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How to Drop Test the Field Coils in a DC Motor

The field coils of a DC motor produce the magnetic field which the rotor interacts with to produce rotor rotation. Occasionally, the field coils might overheat and become damaged. A voltage drop test can help identify if a field coil is bad or not. This test compares the field coil operating voltage to the normal voltage of the source. The difference between these two voltages is called the voltage drop. Bad field coils will have a higher voltage drop.

Things You'll Need

Voltage meter
DC power source
DC field coil

Instructions

- 1
  Set the power source to the rated voltage of the motor field coils. Connect the positive terminal of the power source to the positive terminal of the field coil, then connect the negative terminal of the power source to the negative terminal of the field coil.
- 2
  Disconnect one of the two wires to the field coil. Turn the power source on. Measure the voltage across the source by placing the positive and negative probes of the meter on the positive and negative terminals of the power source, respectively.
- 3
  Turn the power source off and reconnect the wire you just disconnected. Turn the power source on and measure the voltage across the field coils. These two voltages should differ by no more than 1 volt. Any reading higher than this indicates a bad field coil.

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