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Resistance Measurement of Electrical Motors/Generators Windings

Measurement Method

Motor winding resistance test uses the “Four-wire” (Kelvin) measurement method. It provides the best possible measurement results since it ensures that the resistance of the connecting current cables is not included in the measurement.

The test current is passed through the windings using the high current cables. The voltage drop across the windings is measured using the sensing cables.

Placing of the cables is very important. The current cables should always be placed outside of the sensing cables. That way, the resistance of both cables and clamps is almost completely excluded from the resistance measurement (Figure 1). The resistance is calculated using Ohm’s law and it equals voltage drop divided by the test current:

R = U/I

Measuring Resistance
Figure 1 – Connecting RMO-M to a test object

Winding Resistance Test

The value of a test current should be selected according to the nominal winding current. Information about nominal winding current could be found on the nameplate of the test object. The test current should not exceed 10% of the nominal winding current. Because of cables heating, higher values of test current will significantly increase the winding resistance.

Winding resistance of three-phase AC motors is measured between their terminals (all three combinations).

Winding Ohmmeter Connection Diagram
Figure 2 – Measuring the stator winding resistance of an AC motor
Resistance measurement of induction motor
Figure 3 – Connection for measuring the stator winding resistance of induction motor

Winding resistance of slip-ring rotor is measured directly on the slip rings (non-linear transition resistance of brushes is not included in the measured winding resistance).

Connection of RMO-M to slip ring motor
Figure 4 – Measuring the winding resistance of the slip-ring rotor
Results menu RMO-M
Figure 5 – Results menu of RMO-M

Discharging Motor After Winding Resistance Test

Be aware of a possibility the energy still remains in a magnetic circuit. After the measurement has been completed the RMO-M device will start the current discharging process automatically. During the current discharging the ”DISCHARGING” message is shown on the device display.

Discharge menu RMO-M
Figure 6 – The discharging message

The leads should never be removed during the testing process. The operator should always wait for a discharging signal and a buzzer sound signal to end. That is an indication the tested motor has been properly discharged.

The current injection and energy discharging process are fully automatically regulated. The safe discharging circuit, equipped with an indicator, rapidly dissipates the stored magnetic energy after the test completion.

CAUTION: The test leads should not be disconnected before the “Discharging” message disappears from the display and the discharging LED is off.

After all the tests have been completed, the test leads are disconnected in the following order:

  1. the test leads are removed from the test object
  2. the test leads are removed from the instrument.

The mains voltage supply cable is removed first from the supply source, and then from the instrument. Finally, the ground (PE) cable is removed from the instrument.

RMO50M and RMO100M

DV Power winding ohmmeters RMO50M and RMO100M are designed for measuring resistances of inductive test objects used in the electric power and other industries.

The RMO50M test current is within the range of 5 mA – 50 A DC. Measuring range spans from 0,1 µΩ to 1000 Ω. The winding ohmmeter RMO100M has the ability of testing with higher values of the test current. The test current of RMO100M is within the range of 5 mA – 100 A DC and the measuring range is between 1 µΩ to 1000 Ω.

The maximum input at the voltage sense channel is 5 V for all test current values. Having this in mind, an operator should select the test current in such a way that for the expected resistance this voltage value would not be exceeded. For example, if anticipated measuring resistance will be around 100 mΩ, the value of the test current should be below 50 A because:

U = I ∙ R

5 V = 50 A ∙ 100 mΩ

Otherwise, the error message “Change Current” is displayed on the device. It indicates the test voltage is too high. In this case, the test current should be reduced and the test repeated.

This message is also displayed if the inductivity of the test object is too high. Again, the test current should be reduced and the test repeated.

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April 1, 2020

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