Back-EMF

The armature resistance of a small 28-volt DC motor is extremely low, about 0.1 ohm. When the armature is connected across the 28-volt source, current through the armature will apparently be

This high value of current flow is not only impracticable but also unreasonable, especially when the current drain is found to be about 4 amperes during normal operation of a motor. This is because the current through a motor armature during operation is determined by more factors than ohmic resistance.

When the armature in a motor rotates in a magnetic field, a voltage is induced in its windings. This voltage is called the back or Back-EMF (electromotive force) and is opposite in direction to the voltage applied to the motor from the external source. Back-EMF opposes the current which causes the armature to rotate. Therefore, the current flowing through the armature decreases as the back-EMF increases. The faster the armature rotates, the greater the Back-EMF. For this reason, a motor connected to a battery may draw a fairly high current on starting. But as the armature speed increases, the current flowing through the armature decreases. At rated speed, the Back-EMF may be only a few volts less than the battery voltage. Then, if the load on the motor is increased, the motor will slow down, less Back-EMF will be generated, and the current drawn from the external source will increase. In a shunt motor, the back-EMF affects only the current in the armature, since the field is connected in parallel across the power source. As the motor slows down and the back-EMF decreases, more current flows through the armature, but the magnetism in the field is unchanged. When the series motor slows down, the Back-EMF decreases and more current flows through the field and the armature, thereby strengthening their magnetic fields. Because of these characteristics, it is more difficult to stall a series motor than a shunt motor.