Direct Current (dc) Machines Flashcards

1
Q

Which three components exist in every generator?

A

Permanent magnets or electromagnets;

An armature winding, which is rotated;

A commutator and brush set.

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2
Q

What are the five main properties of magnetic flux lines?

A

Oriented N to S in the external circuit;

Form complete loops;

Repel each other, never cross;

Become as short as possible;

Follow the path of least reluctance.

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3
Q

What is magnetic reluctance?

A

The opposition to flux in a magnetic circuit.

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4
Q

What is a “Weber” used to measure?

A

Magnetic flux.

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5
Q

How is the left hand coil rule used?

A

The thump points towards the north pole,

The fingers point in the direction of current flow through the coil.

{Oriented as if the pinky finger follows the first wrap of the coil}

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6
Q

How is flux density of a coil affected?

A

Current through the coil;

Number of turns in the coil;

Composition of core material.

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7
Q

Where does the “knee” occur in an air core transformer?

A

Never, only magnetic materials can saturate.

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8
Q

What is magnetomotive force?

A

The measure of a coil’s ability to produce flux.

{measured in amp-turns}

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9
Q

When is flux proportional to current in an iron core coil?

A

Up to just before saturation (at the knee).

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10
Q

Why is a commutator important for a DC machine?

A

To keep the polarity consistent with respect to the armature winding spinning in the field.

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11
Q

What is commutation?

A

A mechanical process which converts induced AC voltage in an armature into a “bumpy” DC voltage.

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12
Q

How is severe arcing at the brushes minimized?

A

By adding interpoles and compensating windings to shift the neutral plane back in line with the brushes.

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13
Q

Why does the neutral plane shift in a generator under load?

Which way does it shift?

A

The flux is distorted by current flowing through the armature.

In the direction of rotation.

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14
Q

Why are interpoles and commutating windings constructed of heavy guage wire?

A

To minimize volt drop as they will share armature current.

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15
Q

What is the force which pushes a current carrying conductor at right angles to a magnetic field?

A

The Lorrentz force.

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16
Q

How is polarity of induced voltage determined in a conductor passing through a magnetic field?

A

Flemming’s left hand rule for generators.

17
Q

What is an “electrical balance”?

A

As a motor speeds up, more cemf is generated. This limits current to be just sufficient enough to balance the torque and counter torque of a load.

18
Q

How is starting current reduced in a DC motor?

A

By placing a resistor in series while starting.

Reducing the voltage at start and gradually increasing it.

19
Q

Which direction does the neutral plane shift to in a Dc motor?

A

opposite the direction of rotation.

20
Q

Which three factors affect the speed of a DC motor

A

Amount of load;

Intensity of the magnetic field;

Amount of armature voltage.

21
Q

Why is runaway dangerous?

A

Because the motor can tear itself apart through centrifugal forces.

22
Q

Why must a shunt motor be started with a resistor in series with the armature?

A

To prevent runaway.

23
Q

Why must a series motor always have a load?

A

It can accelerate to dangerous speeds (runaway).

24
Q

Which Dc motor has the best speed regulation?

A

A shunt motor.

25
Q
A