Question

A motor in normal operation carries a direct current of 0.85 A when connected to a 120 V power supply. The resistance of the motor windings is 11.8 ?.​

Answer 1

Given that,

Direct current = 0.85 A

Voltage = 120 V

Resistance = 11.8 Ω

Suppose, We need to find the back emf generated by the motor

We need to voltage drop across windings due to resistance

Using Ohm's law

$V = IR$

Where, V = voltage

I = current

R = resistance

Put the value into the formula

$V'=0.85\times11.8$

$V' = 10.03\ V$

We need to calculate the back emf generated by the motor

Using formula of back emf

$back\ emf =V-V'$

Put the value into the formula

$back\ emf=120-10.03$

$back\ emf=109.97\ V$

Hence, The back emf generated by the motor is 109.97 V.