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
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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
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