DC MOTORS

What are two ways the speed of a shunt field motor may be controlled??

1.By controlling the resistance of the armature circuit 2. By controlling the resistance of the field circuit

This type of loss occurs when electrons are forced through the windings of the armature and the field producing heat.

Copper Losses General's 10-152

This type of Electrical Loss occurs because the iron core of the armature is a conductor revoling in a magnetic field. This sets up an EMF across portions of the core, causing current to heat the core.

Eddy Current Losses General's 10-152

residual voltage is a result of magnetism in the... Field Windings Field Shoes or Armature

Field Shoes

What are the brushes that ride along the commutator made of??

Graphitic carbon

This type of loss is caused by the armature revolving in an alternating magnetic field. When the armature core is rotating the magnetic field keeps changing direction, producing heat which increases resistance.

Hysteresis Loss General's Book 10-152

In a series DC motor the speed of the motor is dependant upon what??

The speed of the motor is dependant upon the load applied

What role do the springs of the brushes in a DC motor play??

The springs hold the brushes firmly against the commutator. But also allows some play room for irregularities of the commutator

Where is the end frame located in a DC motor??

Typically opposite of the commutator, and usually connected to the drive unit

When field current is decreased, what happens to the field strength and the motor speed.

When field current id decreased, the field strength decreases and the motor speeds up because the counter-emf is reduced

When current flowing through the field windings is increased what happens to the field strength and the speed of the motor??

When the current flowing through the field windings is increased, the field strength increases but the motor slows down due to greater amount of CEMF being generated in the armature windings

What happens to the motor speed and field strength when the field current is decreased??

When the field current is decreased the field strength decreases and the motor speeds up because the CEMF is reduced

What are practical uses Shunt (Parallel) Dc Motors?

Where Continuous Loads are desired and also used for constant speed control

Where is the field frame located?

along the inner wall of the motor housing

How are electrical losses classified??

as either copper losses or iron losses

Field Poles: As few as ___, as many as ___

as few as 2, as many as 8

The starting current, in passing through both the field and armature windings produces a high or low starting torque??

high starting torque

what is the commutator made of??

insulated copper bars

In the armature assembly what is the core made of?

laminated soft iron

What is the purpose of the carbon brushes?

the carbon brushes ride the commutator to carry current into or out of it

Where is the commutater located in the armature assembly?

the commutator is mounted on the rotating shaft

What are the major parts of a DC motor??

Armature Assembly Field Assembly Brush Assembly End Frame

How can the direction of rotaion be changed in a DC motor??

By reversing the electrical connections to either the field or armature windings

How is the motor speed in a DC motor maintained?? By varying the resistance with either....

By varying the resistance with either the field coils in series or the armature coil in series

What are two ways a shunt field DC motor can be reversed??

1. By using two seperate field windings, wound in opposite directions 2. By using a selector switch to reverse direction of the current flowing through the field coils

Why is the armautre made of LAMINATED soft iron core?? (2 reasons)

1. laminated to reduce eddy currents 2. soft iron core has low retentivity

Why must a series field motor always be operated into some form of mechanical load??

If a series motor is not operated into a mechanical load, it will continue to accelerate until it reaches a self-destructive speed

What happens to the motor speed when the armature circuit resistance is increased??

Increasing the armature circuit resistance will decrease the motor speed

What is the effect on the motor speed if you increase the shunt field circuit resistance??

Increasing the shunt field circuit resistance will increase the motor speed

What are 3 types of connectors between the armature and the field used for DC electric motors??

Series Shunt Compund

What is the advantage of a series-wound dc motor?

Series-wound DC motors have a high starting torque

Why do some electric motors have 2 sets of field windings wound in opposite directions??

So that the motor may be operated in either direction.

In a shunt motor how is speed controlled??

Speed is controlled by a rheostat in series with field windings

In Shunt DC motors how does speed vary??

Speed varies slightly with changes in the load

In a DC motor is the armature stationary or does it rotate??

The armature rotates on bearings in between the field poles

In a DC motor do the field poles rotate or are they stationary??

The field poles are stationary