Unit 31

DC Motors

Unit 31 DC Motors

Objectives:

• Discuss the operation of direct-current

motors.

• Discuss different types of DC motors.

• Review schematic diagrams of different

types of DC motors.

Unit 31 DC Motors

Construction

• A motor is a device used to convert electrical

energy into mechanical energy.

• A DC motor and a DC generator use the

same magnetic principles of attraction and

repulsion.

• The basic construction of both devices is

very similar.

Unit 31 DC Motors

Basic one-loop motor.

Unit 31 DC Motors

Construction

• The turning force or torque of a motor is

caused by the interaction of the magnetic

fields surrounding the wire loops and the

pole pieces.

• Torque is determined by two factors: the

magnetic strength of the pole pieces, and

the magnetic strength of the armature.

Unit 31 DC Motors

Torque is caused by interaction of magnetic fields.

Unit 31 DC Motors

Construction

• The commutator is a mechanical rectifier

converting AC into DC.

• The commutator also provides pathways

and connections to the external circuit.

• DC motors can use schematics similar to

the ones used for DC generators.

Unit 31 DC Motors

Increasing the number of loops and turns

increases output torque.

Unit 31 DC Motors

Construction

• The armature is the rotating part of a DC

motor.

• Three classes of DC motors are:

1. shunt motors

2. series motors

3. compound motors

Unit 31 DC Motors

Shunt motor schematic.

Unit 31 DC Motors

Series motor schematic.

Unit 31 DC Motors

Compound motor schematics.

Unit 31 DC Motors

Construction

• Speed regulation is the amount that speed

decreases as mechanical load is increased.

• Speed regulation is proportional to the

resistance of the armature.

• Lower armature resistance means better

speed regulation.

Unit 31 DC Motors

Comparison speed / current curves.

Unit 31 DC Motors

Comparison torque / current curves.

Unit 31 DC Motors

Direction of Rotation

The direction of rotation of a DC motor

can be reversed by changing the

connections of the armature leads or the

field leads.

Unit 31 DC Motors

Changing rotation of series motors.

Unit 31 DC Motors

Changing rotation of shunt motors.

Unit 31 DC Motors

Changing rotation of compound motors.

Unit 31 DC Motors

Reversing rotation of shunt motor with a switch.

Unit 31 DC Motors

Forward-reversing control schematic.

Unit 31 DC Motors

Horsepower

• James Watt determined that the average horse could do work at a rate of 550 foot/pounds per second. This was standardized as one horsepower.

• Electrical power can be compared to mechanical power using this formula: 1 horsepower = 746 watts

Unit 31 DC Motors

Horsepower

These are some more standard

conversion formulas:

– 1 watt = 3.42 BTUs per hour

– 1055 watts = 1 BTU per second

– 4.18 watts = 1 calorie per second

– 1.36 watts = 1 foot/pound per second

Unit 31 DC Motors

Motor Types

There are many different ways of

designing DC motors to enhance

particular characteristics.

– brushless DC motors

– permanent magnet motors

– servomotors and ServoDisc® motors

Unit 31 DC Motors

Brushless DC motor cutaway view.

Unit 31 DC Motors

Basic ServoDisc® motor.

Unit 31 DC Motors

Right-Hand Motor Rule

The fingers of the right hand can be used to

determine the direction of rotation of the

armature when the magnetic field polarity of

the pole pieces and the direction of current

flow through the armature are known.

Unit 31 DC Motors

Right-hand motor rule.

Unit 31 DC Motors

Review:

1. A motor is a machine that converts electrical energy into mechanical energy.

2. Direct-current motors operate on the principle of attraction and repulsion or magnetism.

3. DC motors can use schematics similar to the ones used for DC generators.

Unit 31 DC Motors

Review:

4. Two factors that determine the torque

produced by a motor are:

a. the strength of the magnetic field of the

pole pieces.

b. the strength of the magnetic field of the

armature.

Unit 31 DC Motors

Review:

5. Three basic types of DC motors are the

series, shunt, and compound.

6. Brushless DC motors do not contain a

wound armature, commutator, or brushes.

Unit 31 DC Motors

Review:

7. When full voltage is applied to both the

armature and shunt field, the motor will

operate at base speed.

8. When full voltage is applied to the field

and reduced voltage is applied to the

armature, the motor will operate below

base speed.

Unit 31 DC Motors

Review:

9. When full voltage is applied to the armature and reduced voltage is applied to the shunt field, the motor will operate above base speed.

10.The direction of rotation of a direct current motor can be changed by reversing the connection of either the armature or the field leads.

Unit 31 DC Motors

Review:

11. It is common practice to reverse the

connection of the armature leads to

prevent changing a compound motor from

a cumulative to a differential compound.

12.A shunt field relay is used to disconnect

power to the armature if shunt field current

drops below a certain level.