PP104

What are the major parts, or assemblies, of a DC generator?

The major parts, or assemblies, of a DC generator are a field frame (or yoke), a rotating armature, and a brush assembly.

Why are the poles usually laminated?

The poles are usually laminated to reduce eddy current losses and serve the same purpose as the iron core of an electromagnet; that is, they concentrate the lines of force produced by the field coils.

Name two general kinds of armatures.

The ring and the drum

Define armature reaction.

This distortion of the magnetic field is called armature reaction.

When lines of magnetic force are cut by a conductor passing through them, ________ is induced in the conductor.

Voltage

A generator is rated in power output. The rating usually is given as the number of __________________ the generator can safely supply at its rated __________________.

amperes, voltage.

The _________________ assembly of a generator consists of many armature coils wound on an iron core, a commutator and associated mechanical parts.

armature

The rotating parts of a DC generator (coil and commutator) are called an _____________________.

armature

If the current is collected by a ___________________, it is direct current, and the generator is called a DC generator?

commutator

As the number of loops is __________________, the variation between maximum and minimum values of voltage is reduced, and the output voltage of the generator approaches a steady DC value.

increase

What is the purpose of the pigtail?

is to conduct the current, rather than subjecting the brush spring to currents that would alter its spring action by overheating.

_________________ windings are used in generators that are designed for high current.

lap

Name two methods drum type armatures are constructed?

lap winding and the wave winding

The drum type advantage is that each winding completely surrounds the core so that the entire _____________ of the conductor cuts through the magnetic flux.

length of the conductor

Define flux (magnetism).

lines of magnetic flux are the same as lines of magnetic force.

The variation of DC voltage is called "ripple" and may be reduced by using more ____________, or _______________.

loops, coils

An Interpol is a pole places between the ________________ poles of a generator.

main

When a brush has worn to the _______________ length permissible, it must be replaced.

minimum

A flexible, braided copper conductor, commonly called a _______________, connects each brush to the external circuit.

pigtail

What determines the output voltage of a DC generator?

product of the number of turns per loop, the total flux per pair of poles in the machine, and the speed of rotation of the armature.

At position D, commutator action __________________ the current in the external circuit, and the second half cycle has the same waveform as the first half cycle.

reverses

Name three types of DC generators:

series wound, shunt wound, and shunt series or compound wound.

If the current is taken form the loop by _______________ _______________, it is called an AC generator, or alternator.

slip rings

The positive end of the shunt field winding connects to terminal _______________, and the opposite ends connects to the ________________ terminal al bush.

A

If the ends of the conductor are connected, what does it form?

A complete circuit

Define commutator:

A mechanical rectifier mounted on the armature shaft of a DC generator or motor. It consists of a cylindrical arrangement of insulated copper bars connected to the armature coils. carbon brushes ride on the copper bars to carry current into or out of the commutator, providing a unidirectional current from a generator or a reversal of current in the coils of of a motor.

A practical DC generator uses __________________ instead of permanent magnets.

A practical DC generator uses electromagnets instead of permanent magnets.

Where is the commutator located and what does it consists of?

the commutator is located at the end of an armature and consists of wedge shaped segments of hard drwn copper, insulated from each other by thin sheet of mica.

What will happen if you replace a generator with one that has the incorrect direction of rotation?

the voltage is reversed.

The __________ winding is used in generators that are designed for high voltage outputs.

wave

If the voltage induced throughout the entire 360 degrees of rotation is plotted, this voltage is called an ______________ ________________ because of its reversal from (+) to (-) value, first in one direction and then in the other.

Alternating voltage

The rotation of generators is termed either _____________ or _________________, as viewed from the driven end.

Clockwise or counterclockwise

The _________________ windings consist of a series of coils embedded in slots in the pole faces. These coils are also connected in _______________ with the armature.

Compensating, Series

__________________ generators are usually designed to be overcompounded. This feature permits varied degrees of compounding by connecting a variable shunt across the series field.

Compound

As the loop approaches the vertical position, the induced voltage ________ because both sides of the loop are approximately parallel to the lines of force and the rate of cutting is reduced.

Decreases

The negative armature lead connects to the _______________ terminal.

E

Since there is always one north pole for each south pole, there must always be an __________________ number of poles in any generator.

Even

What information is stamped on the nameplate attached to the generator?

Generator rating and performance data

Which kind of armature is not widely used?

Gramme ring armature

The strength of the induced voltage is dependent upon the speed of the conductor and the strength of the _________________.

Magnetic feild

Looking at position A, is EMF generated?

No EMF is generated, since no lines of force are cut by the coil sides

What is "high mica"?

Operation over extended periods of time often results in the mica insulation between commutator bars protruding above the surface of the bars.

________________ generators have very ______________ voltage regulation under changing load, since the greater the current through the field coils to the external circuit, the greater the induced EMF and the greater the terminal or output voltage.

Series, poor

The terminal voltage of a ______________ generator can be controlled by means of a ________________ inserted in series with the field windings.

Shunt generator, rheostat

Define slip rings:

Smooth, continuous rings of brass or copper mounted on the rotor shaft of an electrical generator or alternator. current used to produce the magnetic field in the rotor of a DC alternator flows into the rotor coil through slip rings. slip rings also carry current into the deicer heating coils on an airplane propeller blade.

________________ of brushes quickly reduces the effective brush area in contact with the commutator bars.

Sparking

__________________________ is an indication of improper brush placement, which is the main cause of improper commutation.

Sparking

What two functions does the frame have?

The frame has two function: It completes the magnetic circuit between the poles and acts as a mechanical support for the other parts of the generator.

The positive armature lead in the generator connects to the _________ terminal.

B

On most large 24-volt generators, how are the terminals marked?

B, A, and E

Looking at position B, is the induced EMF at maximum or minimum?

In position B, the flux is now being cut at a maximum rate, which means that the induced EMF is maximum.