Speed Control of DC Series Motors

Speed Control of DC Series Motors Points : Speed Control of DC Series Motors, Armature Control Methods, Shunted Armature Control, Armature Terminal Voltage Control, Series Parallel Control Speed control of dc series motor may be accomplished through either armature control, field control or series-parallel control. (i) Armature Control Methods Speed adjustment of dc series motors by armature control may be had by any one of the following three methods.
(i) Armature resistance control.
(ii) Shunted armature control and.
(iii) Armature terminal voltage control. 1. Armature Resistance Control This is the most common method employed. It is obtained in the same way as for a dc shunt motor with the exception that the control resistance may be connected directly in series with the supply to the complete motor. Control of armature voltage for the series motor is the same as the voltage applied to the complete motor. The drawbacks of armature resistance control for machines with shunt fields are not as important in the speed .control of dc series motors.

The poor speed regulation that is inherent in this method has no significance for the control of dc series motors, since the speed characteristic of a dc series motor is a rapidly dropping curve. The power loss in the control resistance for many applications of dc series motors is not too serious, since in these applications the control is utilized for a large portion of time for reducing the speed under light-load conditions and is only employed intermittently when the motor is carrying full-load. The speed-torque characteristics of dc series motors with resistance control. The maximum range of speed controls of about will be available depending on the load. This method of speed control is most economical for constant torque drives. This method of speed control is employed chiefly for dc series motors driving cranes, hoists, trains etc. because such drives operate on intermittent duty.

2. Shunted Armature Control The combination of a rheostat shunting the armature and a rheostat in series with the armature used to give slow speeds at light loads. Such a scheme accomplishes the speed control both by lowering the voltage applied to the motor armature and by v drying the flux. The voltage applied to the armature terminals is varied by varying series rheostat R1. The exciting current can be varied by varying the armature shunting resistance R₂ for the same armature current I_a.

3. Armature Terminal Voltage Control The speed control of dc series motors can be accomplished by supplying the power to the motor from a separate variable voltage supply. This method .is seldom used because of high cost of control equipment. (ii) Field Control Method The speed of a dc series motor can be controlled by varying the flux in any one of the following manners. 1. Field Diverter Method The field flux can be reduced by shunting a portion of motor current around the series field, thus reducing the excitation emf and weakening of field. This method gives speeds above normal because flux is reduced by this method. Lesser the diverter resistance, less the field current, less flux and, therefore, more the speed. This method is convenient as well as economical and provides the speed control range usually not exceeding. This method of speed control is used in electric drives in which the speed should rise sharply as soon as the load falls.

For satisfactory operation under violently changing loads, the diverting resistance should be highly inductive because it is connected across the series field winding which normally has very low resistance. If the field winding is shunted by means of non inductive resistance, any sudden change in the load current will not immediately affect the field winding owing to the high inductance of the field circuit impeding any violent change in its current.

2. Tapped Field Control This is another method of increasing the speed by reducing the flux and it is accomplished by reducing the number of turns of the field winding through which the current flows. In this method of speed control of dc series motors a number of tapings from the field winding are brought outside, as illustrated. A number of series field turns can be short-circuited according to the requirement. When all field turns are in circuit, the motor runs at the lowest speed and speed increases with cutting out some of the series field turns. This method is usually employed in electric traction.

3. Paralleling Field Coil Method In this method, used in fan motors, several speeds can be obtained by re-grouping of field coils, as illustrated.

(iii) Series Parallel Control When two or more similar series motors are employed in pairs, as in electric traction, speed control can be obtained by combining series resistance with series and parallel connections, as illustrated.

The machines are started up in series with each other and a starting or control resistance as illustrated.

a. The additional resistance is gradually cut-out by the controller as the motors attain speeds and finally the control resistance is totally removed, then each motor has one half of the line voltage across it.

(b). This is the first running position. In this position for may given value of armature current, each motor will run at half of its normal speed. Since there is no external resistance in the circuit, there is no waste of energy and so motors operate at an efficiency nearly equal to that obtainable with full line voltage across the terminal of each motor.

(c) When it is desired to increase the speed of the combination, two motors are connected in parallel and in series with a variable resistant R.
This resistance is gradually cut out as the motors attain the speed and finally when this resistance is totally removed from the circuit, as illustrated.

(d) The second running position is obtained. In this position each motor is connected across the full line voltage.