Impedance Measurements and the Rotor Influence Check

This page is an overview of iTIG impedance measurements and the rotor influence check or RIC test.

Why Are Impedance Measurements Used?

Impedance measurements such as capacitance, inductance and phase angle, are taken to calculate the level of balance between the phases in a motor. The measurements are repeated at regular intervals for the purpose of comparing the results at each interval over time.

Because standard measurements for impedance and its components are not published by motor manufacturers, tracking a motor’s impedance or calculating its imbalances over time is required to establish a performance record for the device. By having a record of the motor’s measurements over time, it is possible to compare its latest results to its earlier results to detect changes or degradation in performance.

Assembled motors can have significantly higher imbalances than stators due to rotor influence.

The measure of capacitance from the windings to the frame can be used to determine how “dirty” the windings are. Capacitance must be tracked over time on the same device since good and bad readings vary by motor. Most people will use the IR or megohm measurement for this purpose.

The rotor test, also called a rotor influence check or RIC test, is used to test squirrel cage rotors for broken rotor bars.

iTIG low voltage impedance test screen
iTIG low voltage impedance test screen.
Learn more about impedance measurement and the rotor test

How Impedance Measurements Work

Impedance, phase angle, capacitance and inductance are measured by applying an oscillating low voltage to the winding (DUT). The default voltage frequency is 1000Hz. Sometimes frequencies around 100Hz can be used. The voltage, current, and phase angle differences are measured, then the impedance is calculated.

Although impedance is measured in ohms, impedance is different from a winding resistance measurement since winding resistance is measured using a DC voltage. Since impedance includes measurements of resistance, inductance, and capacitance, it can provide some additional imbalance information when compared to DC resistance alone.

The dissipation factor (D) and the quality factor (Q) are calculated. The dissipation factor is also called the tan delta, not to be confused with tan delta measurements taken on large machinery, typically using AC hipot type equipment. The dissipation factor tan delta is a multimeter type of measurement.

Q is a measurement of the dampening effect of a circuit. A tuning fork has a very high Q factor whereas an assembled motor has a very low Q factor. The Q factor has limited known use when it comes to motor testing.

Rotor Test or Rotor Influence Check (RIC Test)

The RIC test uses inductance measurements to plot a sinusoidal wave for each phase of an assembled motor. The rotor is turned between each measurement. Distorted waveforms produced during the test are indications of rotor bar problems. The RIC test works well on some motors, but not all.

iTIG rotor influence check screen
iTIG rotor influence check screen.