# DC Machines: DC Generators--Summary / QUIZ

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### Summary

++ A generator is a machine that converts mechanical energy into electric energy.

++ Generators operate on the principle of magnetic induction.

++ AC is produced in all rotating armatures.

++ The commutator changes the AC produced in the armature into DC.

++ The brushes are used to make contact with the commutator and to carry the output current to the outside circuit.

++ The position at which there is no induced voltage in the armature is called the neutral plane.

++ Lap-wound armatures are used in machines designed for low-voltage and high-current operation.

++ Wave-wound armatures are used in machines designed for high-voltage and low-current operation.

++ Frogleg-wound armatures are the most used and are intended for machines designed for moderate voltages and current.

++ The loops of wire, iron core, and commutator are made as one unit and are referred to as the armature.

++ The armature connection is marked A1 and A2.

++ Series field windings are made with a few turns of large wire and have a very low resistance.

++ Series field windings are connected in series with the armature.

++ Series field windings are marked S1 and S2.

++ Shunt field windings are made with many turns of small wire and have a high resistance.

++ Shunt field windings are connected in parallel with the armature.

++ The shunt field windings are marked F1 and F2.

++ Three factors that determine the voltage produced by a generator are…

a. the number of turns of wire in the armature.

b. the strength of the magnetic field of the pole pieces.

c. the speed of the armature.

++ The voltage regulation of a DC generator is proportional to the resistance of the armature.

++ Compound generators contain both series and shunt field windings.

++ A long shunt compound generator has the shunt field connected in parallel with both the armature and series field.

++ A short shunt compound generator has the shunt field connected in parallel with the armature but in series with the series field.

++ When a generator is overcompounded, the output voltage is higher at full load than it’s at no load.

++ When a generator is flat-compounded, the output voltage is the same at full load and no load.

++ When a generator is undercompounded, the output voltage is less at full load than it’s at no load.

++ Cumulative-compound generators have their series and shunt fields connected in such a manner that they aid each other in the production of magnetism.

++ Differential-compound generators have their series and shunt field winding connected in such a manner that they oppose each other in the production of magnetism.

++ Armature reaction is the twisting or bending of the main magnetic field.

++ Armature reaction is caused by the interaction of the magnetic field produced in the armature.

++ Armature reaction is proportional to armature current.

++ Interpoles are small pole pieces connected between the main field poles used to help correct armature reaction.

++ Interpoles are connected in series with the armature.

++ Interpoles used in a generator must have the same polarity as the main field pole directly ahead of them in the sense of rotation of the armature.

++ Interpoles used in a motor must have the same polarity as the main field pole directly behind them in the sense of rotation of the armature.

++ Interpole leads are sometimes marked C1 and C2 or S3 and S4.

++ Interpole leads are not always brought out of the machine.

++ The neutral plane can be set by connecting an AC voltmeter to the shunt field and a source of low-voltage AC to the armature. The brushes are then adjusted until the voltmeter indicates the lowest possible voltage.

++ When a generator supplies current to a load, countertorque is produced, which makes the armature harder to turn.

++ Countertorque is proportional to the armature current if the field excitation current remains constant.

++ Countertorque is a measure of the useful electric energy produced by the generator.

### QUIZ

1. What is a generator?

2. What type of voltage is produced in all rotating armatures?

3. What are the three types of armature winding?

4. What type of armature winding would be used for a machine intended for high-voltage, low-current operation?

5. What are interpoles, and what is their purpose?

6. How are interpoles connected in relation to the armature?

7. What type of field winding is made with many turns of small wire?

8. How is the series field connected in relation to the armature?

9. How is the shunt field connected in relation to the armature?

10. What is armature reaction?

11. What is armature reaction proportional to?

12. What are eddy currents?

13. What condition characterizes overcompounding?

14. What is the function of the shunt field rheostat?

15. What is used to control the amount of compounding for a generator?

16. Explain the difference between cumulative- and differential-compounded connections.

17. What three factors determine the amount of output voltage for a DC generator?

18. What is the voltage regulation of a DC generator proportional to?

19. What is countertorque proportional to?

20. What is countertorque a measure of?

On the Job…

You are working as an electrician in a large steel manufacturing plant, and you are in the process of doing preventive maintenance on a large DC generator. You have megged both the series and shunt field windings and found that each has over 10 MV to ground. Your ohmmeter, however, indicates a resistance of 1.5V across terminals S1 and S2. The ohmmeter indicates a resistance of 225V between terminals F1 and F2. Are these readings normal for this type machine, or is there a likely problem? Why?

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