What is it used for?

Impedance measurements and components of impedance such as capacitance, inductance and phase angle, are mainly used to calculate the level of balance between the phases in a motor and to compare results over time.

impedancerotor_lowvoltImpedance and its components are not published by motor manufacturers, therefore tracking it over time and/or calculating imbalances are required.

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

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

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

How it works

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, and math is used to calculate the various components.

Impedance is measured in Ohms. It is different from a winding resistance measurement since this is done with a DC voltage. Since impedance includes components of resistance, inductance and capacitance, it can provide some additional imbalance information when compared to DC resistance only.

The D factor and Q factor are calculated. The Dissipation factor is also called Tan Delta. Tan Delta measurements on large machinery typically uses AC hipot type equipment to make the measurement, and should not be compared to this measurement which is a multimeter type.

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

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 are indications of rotor bar problems. This test works well on some motors, but not all.