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  Maintenance of AC Motors

Introduction to Maintenance of A.C. Motors

A.C. (alternating current) Motors are run for very long periods without repairs. If the bearings are appropriately lubricated and air passages are reserved clear, then most of the A.C. Motors do not contain failure problems, when the motors have no commutators that cause motor failure. There are some motors that have commutators, like universal motor, repulsion motors in single phase system.

Squirrel Cage Induction Motor:

Overheating and shock might harm the squirrel cage rotor that may result fractures in bars in the slots and end ring connections and joint in the rotor cage. Suitable operation of motor is hard with fractured rotor cage and end rings. In large motors the bars are bolted or wedged in slots and can be make tighter readily if these become loose. Loose coils could be detected with the help out of growler. If it is not feasible to disassemble the motor, connect an ammeter in series along with one phase and apply 25 per cent of full voltage to one of the stator phase winding and turn the rotor slowly through hand. If the ammeter reading changes in excess of 3 per cent, it can be supposed that there is loose bar in rotor.

Wound Rotor Motors:

Working standard is similar as 3 phase sq. cage rotor induction motor, comprising better torque and speed. Wound rotor might have low speed with starter resistance cut off from rotor circuit. If there are no openings in control and starter circuit, the rotor coils have to be checked for continuity. Growler test might reveal open are short circuit in the coils. There is as well probability of brushes sticking in the holder or brushes may not have enough tension (sparking and overheating at contact area).

Growler is an electromagnet that comprising  110 V /240 V supply voltage and properly shaped for testing armature or rotor, through placing the growler core (or rotary growler for testing stator winding) and slowly moving on/in it shorted or broken coils can be noticed through noting the change in the humming noise of the growler.

Synchronous Motors:

Construction is identical to D.C. (direct current) generator (apart from commutator) or auto synchronous motor. Centrifugal force has substantial influence on the leads that should be securely fastened to the shaft. Alteration in air gap caused through misalignment of frame or wear in the bearings causes great troubles in the synchronous motors. Other points of care and maintenance are identical to other ac (alternating current) motors.

Single Phase Motors:

Centrifugal (CF) switch is one of the major causes of difficulties in single phase motors. If the springs of centrifugal switch turn into weak, the C.F. switch will run before reaching the full speed that will cause motor to operate at sub normal speed and stop. If the switch sticks closed, the beginning winding will stay in the circuit, overheat and harm the starting winding. Commutator motors require approximately similar type of care and maintenance as in dc motors.

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