309A Circuit Theory and Calculations 3

Question 1

What is the inductive reactance of a 50 mH inductor at 60 Hz?

Accepted Answer

18.85 Ω

Answer

Inductive reactance XL = 2πfL = 2 × 3.14159 × 60 × 0.05 = 18.85 Ω. Reactance increases with both frequency and inductance.

Question 2

A 120 V circuit has a load that draws 10 A. What is the power dissipated if the power factor is 0.9?

Accepted Answer

1080 W

Answer

Real power P = V × I × PF = 120 × 10 × 0.9 = 1080 W. The power factor accounts for the phase difference between voltage and current.

Question 3

In a wye-connected three-phase system, what is the relationship between line voltage and phase voltage?

Accepted Answer

VL = VP × √3

Answer

In a wye (star) connection, the line voltage is √3 times the phase voltage: VL = √3 × VP. Line current equals phase current in a wye connection.

Question 4

What is the capacitive reactance of a 50 µF capacitor at 60 Hz?

Accepted Answer

53.05 Ω

Answer

Capacitive reactance XC = 1/(2πfC) = 1/(2 × 3.14159 × 60 × 0.00005) = 1/0.01885 = 53.05 Ω.

Question 5

A conductor has a resistance of 2 Ω at 20°C. If its temperature rises to 75°C and the temperature coefficient is 0.00393/°C, what is the new resistance?

Accepted Answer

2.43 Ω

Answer

R2 = R1 × [1 + α(T2 - T1)] = 2 × [1 + 0.00393(75 - 20)] = 2 × [1 + 0.216] = 2 × 1.216 = 2.43 Ω.

Red Seal Electrician 309A Practice Test

Red Seal Electrician 309A Practice Test

309A Circuit Theory and Calculations 3