Armature and Rotor Removal.- The removal of armatures and rotors from within the frame of the unit requires considerable care to avoid damage to the parts.

For removing an armature or rotor, follow these suggestions:

1. Support the armature or rotor only by its shaft when possible.

2. Slide a thin piece of cardboard between the underside of the rotor and stator to protect the laminations and windings during rotor removal. 3. In a shop, a hoist should be used to remove the rotors of large motors.

TESTING COMPONENTS.- After a motor is disassembled, you perform certain tests to determine which components are faulty.

Field Winding.- To locate a grounded field winding, disconnect and separate the internal connections between the windings. With this done, position one lamp prod of a series test lamp to the housing. With the other test lamp prod, touch each winding lead individually. If the test lamp lights, that particular winding is grounded. Test all the windings. You may also perform this test with an ohmmeter. A reading of continuity indicates a short; no reading indicates that the field winding is not grounded.

The test for an open circuit in the field windings of a motor may also be done with a series test lamp. Touch one test lead to one coil terminal and the other lead to the opposite coil terminal. If the test lamp doesn't light, the winding is open. If it does light, an open circuit doesn't exist, and the winding is serviceable.

To test for shorts in the field winding of a motor, you can compare the relative voltage drop in each field winding section with a voltmeter. You should get the same reading for each section. A decrease in voltage drop in a section indicates a short circuit.

Armature Winding.- The first test on an armature winding should be to locate grounded circuits.

This test is also performed with a series test lamp. Touch one test prod to the armature core or shaft, as shown in figure 7-58. Using the other test prod, touch each commutator segment. If the armature winding is grounded, the test lamp will light when you apply the lamp prod to the grounded armature winding or commutator segment. Replace the grounded armature when all attempts to remove the ground have failed.

When checking for a shorted armature, place the armature in an armature test set (growler). Lay the test blade lengthwise with the flat side loosely in contact with the armature core, as shown in figure 7-59. Turn the test stand to the ON position and slowly rotate the

CAUTION Place the test set switch in the off position before removing the armature, and never leave the test set turned on unless there is an armature placed in the core.

When you are testing an armature for an open circuit, place the armature in an armature test set and turn the test set ON. Place the armature double prods on two adjoining commutator segments and note the reading on the ammeter, as shown in figure 7-60. Rotate the armature until each pair of adjoining commutator segments have been read. All the segments should read the same. No reading indicates an open circuit, and a high reading indicates a short circuit.

Figure 7-58.- Testing for grounds in armature windings. armature while you hold the test blade stationary. If there is a short in the armature windings, the test blade will be attracted to the armature (magnetized) and will vibrate.

CAUTION Place the test set switch in the off position before removing the armature from the test stand.

Check the commutator for broken leads. Repair and resolder any you find. If you find an open winding, check the commutator for burned spots. They reveal the commutator segment to which the open winding is connected. Open circuits in the windings generally occur at the commutator and can be found by a visual inspection. If there is excessive sparking at the brushes with the motor reassembled, disassemble it and replace the armature.

Figure 7-59.- Testing for shorts in armature windings

Figure 7-60.- Testing for opens in a commutator.

In testing for a grounded brush holder or rigging, touch one test lamp prod of the armature test set to the motor housing. With the other test prod, touch each brush holder individually. If the lamp lights, there is a ground in the brush holder.

CAUTION Remove all leads to the brush holders and brushes before you attempt this test.

The color of the commutator and slip rings will indicate the type of trouble. An even chocolate-brown color indicates a normal condition and a black color indicates brush arcing. You can remove slight burns on the commutator segments by polishing the commutator as the armature rotates. Use a canvas pad, as shown in figure 7-61. To remove the deeper burns, use fine sandpaper instead of the canvas pad. When a commutator is deeply scored, it must be reconditioned in a lathe or with a special tool.

CAUTION Never use emery cloth to polish commutators because the emery particles can lodge between the segments and cause the commutator circuits to short.

Slip rings used on rotors are usually made of bronze or other nonferrous metals. Under normal conditions, the wearing surface should be bright and smooth. When the rings are pitted, they should be polished. When excessively worn and eccentric, they should be trued with a special tool.