Transformer circuit calculations
This worksheet and all related files are licensed under the Creative Commons Attribution License,
version 1.0.
To view a copy of this license, visit , or send a
letter to Creative Commons, 559 Nathan Abbott Way, Stanford, California 94305, USA. The terms and
conditions of this license allow for free copying, distribution, and/or modification of all licensed works by
the general public.
1

Questions
Question 1
If an
isolation transformer
(a transformer with the same number of “turns” in the primary and secondary
coils) is connected between an AC source and an AC load, we will measure the same voltage and the same
current at both source and load terminals:
A
A
V
V
120 VAC
120 VAC
3.5 A
3.5 A
If we calculate power output by the source and power dissipated by the load, the value is the same: 420
Watts (
P
=
IV
).
Now suppose we analyze a circuit containing a
step-up
transformer (one with more turns of wire in the
secondary coil than in the primary coil). With a step-up transformer, the load voltage will be greater than
the supply voltage. In this example, I show a step-up transformer with a 1:2 step ratio:
A
A
V
V
120 VAC
1:2
240 VAC
10 A
Assuming the load resistance is completely different from the first (isolation transformer) circuit, what
can you deduce about the load current and the power (both source and load) in this circuit? Is the load
current less than the source current? Is the load current greater than the source current? Is the load power
greater than the source power? Explain your answers.
file i01253
2

Question 2
Industrial
control power transformers
are used to step down 480 or 240 volts to a level more acceptable
for relay control circuitry: usually 120 volts. Some control power transformers are built with multiple primary

#### You've reached the end of your free preview.

Want to read all 12 pages?

- Fall '19
- Alternating Current, Vsecondary, Isecondary, Vprimary, Iprimary