The refrigeration control circuit on high voltage equipment almost always uses a step-down transformer.
The transformer lowers the voltage to a safer level and allows the circuit to be isolated.
Having a isolated control circuit also makes troubleshooting easier.
With one test lead on the common side of the transformer, its simple to test each control for open contacts.
Have you ever encountered a transformer graveyard?
What do I mean?
I'm talking about opening a control cabinet and finding three or four burned out transformers littering the bottom.
Let's go through a typical repair.
At first it appears the transformer is the problem, it smells and looks burnt.
So you replace it and start the unit, only to watch as the new transformer begins to smoke!
It's not the transformer!
Chances are a contactor or relay coil is grounded or shorted out. In fact, any bad coil in any control can cause a smoked transformer.
So how do you find the bad coil without continually burning up transformers as you search?
Clip your amp meter on the secondary side of the transformer. On a normally operating system, the amp draw is very low. Usually less than an amp.
Start by disconnecting one of the coils in the control circuit. Turn on the power while watching the amp meter.
If the meter reading jumps, quickly turn the power off. Reconnect the coil and disconnect another. If the meter jumps, quickly turn the power off and reconnect the coil and disconnect another.
Repeat this procedure until disconnecting a control coil doesn't cause the amp meter clipped to the secondary lead of the trasformer to jump.
You've now isolated the bad coil without burning up another transformer.
Replace the control with the bad coil and reapply the power. You should now have a very low amp reading and the refrigeration equipment should be operating normally.
Refrigeration control transformers rarely go out. They last years and years.
When you find a burnt transformer, always follow the above procedure!
Do your part. Help stamp out "transformer graveyards."
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