Chapter

7

DC Electrics - DC Motors

Electric Motors . . . . . . . . . . . .	. .	. .	. .	. . . . . .	. .	. .		.105
Fleming’s Left Hand Rule								105
Practical DC Motor . . . . . . . . . .	. .	. .	. .	. . . . . .	. .	. .	.	106
Back EMF . . . . . . . . . . . . . .	. .	. .	. .	. . . . . .	. .	. .		.107
Slow Start Resistor . . . . . . . . . .	. .	. .	. .	. . . . . .	. .	. .	.	107
Commutation . . . . . . . . . . . .	. .	. .	. .	. . . . . .	. .	. .	.	107
Series Wound Motors								108
Shunt Wound Motors								108
Starter-generator Systems . . . . . . . .	. .	. .	. .	. . . . . .	. .	. .		.109
Actuators . . . . . . . . . . . . . .	. .	. .	. .	. . . . . .	. .	. .		.110
Solenoid Actuators . . . . . . . . . .	. .	. .	. .	. . . . . .	. .	. .	.	110
Motor Actuator Construction . . . . . .	. .	. .	. .	. . . . . .	. .	. .	.	110
The Split Field Series Actuator . . . . . .	. .	. .	. .	. . . . . .	. .	. .	.	111
The Split Field Series Actuator Operation . .	. .	. .	. .	. . . . . .	. .	. .	.	111
Motor Actuators . . . . . . . . . . .	. .	. .	. .	. . . . . .	. .	. .	.	112
Rotary Actuators . . . . . . . . . . .	. .	. .	. .	. . . . . .	. .	. .	.	112
Linear Actuators . . . . . . . . . . .	. .	. .	. .	. . . . . .	. .	. .	.	113
Actuator Brakes								114
Actuator Clutches								114
Visual Indicators Used with Linear Actuators .	. .	. .	. .	. . . . . .	. .	. .	.	114
Visual Indicators Used with Rotary Actuators								114
Indicator Lights . . . . . . . . . . . .	. .	. .	. .	. . . . . .	. .	. .		.114
Electromagnetic Indicators								115
Questions . . . . . . . . . . . . . .	. .	. .	. .	. . . . . .	. .	. .		.116
Answers . . . . . . . . . . . . . .	. .	. .	. .	. . . . . .	. .	. .	.	118

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7 DC Electrics - DC Motors

Motors DC - Electrics DC 7

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DC Electrics - DC Motors 7

Electric Motors

An electric motor is a machine for converting electrical energy into mechanical energy. Its function is, therefore, the reverse of that of a generator. There is little difference between the construction of DC motors and DC generators; both have essentially the same parts and they look alike. In fact, in many cases, a DC machine can be used either as a motor or a generator.

Remember back to magnetic principles, a current flowing through a wire placed in a magnetic field causes the wire to move due to a force acting on the wire; a motor works on this principle.

Fleming’s Left Hand Rule

The direction of rotation of a motor can be determined by Fleming’s Left Hand Rule (Figure 7.1). To do this, align the first finger with the field from the North Pole to the South Pole. Point the second finger in the direction of the current flowing into or out of the armature and the thumb will indicate the direction of motion.

For example in Figure 7.1 the first finger is aligned with the field and the second finger is pointing in the direction of the current coming out of the red (negative) half of the armature. The thumb is pointing upward indicating that the motion is upwards and therefore anticlockwise. In the blue (positive) half of the armature the current is flowing into the armature. Therefore, with the first finger still aligned with the field if the hand rotated through 180 degrees, the thumb will now be pointing downward confirming anticlockwise rotation of the armature.

If the current or the field polarity is reversed, then so will be the direction of rotation of the motor. However, if both are reversed the direction of rotation of the motor remains unchanged.

ThuMb

Motion

First

Finger

Field

SeCond

Finger

Current

Figure 7.1 Fleming’s left hand rule for motors

DC Electrics - DC Motors 7

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7 DC Electrics - DC Motors

Practical DC Motor

The simple DC generator shown earlier and the DC motor below are not practical and can be improved by adding further armature/s and improving the shape of the pole pieces. (Figure 7.2b.) Generator voltage output and motor speed can be controlled by the addition of field windings which enable the field strength to be adjusted. Figure 7.3 shows a sectional view of a practical DC generator which is similar to a DC motor.

Motors DC - Electrics DC 7

	Figure 7.2
a: Simple DC motor	b: Improved DC motor

Figure 7.3 Sectional view of DC rotating armature generator

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