7. Read and translate the text

DC GENERATORS TROUBLESHOOTING AND REPAIR

Testing dc generators is similar to testing dc motors. The faults and trou­bles that occur in dc generators, but in dc motors are described below.

I. If it does not generate the trouble may be:

a. Loss of residual magnetism. If the field poles lose residual magnetism, it is impossible for the armature to cut lines of force, and therefore no current can be generated. To remedy this condition, the shunt field is connected to a source of different current for a few seconds.

b. Too much resistance in the field circuit. Because the building-up pro­cess of a generator depends on the continued increase in the strength of the field, It is obvious that the voltage cannot build up if a high resistance in the Held circuit prevents sufficient current from flowing in the field coils to increase the flux. The high resistance may be due to the field rheostat, open circuit in the field, loose connections, poor brush contact, or broken brush pigtails.

c. Wrong field connection. The residual magnetism in a generator produces lines of force from a north pole to a south pole. If the current in the field coils is in the wrong direction, lines of force will be produced opposite to the residual lines, and a cancellation of flux will result which will prevent the generator from building up. To remedy this trouble, reverse the shunt-field connections or reverse the direction of rotation of the generator.

d. Wrong rotation. Wrong direction of rotation is similar to reversed field polarity because it causes the current in the shunt field to flow in the wrong direction. To correct this situation, reverse the direction of rotation or interchange the shunt-field leads.

e. Shorted armature or field. A shorted armature or field may allow only a low voltage to build up. If completely shorted, the voltage will not increase, and the armature will smoke. If all other faults are eliminated, test the armature and field for shorts according to the method described for dc motors.

2. If the voltage drops considerably as the load is placed on the generator, the trouble may be:

a. Differential connection

b. Shorted armature

c. Overload

3. If the voltage does not build up to a maximum, the trouble may be:

a. Wrong brush position. Check for the neutral position, as you would for dc motors, for interpole generators, the neutral point is directly under the cen­ter of the interpole

b. Shorted armature or field coils

c. resistance in the field circuit

d. Speed of generator too low

All the troubles listed above are in addition to those usually found in a dc motor. For instance, sparking at the brushes of a generator may be due to the same causes as in a dc motor.

8. Answer the questions.

1. Why should a new motor be tested before installation?

2. When are the tests applied?

3. What points does the test include?

4. What is the difference between a symptom and a trouble?

5. What symptoms mentioned do you remember?

6. What possible troubles to each symptom can you remember?

7. Is there any difference between testing dc generators and dc motors?

8. What are the faults and troubles that occur in dc generators and not in dc motors?

9. What is the difference between a fault, a trouble and a symptom?