S3 - Sample Questions Transformers

Question 1

What is the primary voltage of a Voltage Transformer that has a marked ratio of 60:1 and the measured secondary voltage is 124.5 Volts?

Answer 1

Ep = Np/Ns * Es
= 60 * 124.5
= 7,470.0 Volts

Question 2

What is the primary amps of a 7200/240 Volt Transformer when the secondary current is measured at 495.4 Amps?

Answer 2

Ip/Is = Es/Ep
Ip = Es/Ep * Is
= 240/7200 * 495.4
= 16.5 Amps

Question 3

What must the no-load secondary voltage of a transformer be with a 98.4% regulation when the voltage at full-load needs to be 120.0 Volts?

Answer 3

Question 4

A transformer is rated at an efficiency of 98.7 percent. If the output of the transformer is rated at 100 kVA, what is the input power to the transformer?

Answer 4

Here we need to rearrange the equation to solve for the input power
input = output / Efficiency
= 100,000 Volt-Amperes / 0.98
= 102,040.8 Volt-Amperes or 102.0 kVA