|Electrical Machines - Basic vocational knowledge (Institut für Berufliche Entwicklung, 144 p.)|
|6. Direct current machines|
|6.5. Circuit engineering and operational features of customary direct current motors|
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.
Sound speed control is possible by altering the exciter flow with the aid of the strain field actuator and varying the applied mains voltage.
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?