Troubleshooting Electric Motors

 

Troubleshooting Electric Motors

 

Electric motors operate on the principle of electromagnetic induction. An electric motor has a stationary magnet, or stator, with windings connected to the supply conductors, and a rotating magnet. There is no electrical connection between the stator and rotor. The magnetic field produced in the stator windings induces a voltage in the rotor.

When an electric motor malfunctions, the stator (stationary) windings are often defective, and must be repaired or replaced. Stator problems are usually caused by one or more of the following:

Ø  Worn bearings

Ø  Moisture

Ø  Overloading

Ø  Poor insulation

Ø  Single-phase operation of a three-phase motor

Troubleshooting Motors

To detect defects in electric motors, the windings are normally tested for ground faults, opens, shorts, and reverses. The exact method of performing these tests depends on the type of motor being serviced. However, regardless of the motor type, a knowledge of some important terms is necessary to properly troubleshoot motors:

Ground           : A winding becomes grounded when it makes an electrical contact with the iron frame of the motor. The usual causes of grounds include bolts securing the end plates coming into contact with the winding; wires press against laminations at the corners of the slots; or the centrifugal switch becoming grounded to the end plate.

Open circuits  : Loose or dirty connections, as well as a broken wire, can cause an open circuit in an electric motor.

Shorts             : If two or more turns of a winding contact each other, the result is an electrical short circuit. This condition may develop in a new winding if the winding is tight and pounding is necessary to place the wires in position. In other cases, excessive heat caused by overloads degrades the insulation and causes a short. A short circuit is often detected by observing smoke from the windings as the motor operates, or if the motor draws excessive current at no load.

The chart in Figure 6-1 lists tools and equipment used in maintenance and troubleshooting of electric motors. The following sections describe common causes of motor malfunctions.

 

6-1 Tools for electric motor maintenance.

 

Grounded Coils

A grounded coil in a motor winding typically causes repeated tripping of the circuit breaker. Follow these steps to test for a grounded coil using a continuity tester:

1.      Open and lock out the disconnecting means, to insure the motor is de-energized.

2.      Place one test lead on the frame of the motor and the other in turn on each of the ungrounded (power) conductor supplying the motor. If there is a grounded coil at any point in the winding, the lamp of the continuity tester will light, or the meter display will indicate infinity.

3.      For a three-phase motor, test each phase separately, after disconnecting the star or delta connection.

4.      Sometimes moisture on old insulation around the coils causes a high-resistance ground that is difficult to detect with a test lamp. A megger can be used to detect such faults.

5.      Test the armature windings and commutator for grounds in a similar manner.

6.      On some motors, the brush holders are grounded to the end plate. Before the armature is tested for grounds, lift the brushes away from the commutator.

 

Shorted Coils

Shorted turns within coils are usually the result of failure of the insulation on the wires, caused by oil, moisture, and the like. One inexpensive way of locating a shorted coil is by the use of a growler and a thin piece of steel, as shown in Figure 6-2.

6-2  Growler used to test a stator of an AC motor.

1.      Place the growler in the core as shown, with the thin piece of steel at the distance of one coil span from the center of the growler.

2.      Test the coils by moving the growler around the bore of the stator and always keeping the steel strip the same distance away from it.

3.      If any coil has one or more shorted turns, the piece of steel will vibrate very rapidly and cause a loud humming noise. By locating the two slots over which the steel vibrates, both sides of the shorted coil can be found.

4.      Sometimes one coil or a complete coil group becomes short-circuited at the end connections. The test for this fault is the same as that for a shorted coil.

 

 

 

Open Circuit

1.      When one or more coils become open-circuited by a break in the turns or a poor connection at the end, they can be tested with a continuity tester as previously explained. If this test is made at the ends of each winding, an open can be detected by the lamp failing to light. Remove the insulation from the pole-group connections, and test each group separately.

2.      An open circuit in the starting winding may be difficult to locate, since the problem may be in the centrifugal switch instead of the winding itself. In fact, the centrifugal switch is more likely to cause trouble than the winding since parts become worn, defective, and more likely, dirty. Insufficient pressure of the rotating part of centrifugal switches against the stationary part will prevent the contacts from closing and thereby produce an open circuit.

 

Reversed Coil Connections

Reversed connections cause current to flow through coils in the wrong direction. This causes disturbance of the magnetic circuit, which results in excessive noise and vibration.

The fault can be located by the use of a magnetic compass and a direct current power source, as follows:

1.      Adjust to send about one-fourth to one-sixth of the full-load current through the winding, with the DC leads placed on the start and finish of one phase.

2.      If the winding is a three-phase, star-connected, winding this is at the start of one phase and the star point. If the winding is delta-connected, disconnect the delta point and test each phase separately.

3.      Place a compass on the inside of the stator and test each coil group in that phase. If the phase is connected correctly, the needle of the compass will reverse definitely as it is moved from one coil group to another. However, if any one of the coils is reversed, the reversed coil will build up a field in the direction opposite to the others, thus causing a neutralizing effect that is indicated by the compass needle refusing to point definitely to that group. If there are only two coils per group, there will be no indication if one of them is reversed, as that group will be completely neutralized.

4.      When an entire coil group is reversed, current flows in the wrong direction in that whole group. The test for this fault is the same as that for reversed coils. Magnetize the winding with DC, and when the compass needle is passed around the coil group, it should alternately indicate North-South, North-South, and so on.

 

 

 

Reversed Phase

Sometimes in a three-phase winding a complete phase is reversed by either having taken the starts from the wrong coils or connecting one of the windings in the wrong relation to the others when making the star or delta connections.

Delta connection: In a delta-connected winding, disconnect any one of the points where the phases are connected together and pass current through the three windings in series. Place a compass on the inside of the stator and test each coil group by slowly moving the compass one complete revolution around the stator. The reversals of the needle in moving the compass one revolution around the stator should be three times the number of poles in the winding.

Wye connection: In a star- or wye-connected winding, connect the three starts together and place them on one DC lead. Then connect the other DC lead and star point, thus passing the current through all three windings in parallel. Test with a compass in the same way as the delta winding. The result should then be the same, or the reversals of the needle in making one revolution around the stator should again be three times the number of poles in the winding.

These tests for reversed phases apply to full-pitch windings only. If the winding is fractional-pitch, a careful visual check should be made to determine whether there is a reversed phase or mistake in connecting the star or delta connections.