Question

in a dc shunt motor operating at 380v; it draws an armature current of 55a at full load, running at 1200 rpm. given an armature resistance of 0.5 ohms and shunt field winding resistance of 250 ohms, determine the no-load speed of the dc shunt motor?

Answer 1

We find that the estimated no-load speed is approximately 1291 rpm. The no-load speed of the DC shunt motor can be estimated by calculating the back EMF at full load and assuming that the shunt field current remains constant. After calculating the voltage drop across the armature and the back EMF.

Step-by-step explanation:

To determine the no-load speed of the DC shunt motor, we need to calculate the back EMF (Electromotive Force) at full load conditions and then use that to estimate the no-load speed, considering the armature resistance and the shunt field winding resistance. First, we calculate the back EMF when the motor is running at full load:

Calculate the voltage drop across the armature resistance at full load:

Vdrop = Ia × Ra

= 55 A × 0.5 Ω

= 27.5 V.

Calculate the back EMF at full load:

Eb(full load) = V - Vdrop

= 380 V - 27.5 V

= 352.5 V.

Assuming the motor is an ideal machine and the shunt field current remains constant, the no-load speed can be estimated as proportional to the back EMF.

Compute the no-load back EMF: Eb(no load) ≈ Rated supply voltage (since Ia is negligible, there is no significant voltage drop across the armature).

The no-load speed N0 is proportional to the back EMF,

so N0 = Nfull load × (Eb(no load) / Eb(full load)).

Using the full load speed of 1200 rpm:

N0 = 1200 rpm × (380 V / 352.5 V)

≈ 1291 rpm.

Therefore, the estimated no-load speed of the DC shunt motor is approximately 1291 rpm.