BACK ELECTROMOTIVE FORCE

by the Boom

When an electric motor is running, its armature windings are cutting through the magnetic field of the stator. Thus the motor is acting also as a generator. According to Lenz's Law, the induced voltage in the armature will oppose the applied voltage in the stator. This induced voltage is called BACK EMF. The strength of the Back EMF depends upon the number of magnetic lines cut per second. So the faster the motor turns, the greater the Back EMF.

Back EMF is very important in electric motors. When the motor is first turned on, there is no Back EMF inasmuch as the armature is not yet turning. This means that the motor will have a high starting torque since there is no opposition to the applied voltage. Then when the motor is running at speed, the Back EMF will oppose much of the applied voltage and the net result is a relatively small amount of power consumption. A big load will slow the motor, reducing the Back EMF, so more power is applied to maintain the torque. (The inverse feedback principle). This is the reason why a motor will burn out if it is not allowed to run. The windings are designed to operate at the net voltage determined by taking the difference between the Applied Voltage and the Back EMF. The starting motor takes several times as much current as the windings can tolerate for more than a few seconds.

Set up your 120 volt power supply, switch, and light sockets and bulbs (see the apparatus in ELECTRIC CIRCUITS).

Use an ordinary 120 volt AC motor (such as an electric fan or power drill). Connect it in SERIES with a large clear glass light bulb and your switch. With the bulb in series with the motor, you should be able to prevent the motor from starting with your hand (better use a cloth on the shaft just in case it's stronger than normal).

Start the power, and note the intensity of the bulb. Then allow the motor to run at full speed. The bulb should be brightest when the motor is stopped because there is no Back EMF to oppose current flow. At full speed, the Back EMF will cut down on the power consumption, and the bulb will dim. You may need to experiment with different wattage bulbs to find the one that works best for your motor.

Add load to the motor with friction, and note the bulb gets brighter as Back EMF is reduced.

You may wish to use AC meters to measure quantitatively the above results.