The direction of rotation of a split-phase motor may be reversed by reversing the connections leading to the starting winding. This action can usually be done on the terminal block in the motor. Figure 7-30 shows a diagram of the connections of a split-phase motor.
Motors require occasional repairs, but many of these can be eliminated by following a preventive maintenance schedule. Preventive maintenance, in simple terms, means taking care of the trouble before it happens. For example, oiling, greasing, cleaning, keeping the area around the equipment clean, and seeing that the equipment has the proper protective fuses and overload protection are preventive maintenance steps that eliminate costly repairs.
To analyze motor troubles in a split-phase motor, the first check is for proper voltage at the terminal block. If you have the proper voltage, check the end bells for cracks and for alignment. The bolts or screws may be loose and the ends may be out of line. The next check is for a ground With the motor disconnected, check the connections from the terminal block to the frame with an ohmmeter or megger. If you find a ground in this test, remove the end bell with the terminal block and centrifugal switch and separate the starting winding and running winding and make another ground check on each of these windings. In many cases you will find the ground in the loops where the wires are carried from one slot to the next one. This situation can sometimes be repaired without removing the winding. In some cases, the ground may be in the centrifugal switch due to grease that has accumulated from over- greasing.
If the first test does not show a ground in the motor, check to see that the rotor revolves freely. If the rotor turns freely connect the motor to the source of power
Figure 7-30. - Diagram of the connections of a split-phase motor.
and again check to see that the rotor turns freely when energized. If the rotor turns freely with no voltage applied, but locks when it is applied, you will know that the bearings are worn enough to allow the iron in the rotor to make contact with the iron in the pole pieces.
If the trouble is a short, either the fuse will blow or the winding will smoke when the motor is connected to the line. In either event the motor will have to be disassembled A burned winding is easily recognizable by its smell and the burned appearance. The only remedy is to replace the winding. If the starting winding is burned, it can usually be replaced without disturbing the running winding, but check closely to be sure that the running winding is not damaged. In making a check for a shorted coil, the proper procedure is to use an ohmmeter to check the resistance in the coil that you suspect to be bad. Then check this reading against a reading from a coil that is known to be good.
An open circuit can be caused by a break in a wire in the winding, or by the centrifugal switch not closing properly when the motor is at a standstill. Too much end play in the rotor shaft may cause the rotating part of the centrifugal switch to stop at a point where it allows the contacts on the stationary part of the switch to stand open. Should the rotor have more than 1/64-inch end play, place fiber washers on the shaft to line the rotor up properly.
If the motor windings are severely damaged, the motor must be sent to a motor shop for repairs. The repairs will usually be done in a shop operated by Public Works or the motor may be sent outside the base to a civilian operated motor shop. For this reason only the basic principles of the winding procedure will be covered.
Figure 7-31. - The pitch of a coil.Continue Reading