Electrical Machines: Transformers, DC, Induction and Synchronous — Practice

Question 1

A 4-pole 50 Hz induction motor runs at 1440 rpm. Find the slip and rotor frequency.

Accepted Answer

Ns = 1500 rpm, s = 60/1500 = 0.04, rotor frequency = 0.04 x 50 = 2 Hz.

Question 2

By what factor does star-delta starting reduce the starting current and torque of a delta-connected motor?

Accepted Answer

Both fall to one-third.

Question 3

A 100 kVA transformer has iron loss 1 kW and full-load copper loss 2 kW. Find the load for maximum efficiency and the efficiency at 0.8 power factor.

Accepted Answer

x = root(1/2) = 0.707, so 70.7 kVA. Output 56.6 kW, losses 2 kW, efficiency 56.6/58.6 = 96.6 percent.

Question 4

An induction motor has air-gap power 10 kW and slip 0.04. Find rotor copper loss and mechanical power.

Accepted Answer

Rotor copper loss = 0.04 x 10 = 0.4 kW. Mechanical power = 9.6 kW.

Question 5

A synchronous generator has E = 1.2 pu, V = 1 pu, Xs = 1 pu and delta = 30 degrees. Find P.

Accepted Answer

P = 1.2 x 1.0 x 0.5/1.0 = 0.6 pu.

Question 6

A DC shunt motor runs at 1000 rpm with Eb = 220 V. The field flux is reduced by 10 percent with the same Eb. Find the new speed.

Accepted Answer

Speed is proportional to Eb over flux, so N = 1000/0.9 = 1111 rpm.

Question 7

Why does adding resistance to the rotor of a slip-ring induction motor raise its starting torque but not its maximum torque?

Accepted Answer

Maximum torque occurs at slip R2/X2 and its value does not depend on R2. More R2 moves the peak toward slip 1, so starting torque rises, up to the point where the peak sits at standstill.

Question 8

What does an over-excited synchronous motor do to the system power factor?

Accepted Answer

It draws leading current and improves the lagging power factor of the system.

Practice — Electrical Machines: Transformers, DC, Induction and Synchronous

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