Page 399 - Basic Electrical Engineering

Example 3.38 A coil of inductance 1 mH and resistance 50 Ω connected in
series with a capacitor is fed from a constant voltage, variable frequency
supply source. If the maximum current of 5 A flows at a frequency of 50 Hz,

calculate the value of C and the applied voltage.

Solution:

Resonant frequency,

or,

or,

At resonance, X = X , Z = R
L
C

Voltage drop across R = Supply voltage = I R = I R
m
0
Thus, the applied voltage = 5 × 50 = 250V

Example 3.39 An inductive coil has a resistance of 2.5 Ω and an inductive
reactance of 25 Ω. This coil is connected in series with a variable capacitance
and a voltage of 200 V at 50 Hz is applied across the series circuit. Calculate

the value of C at which the current in the circuit will be maximum. Also
calculate the power factor, impedance, and current in the circuit under that

condition.

Solution:

When current is maximum in an R, L, C series circuit, the circuit is under the

resonance condition. At resonance, X = X and Z = R.
L
C