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REVERSING DIRECTION OF ROTATION OF UNIVERSAL MOTOR

The direction of rotation of a universal motor can be changed by either: (i) Reversing the field connection with respect to those of armature; or (ii) By using two field windings wound on the core in opposite directions so that the one connected in series with armature gives clockwise rotation, while the other in series with the armature gives counterclockwise rotation.

The second method, i.e, the two field method is used in applications such as motor operated rheostats and servo systems. This method has somewhat simpler connections than the first method.
For simple applications like portable drills etc. manual switches are frequently used for reversing the direction of rotation of the motor. Figure 1(a and b) shows how a DPDT (Double Pole Double Throw) switch and a three position switch may be used for reversing the direction of rotation of single field and double field type of motors respectively.
Figure 1 Reversing of a universal motor (a) Armature reversing method using a reversing switch (b) Two-field method using a three-position switch
In Figure 1 (a), when the DPDT switch is in the position shown, the switch blade bridges the switch terminals A – A1 and B – B1, current through series field and armature flows in the direction as indicated by arrows. When direction of rotation of the motor is to be reversed the switch is thrown to position 2.

Now, terminal A gets connected to terminal A2 and terminal B gets connected to B2.  Current through the armature reverses while direction of current through the series field remains unchanged. This leads to reversal of direction of rotation of the motor. As shown in Figure 1(b) the direction of rotation of the motor is reversed by moving the selector switch in position 1 or 2. In position 1, the FORW-field runs the motor in one direction and in position 2, REV-field runs the motors in the reverse direction. Power and control circuit for a double field universal motor has been shown in Figure 2.
Figure 2 Power and control circuit for reversing of a two-field universal motor
When the FOR-push button is pressed, the forward contactor F get energized and its contacts F1 and F2 close to energies the armature and the FORW-field winding. Similarly when the reverse contactor is energized the REV-field winding and the armature get energized by receiving supply through contacts R1 and R2. In the control circuit, push button interlocking, as also auxiliary contact interlocking have been provided to avoid simultaneous energization of contactors F and R.

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