5K40.72 - Back EMF
Demonstrate back emf in a motor.
The large DC motor with its starter box is connected as shown to the DC supply with the 5-A projection meter connected in series. The motor is started by slowly moving the arm on the starter box up to the stop. Running with no load, the motor will draw about two amps, but if it is braked with a block of wood to produce a load, the current will go up to six amps or more, showing that the back emf has decreased. The effect of the back emf can also be shown by pushing the starter arm up and holding it; the current will drop as the motor gains speed.