How a Transformer Works 3D
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In this case, the transformer primary has three taps: common, 208V, and 240V; you will tap a transformer to match the voltage of the power entering the transformer. Power enters a transformer through the primary. The primary and secondary windings don’t touch each other; they rely on an electromagnetic field to transfer energy from the primary to the secondary (induction).
The primary and secondary are both coils of copper wire wrapped around an iron core. The ratio of wraps between the primary and secondary determines how much a transformer steps up or steps down the voltage. When there are 10x fewer wraps on the secondary winding in a transformer with 240V incoming power, the stepped-down voltage will be 24V.
You may also use an ohmmeter to ohm out the primary if the transformer is de-energized and has its primary and secondary disconnected; on a step-down transformer, the resistance will be higher on the primary than on the secondary due to the greater number of wraps. The resistance will also be higher on a transformer tapped to 240V than 208V with the same secondary voltage.
In the case of a failed primary, the ohmmeter would read OL. If there is a short, you will read a path between the terminals and the transformer casing. Blown fuses are commonly misdiagnosed as failed transformers.
You can also measure the voltage inputs and outputs while a transformer is energized to help troubleshoot it.