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close this bookElectrical Machines - Basic vocational knowledge (Institut für Berufliche Entwicklung, 144 p.)
close this folder6. Direct current machines
close this folder6.5. Circuit engineering and operational features of customary direct current motors
View the document6.5.1. Direct current motor with permanent excitation
View the document6.5.2. Direct current series motor
View the document6.5.3. Direct current shunt motor

6.5.1. Direct current motor with permanent excitation

Circuitry

The permanently excited direct current motor is a small machine in which the exciter field is established through permanent magnets.


Figure 104 - Circuitry and terminal board of a direct current motor with permanent excitation

Dependence of current take-up on the torque (load)

As the curve I = f (M) indicates in Figure 105, current take-up is directly proportional to the torque because of the constant exciter flow.


Figure 105 - I = f (M); Current take-up dependence on torque

Dependence of speed on the torque (load)

Direct proportionality exists between speed and torque in speed-load behaviour.


Figure 106 - n = f (M); Speed dependence on torque

Speed control

Speed control ensues by changing the applied voltage.

Application

Direct current motors with permanent excitation are used mainly for power ratings up to 500 W. Such motors find application in the toy industry, household appliances, measurement and control technology.

6.5.2. Direct current series motor

Circuitry

The direct current series motor is a direct current motor whose exciter windings (D1, D2) have been series-switched to the rotor winding.


Figure 107 - Circuitry and terminal board of a direct current series motor

1 Clockwise, 2 Anti-clockwise

Dependence of current take-up on the torque (load)

Current take-up by the motor depends on the load. As curve I = f (M) indicates, during idling the motor only takes up minimal current. Current take-up increases through greater load. Thereby, however, the increase in current intensity is greater than the load growth.


Figure 108 - I = f (M); Dependence of current take-up of a direct current series motor on torque

1 Rated current, 2 Rated torque

Dependence of speed on the torque (load)

The speed-torque curve (Figure 109) shows that the speed depends to a considerable extent on the load. Whilst idling speed assumes greater values. Given reduction the motor may “race” under certain circumstances.


Figure 109 - n = f (M); Speed dependence on the torque of a direct current series motor

1 Rated speed, 2 Rated torque

The considerable centrifugal power which then arises can destroy the motor. Therefore the motor must be securely attached to the drive machine. Speed declines markedly as the load increases. The direct current series motor develops a considerable initial torque during starting. It can, therefore, also start given excessive load.

Speed control

Speed can be controlled by

- a series resistor
- a strain field actuator parallel to the exciter winding and by
- changing mains voltage.

Application

Direct current series motors are used where considerable speed ranges and excessive torques are in evidence and “racing” is not possible (e.g. for driving electric railways, cranes, escalators).

6.5.3. Direct current shunt motor

The direct current shunt motor is a direct current motor whose exciter windings (E1, E2) have been series-switched to the rotor winding.


Figure 110 - Circuitry and terminal board of a direct current shunt motor

(1) Clockwise, (2) Anti-clockwise

Dependence of current take-up on the torque (load)

Current take-up is made up of rotor current IL and exciter current Ie : I = IL + Ie. The curve I = f (M) (Figure 111) shows that exciter current flows as M = 0. Current take-up increases as load increases. Very considerable current flows during overloading.


Figure 111 - I = f (M); Current take-up dependence on the torque of a direct current shunt motor

1 Rated current, 2 Rated torque, 3 Idling current (exciter current), 4 Idling torque

Dependence of speed on the torque (load)

Speed behaviour is characterised through minimal linear speed reduction as load increases.


Figure 112 - n = f (M); Speed dependence on the torque of a direct current shunt motor

1 Rated speed, 2 Rated torque, 3 Idling speed, 4 Idling torque

The unloaded motor runs at an idling speed of N0 and the rated speed of nn is less than 10 per cent.

The speed change between the idling speed n0 and the rated speed of nn is less than 10 per cent.

Speed control

Sound speed control is possible by altering the exciter flow with the aid of the strain field actuator and varying the applied mains voltage.

Application

The direct current shunt motor is used as a drive for machine tools and automation equipment because of its virtually constant speed.

Questions for repetition and control

1. Describe the construction and mode of operation of a direct current generator.

2. Differentiate between the different types of direct current machines.

3. How can the speed of a direct current shunt motor be changed?

4. Explain why a starter is required to start up a direct current motor?

5. How can one alter the rotational direction of a motor?