Circuits, Three-Phase Power and Electrical Measurements — Practice

Question 1

A source has Vth = 12 V and Rth = 4 ohm. Find the maximum power delivered to a load.

Accepted Answer

At RL = 4 ohm, P = 144/16 = 9 W.

Question 2

What is the impedance of a series RLC circuit at resonance?

Accepted Answer

It is a minimum and equals R.

Question 3

R = 10 ohm, L = 0.1 H, C = 100 microfarad in series. Find omega0, Q and bandwidth.

Accepted Answer

omega0 = 1/root(0.1 x 1e-4) = 316 rad/s. Q = 316 x 0.1/10 = 3.16. BW = R/L = 100 rad/s.

Question 4

A 100 kW load at 0.8 lagging is corrected to unity. Find the capacitor rating.

Accepted Answer

tan(phi1) = 0.75, so Qc = 100 x 0.75 = 75 kVAr.

Question 5

Wattmeters read 2000 W and 1000 W. Find total power and power factor.

Accepted Answer

P = 3000 W. tan(phi) = root3 x 1000/3000 = 0.577, so phi = 30 degrees and pf = 0.866.

Question 6

A 100 ohm, 1 mA meter is to read 100 mA full scale. Find the shunt.

Accepted Answer

Rsh = 100 x 1/(100 - 1) = 1.01 ohm.

Question 7

Three identical impedances are first connected in star and then in delta across the same supply. Compare the power drawn.

Accepted Answer

The delta load draws three times the power of the star load.

Question 8

Which bridge measures capacitance and dielectric loss?

Accepted Answer

The Schering bridge.

Practice — Circuits, Three-Phase Power and Electrical Measurements