309A Circuit Theory and Calculations 2

Question 1

What is the resonant frequency of a series RLC circuit with L = 10 mH and C = 100 µF?

Accepted Answer

159.2 Hz

Answer

Resonant frequency f = 1/(2π√(LC)) = 1/(2π√(0.01 × 0.0001)) = 1/(2π × 0.001) = 159.2 Hz. At resonance, XL = XC.

Question 2

A three-phase balanced load draws 30 A per phase at 600 V line-to-line. What is the total apparent power?

Accepted Answer

31.18 kVA

Answer

For a three-phase system: S = √3 × VL × IL = 1.732 × 600 × 30 = 31,176 VA ≈ 31.18 kVA.

Question 3

What happens to the total capacitance when capacitors are connected in parallel?

Accepted Answer

It increases (values add)

Answer

When capacitors are connected in parallel, the total capacitance is the sum of individual capacitances: Ct = C1 + C2 + C3. This is opposite to how resistors behave.

Question 4

A motor draws 25 A at 480 V three-phase with a power factor of 0.85. What is the real power consumed?

Accepted Answer

17.66 kW

Answer

Real power P = √3 × V × I × PF = 1.732 × 480 × 25 × 0.85 = 17,658 W ≈ 17.66 kW.

Question 5

What is the time constant of a circuit with a 500 µF capacitor and a 2 kΩ resistor?

Accepted Answer

1.0 s

Answer

The time constant τ = R × C = 2000 Ω × 0.0005 F = 1.0 second. After one time constant, a capacitor charges to approximately 63.2% of the supply voltage.

Red Seal Electrician 309A Practice Test

Red Seal Electrician 309A Practice Test

309A Circuit Theory and Calculations 2