MOTOR MAINTENANCE, TESTING, AND REPAIR

An electric motor must be checked, maintained, and repaired just like any other piece of mechanical equipment. With proper servicing, a motor will last longer and give more efficient service. Included in maintenance services are cleaning, lubrication, ventilation, and testing.

Cleaning Inspect motors internally and externally for foreign materials, such as dust, dirt, corrosion, and paint. Open-frame motors may be blown out with compressed air. You should not apply too many coats of paint to motors. A thick coat of paint will interfere with heat dissipation.

Figure 7-55.- Part-winding starter.

CAUTION Air pressure used for cleaning should not exceed 25 psi nozzle pressure. Excessive pressure can damage the insulation on the windings.

Wipe all excess dirt, grease, and oil from the surfaces of a motor with a cloth moistened with an approved solvent.

WARNING Do not use flammable or toxic solvents when cleaning motors. Solvents may cause injury to personnel or damage to equipment.

Lubrication Lubrication should be done according to the manufacturer's instructions. Improper lubrication causes motor bearings to overheat and eventually causes bearing failure. Check a motor for signs of grease and oil-seal failure. If an inside seal fails, the lubricant can get into the motor windings and deteriorate the insulation. This condition also allows dust to adhere to the windings and restricts air circulation, then the motor windings heat and burn out. Inadequate lubrication causes the bearings to wear excessively and, eventually, to seize. When lubricating a motor, refer to the manufacturer's manual to determine the correct type of lubricant to use. Some motors have bearings lubricated with oil, while others require grease. Many motor bearings are lubricated and sealed at the factory and usually last the life of the bearing.

Ventilation Check the running temperature of all motors. If the motor temperature is hotter than specified on the nameplate, you must find the problem. The normal procedure for diagnosing motor overheats is to check the motor for restricted ventilation. Inspect the area around the motor for any obstructions which could hamper free air circulation. If air circulation is not hampered in any way and the motor continues to run hot, reduce the load on the motor or use a motor with more power capability.

Testing The proper testing of a motor should be done in a logical sequence. Proper testing can prevent unnecessary labor and parts. Testing motors is generally classed under two major methods: visual tests and operational tests.

VISUAL TESTS.- A visual test can discover a great deal about the condition of a motor and the possible causes of trouble. Read the nameplate data and be sure that the motor connections are correct for the supplied voltage.

Look at the windings to see if the insulation has overheated (or has been overheating). You can tell when the insulation is burned by the odor within the motor. If you aren't sure of the condition of the windings, test them with a megger to determine if the windings have been damaged beyond use. Connect the leads of the megger to each set of windings.

CAUTION Disconnect the motor leads from each other to ensure reading only one winding at a time. If the winding is good, you will get a reading of continuity. If the winding indicates a large amount of resistance, it is damaged and must be replaced.

Now connect one lead from the megger to the frame of the motor. Connect the other lead of the megger to each lead of the motor, one at a time. A low-resistance reading means insulation breakdown or a short to the motor frame, and replacement of the winding is necessary.

Inspect the commutator for solder thrown from the risers, and for loose, burned, high, and flat bars. Also test for high mica. Notice the surface film on both the commutators and slip rings and the general condition of the brushes. Check the air gap on large motors for any indication of bearing wear or misalignment. For large motors, take an air gap measurement at one reference point on the rotor or armature; then rotate the rotor or armature and measure four points on the stator or field frame to the same reference point. The air gap measurement should be within plus or minus 5 percent at any of these points.

Check the condition and operation of the starting rheostat in dc motors and the starting and control equipment used with ac motors. Also check the terminal connections on all of the control equipment to ensure they are correct and secure. Make sure the proper voltage is at the terminal lead of the motor.

If the visual tests have not revealed the trouble, you should perform some operational tests on the motor.

OPERATIONAL TESTS.- Perform a heat run test, observing the manufacturer's recommendations for that particular motor.

CAUTION Do not attempt to operate a series dc motor without a load.

If the temperature of the motor in normal operation does not exceed the maximum recommended by the manufacturer, the motor is operating satisfactorily.

Always refer to the manufacturer's manual for definite specifications for the motor you are inspecting.

WARNING Be sure the master switch is in the off position before connecting or disconnecting any motor lead connections. Because of their effect on insulating materials, high temperatures shorten the operating life of electric motors. When the windings or the bearings of a motor, not specifically designed for high temperature service, get hotter than 90 degrees centigrade, investigate the operating conditions and relieve the temperature conditions by cooling or relocating the motor. A gradually rising temperature in a motor warrants a shutdown and thorough examination of the unit. The nameplate on the motor usually specifies its normal running temperature in degrees centigrade. Check the current draw of the motor against the data on the nameplate. Excess current causes heating and, in time, will destroy the windings.

7-46 Check the motor for proper speed. A speed above or below that indicated on the nameplate signifies a malfunction in the unit. When a motor's operation is sluggish, check the line voltage to the motor. If you find the voltage low, apply the proper value and continue checking to determine if the motor is overloaded. If it is, reduce the load or replace the motor with one of a larger horsepower. There are other conditions which could make motor operation sluggish. You may find that the brushes have shifted off NEUTRAL, and you must reset them. You may also find that the armature or rotor is dragging on the stator or field poles. To correct this situation, you may need new bearings. Afield pole may be loose, causing it to drag on the armature or rotor.

Other conditions which could cause a motor to be sluggish are shorted field-winding circuits, shorted armature windings, and surface leaks across the commutator segments. After finding the fault in the motor, you may have to replace it. When you replace it, be sure to install a motor of the same size.

CAUTION Be sure to de-energize the motor circuit before disconnecting the unit.

While the motor is running, look for any sparking at the brushes. Many faulty conditions contribute to sparking brushes at the commutator. The two major causes are a faulty armature and malfunctioning brushes. Some of the faults that could develop in an armature include the following: rough commutators, bent armature shafts, and short circuits in the armature windings. Brushes may malfunction because they are off NEUTRAL, they bind in the brush holders, they are wound beyond recommended limits, or they intermittently fail to contact the commutator because of insufficient brush spring tension. Whenever a motor is arcing at the brushes, it is advisable to disassemble it, locate the problem, and make the necessary repairs, There are many causes of motor noise. Listen and feel for any unusual noises. You should first check the motor-mounting bolts for looseness and the alignment of the motor with the driven equipment. If the motor is secure and properly aligned, continue your inspection. Check the motor's balance. Also inspect the motor for loose rotor bars or a bent shaft. If any of these conditions exist, you will have to replace the rotor or armature. Sometimes the centrifugal switch rattles or rubs the interior of the motor housing. Align the switch and tighten the mounting bolts. If the switch has excessive wear, replace it. Check all motor accessories for looseness and tighten as needed. Check the drive pulley and the condition of the belts. Loose pulleys rattle and will damage belts. You will hear a distinct slap when the belt has been damaged.

Motor Repair After you have performed visual and operational tests on a motor and isolated the problem, you may have to disassemble the motor to make the repairs. You should know the procedures and precautions for motor repair.