Simple Electric Motor Impedance Measurement

Accurate impedance measurements of an electric motor over a range of frequencies are critical to forming a model of the machine that is useful for dv/dt simulation. The simulation tool on this website uses a simplified model that does not capture high frequency ringing. This article explains how to populate a more detailed circuit model of the induction machine by taking measurements at the terminals with an impedance analyzer.

Measurement Procedure

The impedance versus frequency of the motor can be measured with an impedance analyzer. This will show the changing impedance of the motor windings as a function of frequency. At higher frequencies the parasitic capacitance of the windings will dominate. If a frequency analyzer is not available, use an LCR meter to take measurements at several different frequencies. Figure 1 shows the measurement configuration to get the impedance measurements. The measurement is phase to neutral, which is easy to obtain for dual voltage induction motors [1]. If it is not possible to get a phase to neutral measurement, take line to line measurements and convert to a single phase equivalent for the model.

Motor measurement configuration
Figure 1 – Motor impedance measurement configuration
Parameter Calculation

The resulting impedance measurements will look like the example in Figure 2. The low frequency impedance will give the motor inductance Ld and the core loss Re. The high frequency parasitic capacitance characteristic is modeled by the Lt, Rt, and Ct current path. This current path is important for accurate high frequency modeling of the motor system. Suggested calculations for determining each element of the circuit are given in [1].

Motor Impedance and Model
Figure 2 – Motor impedance versus frequency plot and a suggested single phase equivalent circuit model

[1] A. F. Moreira, T. A. Lipo, G. Venkataramanan, and S. Bernet, “High-frequency modeling for cable and induction motor overvoltage studies in long cable drives,” IEEE Transactions on Industry Applications, vol. 38, no. 5, pp. 1297–1306, Sep. 2002. (IEEE Xplore Link)

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