Electrical Engineering ⇒ Topic : Starters for Shunt and Compound Motors
Starters for Shunt and Compound Motors
The starters of D.C. motors are generally manufactured in convenient sizes and styles for use as auxiliaries with D.C. shunt and compound motors. Their primary function is to limit the current in the armature circuit during the starting accelerating period.The motor starters are always rated on the basis of output power and voltage of the motors with which they are to be used.There are two standard types of motor starters for shunt and compound motors. These are
(i) Three-point type; and (ii) Four-point type.
Three-point starters are not completely satisfactory when used with motors whose speeds must be controlled by inserting resistance in the shunt field circuit. However, when applications require little or no speed control, either may be employed.
Three-point starter. Refer Fig. (b) The starter has three terminals L,'F and A. The line terminal L must be connected to either side, positive or negative of the D.C. source on the main switch; the field terminal F is connected to one field terminal on the motor; the armature terminal A must be connected to either one of the motor armature terminals. The final connection must then be made from the second line terminal on the main switch to a junction of the remaining two armature and field terminals of the motor. If it is desired that the speed of the motor is controlled, a field rheostat is added as shown in Fig. (a).When the motor is at rest, the starter arm [represented by an arrow in.Fig.(a)] is held in the OFF position by a strong spiral spring.
Starting of motor :
In order to start the motor, one hand is'held on the handle of the open main switch while'the starter arm is moved to the first stud [Figs 1(a) and (b)] with the other hand; then the main switch is closed. If all the wiring is correct and the armature is free to turn, the motor will start
Fig. (1) Three-point starter connected to a shunt motor
After the armature has accelerated sufficiently on the first stud, the starter arm is slowly moved to studes 2, 3, 4 etc. until the arm rests firmly against the iron poles of the holding-coil electromagnet. The entire starting process should take from 5 to 10 seconds. In the final position, the electromagentic pull exerted by the holding coil will be greater than the force exerted by the spiral spring. Should there be a power failure or should the field circuit be opened accidentally, the starter arm will fall back to its OFF position. This function of starter is particularly important because :
if the power fails and starter arm is not restored to the OFF position, the motor might be damaged should the power, come on again; and if the shunt field circuit were opened accidentally and the starter arm did not return to the OFF position, the motor speed might become dangerously high.Often the motors are protected against overloads by thermal overload relays bimetallic is heated by motor current at approximately the same rate at which the motor is itself heating up. Above a certain temperature, this relay trips and,opens the line contractor thereby isolating the motor from the supply.
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