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Starting of D.C. Motors

7.13. Starting of D.C. Motors

7.13.1. Need for starters. A motor at rest has no back or counter e.m.f. At staring therefore, the armature current is limited only by the resistance of the armature circuit. The armature resistance is well low, however, and if full voltage were impressed upon the motor terminals at stand still, the resulting armature current would be many times full-load value-usually sufficient to damage the machine. For this reason, additional resistance is introduced into the armature circuit at starting. As the motor gains speed, its back e.m.f builds up and the starting resistance is cut out.

Note. Very small D.C. motors, either shunt, series or compound wound, have sufficient armature resistance so that they may be started directly from the line without the use of a staring resistance and without injury to the motor.

Fig. 75 shows the connections of a starting resistance in three types of D.C. motors:

a)     A series motor;

b)    A shunt motor; and

c)     A compound motor.

  • In the case of series motor [Fig. 75 (a)], the armature, field and starting resistance are all in series.
    • In the case of shunt motor [Fig. 75 (b)l, it will be seen that the top end of shunt field is connected to the first contact on the starting resistance. This is to ensure that the field winding receives the full supply at the moment of switching on. If the fields were connected to the last stud of the starting resistance, then on starting, the field would receive only a proportion of the supply voltage, the field current would be correspondingly weak and the torque might be too small to start the motor against the friction of the moving parts. 2

      Fig. 75. Circuits incorporating starting resistances.

      • The connections for the compound motor are seen from [Fig. 75 all to be a combination of those of the series and the shunt connections.

      7.13.2. 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. 76. 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. 76 (a). 3

      Fig. 76. Three-point starter connected to a shunt motor.

       

      When the motor is at rest, the starter arm [represented by an arrow in Fig. 76 (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. 76 (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.
      • After the armature has accelerated sufficiently on the first stud, the starter arm is slowly moved to studs 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 electromagnetic 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:

       

      (i)                           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

       

      (ii)                        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 in which 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.

       

      Drawback of a three-point starter. The use of a three-point starter presents a problem. The speed of the motor is controlled by means of the field rheostat. To increase the speed of motor necessitates the setting of the field rheostat to a higher resistance value. The current through the shunt field is reduced, and so is the current through the coil of the holding electromagnet. The reduced current through the coil weakens the strength of the magnet and makes susceptible to line-voltage variations. In the weakened condition a slight reduction in line voltage would further weaken the holding magnet, releasing the arm of the starter and thus disconnecting the motor from the line. Unscheduled stoppages of the motor make the three-point starter quite unpopular.

       

      Four-point starter:

       

      • Fig. 77 shows a simplified diagram of a four-point starter. 4

        Fig. 77. Four-point starter.

        • In this starter the drawback/disadvantage of the three-point starter is eliminated. In addition to the same three-points that were used with the three-point starter; the other side of the line, L2 is the fourth point brought to the starter. The coil of the holding magnet is connected across the line when the arm is moved from the ‘off position. The holding magnet and starting resistors function as in the three-point starter. The possibility of accidentally opening the field circuit is quite remote; hence the greater acceptance of the four-point starter over the three-point starter.

        -        The four-point starter provides the motor with no voltage protection. Should the power fail, the motor must be disconnected from the line. If not, full line voltage will be applied to the armature without the benefit of starting resistors when power is restored. The holding magnet, being connected across the line, releases the arm when the voltage drops below a specific value, thus protecting the motor when the power is restored.

        8. SPEED CONTROL OF D.C. MOTORS