The coil of an AC motor has a resistance of 5 ohms. The motor is plugged into an outlet where V=120V (rms) and the coil develops a back emf of 118V (rms) when rotating at normal speed. The motor is turning a wheela)Find the current (rms) when the motor first starts upb)Find the current when the motor is operating at normal speed
At the moment the motor starts it has no back emf developed, so the current is (from Ohm's law): I = V / R = 120 volts / 5 ohms = 24 amps When the motor is operating normally it produces a back emf of 118 volts, so the total voltage across the motor is 120 volts - 118 volts = 2 volts. Using Ohm's Law again, but with the new voltage, we get: I = V / R = 2 volts / 5 ohms = 0.4 amps
