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The dc motor is reversed by reversing the direction of the current in the armature.
When the armature current is reversed, the current through the interpole is also reversed.
Therefore, the interpole still has the proper polarity to provide automatic commutation.
MANUAL AND AUTOMATIC STARTERS Because the dc resistance of most motor armatures is low (0.05 to 0.5 ohm), and because the counter emf does not exist until the armature begins to turn, it is necessary to use an external starting resistance in series with the armature of a dc motor to keep the initial armature current to a safe value. As the armature begins to turn, counter emf increases; and, since the counter emf opposes the applied voltage, the armature current is reduced. The external resistance in series with the armature is decreased or eliminated as the motor comes up to normal speed and full voltage is applied across the armature. Controlling the starting resistance in a dc motor is accomplished either manually, by an operator, or by any of several automatic devices. The automatic devices are usually just switches controlled by motor speed sensors. Automatic starters are not covered in detail in this module. Q.16 What is the purpose of starting resistors? |
 
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