A circuit model of a cable and motor system is useful for simulation of the voltages at the motor terminals. Figure 1 shows a simple model for the cable and motor system. The model is valid out to frequencies of a few megahertz depending on the design. However, at higher frequencies, additional detail must be added to account for high-frequency effects. Read on to see details of the motor cable model and motor model.
Motor Cable Model
In this simple model, the cable is a series inductance Lc representing the inductance of the cable, and a wye-connected capacitance Cc. This model ignores the series resistance and shunt admittance of the cable. These values often have little effect on simulated the voltage rise times.
Motor Model
In this simple model, the motor is a wye-connected parallel RL circuit. The inductance Lm represents the motor magnetizing inductance. The resistance Re represents the core loss of the motor. High-frequency effects such as turn-to-turn capacitance of the motor can be ignored. These values only affect the high-frequency response which is often at frequencies above the area of interest.
Simulation
In conclusion, this model represents a highly simplified cable and motor system model. However, it can determine estimated values of dv/dt, voltage rise time, and peak voltage from a simulation.
With the cable and motor model and knowledge of the drive voltage waveform, you can now simulate the voltages at the motor terminals. The resulting waveform determines the line-line rise time and peak voltage. Industry standards such as the NEMA MG1 standard are useful to help determine acceptability of the resulting waveforms. Use the simulation tool on this website to perform this analysis.
Finally, if the waveform does not meet the specification, one solution is to apply a passive filter in the system. Use the simulation tool to test several different types of output filters.