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.
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.
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.
DISASSEMBLY. - The careless disassembly of a motor can cause serious damage to the delicate components within the motor. Remove and handle all parts with care and always use the proper tools. It is just as important to tag all parts, take down accurate data, and store the parts in an orderly arrangement in a safeContinue Reading