Question

A shunt-connected, 75-kW, 250-V DC motor has an armature resistance of 45 mΩ and a field resistance of 185 Ω. When operated at 250 V, its no-load speed is 1850 RPM.

A) Calculate the current through the shunt field winding
B) Calculate the back EMF at no load
C) Determine the armature current at no load
D) None of the above

Answer 1

The current through the shunt field winding is 1.35 A. The back EMF at no load is 250 V. The armature current at no load is zero.

Step-by-step explanation:

A) To calculate the current through the shunt field winding, we can use Ohm's Law, which states that V = IR, where V is the voltage across the shunt field winding, I is the current through the winding, and R is the resistance of the winding. In this case, the voltage across the shunt field winding is the same as the supply voltage, which is 250 V. The resistance of the shunt field winding is given as 185 Ω. Therefore, the current through the shunt field winding can be calculated as:

I = V / R = 250 V / 185 Ω = 1.35 A

B) The back EMF (E) at no load can be calculated using the formula E = V - IaRa, where V is the supply voltage, Ia is the armature current, and Ra is the armature resistance. At no load, the armature current is zero, so the back EMF can be calculated as:

E = V - 0 * Ra = 250 V

C) The armature current at no load is zero. Therefore, the answer is option D) None of the above.