---|
| Starting gear may be defective. | Check and correct it. |
| Supply voltage increased suddenly. | Check the voltage with a voltmeter and stop the motor till it falls to normal. |
Motor Runs at Slow Speed with External Resistance Cutoff in case of Slip-Ring Induction Motor :
| Causes | Remedies |
|---|
| Rotor may be open-circuited. | Check the open circuit with the series test lamp and correct it. |
| Brushes are not making contact in case of a slip-ring motor. | Set the brushes to make the contact. |
| Collector rings may be rough. | Check and file them. |
| Rings may be ecentric. | Get the rings to be turned on the lathe. |
| Open circuit in the stator or rotor winding. | Check the windings for an open circuit with a series test lamp and connect the disconnected ends. |
Mechanical Problems in the Three Phase Induction Motor :
In general, a motor is found to develop more mechanical troubles rather than electrical troubles. A through knowledge of bearing and lubrication is a must. The below are some Do's and Dont's for ball bearing assembly, maintenance, inspection, and lubrication.
- Work with clean tools, in clean surroundings, use clean solvents and flushing oils.
- Don't work under the handicap of poor tools, rough bench, or dirty surroundings, and don't use dirty, brittle, or chipped tools.
- The bearings must be wiped with clean, lint-free rags. Don't scratch bearing surfaces.
- Do keep bearing lubricants clean when applying and cover containers when not in use. Don't remove grease or oil from new bearings.
- To be sure the shaft size is within specified tolerances recommended for the bearing. Don't install a bearing on a shaft that shows excessive wear.
- Use a clean, short-bristle brush with firmly embedded bristles to remove dirt, scale, or chips.
- To be certain that, when installed, the bearing is square with and held firmly against the shaft shoulder. Don't pound directly on a bearing or ring, when installing, as this may cause damage to the shaft and bearing.
- Follow the lubricating instructions supplied with the machinery. Use the only grease where grease is specified. Use only oil where oil is specified. Be sure to use the exact kind of lubricant called for.
- Excess greases and oil will ooze out of the over-filled housings past seals and closures, collect dirt and cause trouble. Too much lubricant will also cause overheating, particularly where the bearing operates at high speeds.
- Handle grease with clean paddles or grease guns. Store grease in clean containers. Keep the grease containers covered. Don't permit any machine to stand inoperative for months without running it over periodically. This prevents moisture that may condense in a standing bearing from causing corrosion.
