Monday, December 8, 2014

The elementary dc generator and Answers to questions

THE ELEMENTARY DC GENERATOR


A single-loop generator with each terminal connected to a segment of a two-segment metal ring is shown in figure 1-4. The two segments of the split metal ring are insulated from each other. This forms a simple COMMUTATOR. The commutator in a dc generator replaces the slip rings of the ac generator. This is the main difference in their construction. The commutator mechanically reverses the armature loop connections to the external circuit. This occurs at the same instant that the polarity of the voltage in the armature loop reverses. Through this process the commutator changes the generated ac voltage to a pulsating dc voltage as shown in the graph of figure 1-4. This action is known as commutation. Commutation is described in detail later in this chapter.
Figure 1-4. - Effects of commutation.

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For the remainder of this discussion, refer to figure 1-4,parts A through D. This will help you in following the step-by-step description of the operation of a dc generator. When the armature loop rotates clockwise from position A to position B, a voltage is induced in the armature loop which causes a current in a direction that deflects the meter to the right. Current flows through loop, out of the negative brush, through the meter and

the load, and back through the positive brush to the loop. Voltage reaches its maximum value at point B on the graph for reasons explained earlier. The generated voltage and the current fall to zero at position C. At this instant each brush makes contact with both segments of the commutator. As the armature loop rotates to position D, a voltage is again induced in the loop. In this case, however, the voltage is of opposite polarity.
The voltages induced in the two sides of the coil at position D are in the reverse direction to that of the voltages shown at position B. Note that the current is flowing from the black side to the white side in position B and from the white side to the black side in position D. However, because the segments of the commutator have rotated with the loop and are contacted by opposite brushes, the direction of current flow through the brushes and the meter remains the same as at position B. The voltage developed across the brushes is pulsating and unidirectional (in one direction only). It varies twice during each revolution between zero and maximum. This variation is called RIPPLE.


A pulsating voltage, such as that produced in the preceding description, is unsuitable for most applications. Therefore, in practical generators more armature loops (coils) and more commutator segments are used to produce an output voltage waveform with less ripple.


Q.5 What component causes a generator to produce dc voltage rather than ac voltage at its output terminals?

Q.6 At what point should brush contact change from one commutator segment to the next?

Q.7 An elementary, single coil, dc generator will have an output voltage with how many pulsations per revolution?


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ANSWERS TO QUESTIONS Q1. THROUGH Q24.


A1. Magnetic induction.

A2. The left-hand rule for generators.

A3. To conduct the currents induced in the armature to an external load.

A4. No flux lines are cut.

A5. A commutator

A6. The point at which the voltage is zero across the two segments.

A7. Two.

A8. Four

A9. By varying the input voltage to the field coils.

A10. Improper commutation.

A11. Distortion of the main field due to the effects of armature current.

A12. To counter act armature reaction.

A13. A force which causes opposition to applied turning force.

A14. Resistance in the armature coils, which increases with temperature.

A15. By laminating the core material.

A16. Drum-type armatures are more efficient, because flux lines are cut by both sides of each coil.

A17. Higher load currents are possible.

A18. Series-wound, shunt-wound, and compound-wound.

A19. Output voltage varies as the load varies.

A20. Voltage regulation.

A21. Parallel operation.

A22. It can serve as a power amplifier.

A23. Gain = output ÷ input.

A24. The mechanical force applied to turn the amplidyne, and the electrical input signal.


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