e&m.jpg (12862 bytes)

Current and Voltage of a Motor

 

EQUIPMENT
    redball.gif (326 bytes) Motor
    redball.gif (326 bytes) 10.5 ohm Resistor
    redball.gif (326 bytes) 25 W Lamps
    redball.gif (326 bytes) Large Power Supply



NARRATIVE
       The starting current of a motor is high while the voltage drop across it is low because there is a back EMF being built up as the motor is switched on. Likewise, when large loads are put in the motor, the current draw is high and the voltage drop is low.The motor is equipped with a prony brake to produce a load on the motor. Connect the motor through a switch to 125 VDC. Connect a parallel circuit consisting of the 10.5 ohm resistor (6.4 amp) and a 25 W lamp in series with the motor. This acts as a current measuring device. Connect a 25 W lamp in parallel with the motor. This is the voltage measuring lamp.For a short time after switching on the motor, the current measuring bulb is bright and the voltage bulb is dim. As the motor reaches operating speed, the current draw decreases and the voltage increases. (This takes only about 2 seconds.) Apply the brake and an increased current and decreased voltage is apparent.