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.

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].

References

[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)

P.S. Are you having dv/dt issues? Integrated filters may be a simple, compact solution for dv/dt filtering.

Motor cable impedance is a required parameter for accurate simulation of the voltage ringing and overshoot at the motor terminals. There are methods to calculate the cable inductance and capacitance based on the cable material properties and geometries. However, perhaps the easiest method is to measure the cable impedance directly.

Motor Cable Impedance Measurement

Two impedance measurements are required, the short circuit impedance and the open circuit impedance. The short circuit impedance gives the cable series inductance and resistance. The open circuit impedance gives the cable capacitance and leakage resistance. If an impedance analyzer is not available, use an LCR meter to measure these values at several frequencies. Figure 1 shows the open circuit and short circuit line-line measurements for a three-phase cable. Two of the phase cables are tied together to replicate the standard switching configuration of a 3-phase VSI drive.

Cable Parameter Calculation

Figure 2 shows the cable short-circuit impedance versus frequency plot with the associated circuit parameters. The low-frequency impedance gives the cable series resistance Rs, and the higher frequencies give the cable series inductance Ls. Suggested calculations are given in [1].

Next, Figure 3 shows the cable open-circuit impedance versus frequency plot with the associated circuit parameters. The impedance at lower frequencies gives the cable capacitance Cp and leakage resistance Rp. If required, the impedance above resonance gives the high-frequency capacitance and resistance. Suggested calculations are given in [1].

This method yields parameters for the measured cable. Adjusting for the cable length gives per-unit-length measurements. This allows simulation of the system with any cable length for that cable type. Also, note that the measurements shown here are line-line and require adjustment to match the line-neutral cable model shown.

Finally, note that this simple cable model only works up to the resonant frequencies shown in the impedance plots. For more accurate simulation at higher frequencies (typically multi-megahertz), use a more detailed model to capture the high-frequency effects. In addition, the per-unit-length modeling approach breaks down at very long cable lengths. At these lengths, the cable length is of the same magnitude as the wavelength of the relevant frequency component. However, this cable modeling approach works for the simulation of voltage overshoot and ringing in many VFD systems installed today.

References

[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)

P.S. Are you having dv/dt issues? Integrated filters may be a simple, compact solution for dv/dt filtering.

Some motor and drive systems require output filters to protect the motor from high dv/dt or for EMI compliance. However, these filters are often physically large, sometimes exceeding the size and weight of the drive itself. In order to make these filters smaller, lighter, and cheaper, integrated filters have been proposed [1].

These integrated filters combine the magnetic field of an inductor, the electric field of a capacitor, and damping into a single device. The integrated topology is also simple to manufacture using industry-standard techniques. These integrated filters can be used for output filtering of a motor and drive system.

Integrated Filter Concept

The integrated filter combines the fields of a capacitor and inductor into a single core volume. Figure 1 shows this integrated filter conceptually. Integration may decrease the magnetic field energy density or electric field energy density compared to discrete components. However, at the system level, the integrated filter may be smaller, lighter, and/or cheaper to manufacture.

Integrated Output Filters for Motor Drives

This integrated filter concept can be combined into the output bus bars of a motor drive, in order to realize a simple, compact dv/dt filter [2] as shown in Figure 2. Alternating layers of dielectric film and magnetically permeable foil are wrapped around the bus bar. The film and foil layers contain both an electric field and a magnetic field. These fields are orthogonal to each other and nearly independent. In addition, the parasitic resistances in the circuit provide an appropriate amount of damping.

Figure 3 shows the integrated filter connected in a low pass configuration. The phase current passes through the bus bar from L+ to L-. The capacitor terminal C+ connects to the L- side of the bus bar. The capacitor terminal C- connects in a floating wye neutral configuration for 3-phase operation.

Integrated Filter Benefits for Motor Drives

Finally, this type of integrated dv/dt filter may pair well with inverters used as motor drives. There are several benefits to using these filters for motor protection:

The filter inductance is a result of the current through the bus bar. Therefore, it is effectively a single-turn inductor configuration. This means there is no inductor winding turn-to-turn capacitance. The elimination of this turn-to-turn capacitance allows the filter to handle very high dv/dt pulses from SiC drives, for example.

The filter allows for very low gate resistances, effectively moving switching losses out of the silicon, allowing for higher phase currents from the same module.

The integrated device has a low series impedance due to its single-turn inductor topology. Therefore, the filter scales very well as phase current increases for larger motor and drive systems.

Note: This post is a summary of the peer-reviewed work published in [1] and [2] (see below).

References

[1] Andy Schroedermeier and Daniel C. Ludois, “Integrated Inductor and Capacitor With Co-Located Electric and Magnetic Fields,” IEEE Transactions on Industry Applications, vol. 53, no. 1, pp. 380–390, Jan. 2017. (IEEE Xplore Link)

[2] Andy Schroedermeier and Daniel C. Ludois, “Integrated inductors, capacitors, and damping in bus bars for dv/dt filter applications,” in 2018 IEEE Applied Power Electronics Conference and Exposition (APEC), 2018, pp. 2650–2657. (IEEE Xplore Link)

P.S. Are you having dv/dt issues? The simulation tools on this website can help determine if filtering is needed on your motor and drive system to mitigate voltage ringing and high dv/dt issues.

In this dv/dt simulation example, we will look at how motor cables affect ringing and dv/dt at the motor terminals when the cable between the motor and drive is long. When using silicon IGBT-based drives to control electric motors, the waveforms from the drive may cause high dv/dt and voltage ringing at the motor terminals [1]. The interaction of the voltage waveform with the inductance and capacitance of the cable between the motor and the drive causes the ringing. Figure 1 shows this ringing, which can damage the motor windings or the motor bearings in some situations.

Typically for Silicon IGBT drives, this phenomenon is seen at cable lengths of 10’s or 100’s of meters depending on the voltage rise time [2]. If ringing is an issue, filters can be used to lower the dv/dt and damp the ringing that appears at the motor terminals.

Long Cable dv/dt Simulation Setup

The following example can be simulated in the simulation tool on this website by clicking the ‘Scenario 2’ button. The drive is a 3-phase silicon IGBT drive, the motor is a 5 HP, 3-phase induction machine, and the cable is a 14 AWG unshielded rubber 3-phase cable. The system specifications are below. This system represents a typical industrial drive setup.

Drive specifications:

Si drive voltage rise time trise = 250 ns

DC link voltage VDC = 600 V

460 V 5 HP Motor Specifications:

Motor Magnetizing Inductance Lm = 5 mH

Motor Leakage Resistance Re = 5000 Ω

14 AWG Cable Specifications:

Inductance of Cable Lc = 0.29 μH

Capacitance of Cable Cc = 90 pF

Cable Length = 50 m

dv/dt Simulation Results

Figure 2 shows the simulation results for this system. You can see from the simulation results that the there is significant voltage ringing of the line-line voltage at the motor terminals with no filter due to the long cable (blue line).

Adding Filters

Adding a filter to the circuit at the output of the drive will rectify this situation. Two different filters are specified below to bring the rise time to 2us, and keep the peak voltage below 1000 V. This should make the system compatible with older general-purpose drives. Figure 2 shows the filtered waveforms at the motor terminals as the orange and yellow lines. Note that the designs shown here are not the only possible designs, and other combinations of component values can be used to achieve the desired rise times.

Reactor Specifications:

Inductance Lf1 = 0.4 mH

Resistance Rf1 = 350 Ω

LC dv/dt Filter Specifications:

Inductance Lf2 = 0.15 mH

Capacitance Cf2 = 10 nF

Resistance Rf2 = 120 Ω

Head over to the simulation tool to see the simulated waveforms from this scenario. There you can change the parameters to see how variables like rise time and filter size affect the voltage waveform at the motor terminals.

References

[1] E. Persson, “Transient effects in application of PWM inverters to induction motors,” in IEEE Transactions on Industry Applications, vol. 28, no. 5, pp. 1095-1101, Sept.-Oct. 1992. (IEEEXplore Link)

[2] J. C. G. Wheeler, “Effects of converter pulses on the electrical insulation in low and medium voltage motors,” in IEEE Electrical Insulation Magazine, vol. 21, no. 2, pp. 22-29, March-April 2005. (IEEEXplore Link)

Silicon Carbide (SiC) devices have several advantages over traditional silicon IGBTs, including lower switching losses and higher peak junction temperatures. However, the faster turn-on and turn-off times of SiC devices may cause problems in the motor. These issues include damaging bearing currents, voltage ringing at the motor terminals, and insulation breakdown in the motor windings. As an example, the SiC module [1] shown in Figure 1 is capable of turning on in 14 nanoseconds. Industry standards typically limit voltage transitions to 100 nanoseconds or slower for inverter-rated motors.

These high dv/dt and voltage spike issues are present as well for IGBT drives. However, they typically only appear at very long cable lengths due to the slower switching transitions. With SiC-based motor drives, the over-voltage ringing issue in motor and drive systems can be present at cable lengths as short as 3 meters [2].

SiC dv/dt Simulation Setup

In this dv/dt simulation example, we will look at how SiC drives affect ringing and dv/dt at the motor terminals due to their fast turn-on and turn-off characteristics. The following example can be simulated in the simulation tool on this website by clicking the ‘Scenario 1’ button to preload the values used here. In this example, the drive is a theoretical SiC 3-phase drive, the motor is a 3 HP, 3-phase induction machine, and the cable is a 14 AWG unshielded rubber, 3-phase, 4-wire cable. The specifications are below. This system represents a typical industrial drive setup, with SiC devices instead of the IGBTs typically used in motor drives.

Drive specifications:

Si drive voltage rise time trise = 30 ns

DC link voltage Vdc = 600 V

460 V 3 HP Motor Specifications:

Motor Magnetizing Inductance Lm = 4 mH

Motor Leakage Resistance Re = 1100 Ω

14 AWG Cable Specifications:

Inductance of Cable Lc = 0.29 μH/m

Capacitance of Cable Cc = 90 pF/m

Cable Length = 5 m

dv/dt Simulation Results

Figure 2 shows the time domain simulation results for this system. You can see from the simulation results that the there is significant voltage ringing at the motor terminals with no filter (blue line). Due to the high dv/dt of the drive waveform with SiC devices, this ringing occurs even with the relatively short cable length of 5 meters.

Adding Filters

In order to fix the ringing at the motor terminals, a filter can be added to the circuit at the output of the drive. Two different filters are specified below to bring the rise time to 100 nanoseconds or greater and keep the peak voltage below 1000 V. This should, therefore, make the system compatible with many inverter-rated industrial motors. The first filter is a damped reactor. The resistance can be implemented with a discrete resistor or integrated into the core losses of the inductor. The second filter is a damped LC dv/dt filter. Note that the designs shown here are not the only possible designs, and other combinations of component values can be used to achieve the desired rise times.

Reactor Specifications:

Inductance Lf1 = 10 μH

Resistance Rf1 = 150 Ω

LC dv/dt Filter Specifications:

Inductance Lf2 = 1 μH

Capacitance Cf2 = 5 nF

Resistance Rf2 = 10 Ω

Figure 2 shows the filtered waveforms at the motor terminals as the orange and yellow lines. Note that inductor design for SiC drives is not trivial due voltage ringing in the filter caused by the turn-to-turn capacitance of the filter inductor windings.

Head over to the simulation tool to see the simulated waveforms from this scenario. There, you can change the parameters to see how variables like rise time and filter size affect the voltage waveform at the motor terminals.

[2] Andy Schroedermeier and Daniel C. Ludois, “Integrated inductors, capacitors, and damping in bus bars for dv/dt filter applications,” 2018 IEEE Applied Power Electronics Conference and Exposition (APEC), San Antonio, TX, 2018, pp. 2650-2657. (IEEEXplore)

Passive filters are sometimes used at the output of a drive connected to an electric motor in order to mitigate the negative effects of fast rise times and high peak voltages due to ringing at the motor terminals. These low pass filters are commonly needed when the motor leads are long, or a wide band-gap drive is used. Many different passive filter topologies that are in use. Figure 1 shows a few of the most common types.

LR Filter

A common filter used on motor and drive systems with long motor cables is an LR filter or load reactor. The inductance slows down the voltage rise time. The optional resistor damps the overshoot caused by the interaction of the inductance and the cable capacitance.

RC Snubber

One method common with very large motors is the RC snubber circuit, which is placed across the terminals of the motor to reduce the peak voltage from the voltage spikes. The advantage of this filter is that there is no series inductance, which works well with high power drives. However, it only reduces voltage spikes, and it cannot change the dv/dt of the waveform.

LC dv/dt Filter

Another method of slowing down the voltage rise times (lowering the dv/dt) is to employ a damped second-order LC dv/dt filter at the output of the drive. This filter can increase the rise time and damp out the overshoot. It is possible to accomplish this with a smaller value of inductance than in the other filter types.

LC Sine Wave Filter

It also possible to eliminate dv/dt and overshoot problems by employing a second-order sine wave filter at the drive output. This filter provides near sinusoidal waveforms at the motor terminals. However, it is quite a bit larger than other filters due to the larger L and C values needed to bring the filter corner frequency below the PWM frequency.

One common problem with output filters for motor drives is the large size weight, and cost of these filters. The filters also add additional losses to the system. However motor protection requirements necessitate filters in some situations, especially in systems with very long motor cables.

Head over to the Motor Voltage Simulation to see how the voltage waveforms from the motor drive affect overshoot and ringing at the motor terminals, and how filters can mitigate these challenges.

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.

Due to the negative effects of switched power electronic drives on electric motor lifetime in certain situations, several industry standards have been developed. These standards set recommended limits on the rise time of the voltage waveform and the peak voltages allowed. Two of the most common standards are the NEMA MG 1-2016 standard and the IEC 60034-25 standard.

Both standards provide similar guidelines on the rise time and peak of the leading edge of the voltage waveform. However, the definitions of rise time are slightly different. Therefore, it is important to look carefully at the standard and how to measure the drive waveforms. The rest of this post will focus on the NEMA standard

NEMA MG 1 Definitions

The NEMA standard defines rise time as 10% to 90% of the steady state voltage. The standard defines the peak voltage Vpeak as the zero to peak line-to-line voltage. Figure 1 shows the rise time and peak voltage of the waveform measured line-line at the motor terminals as defined by the NEMA standard. The plot shows a zoomed in view of the leading edge of a voltage pulse.

The allowable limits for the NEMA standard also change depending on the rated voltage of the system. The limits also depend on whether the motor is a general purpose motor or an inverter-rated motor.

General Purpose Motors

NEMA MG1 Part 30 specifies allowable rise times and peak voltages for general purpose electric motors. These motors are typically connected to sinusoidal line voltages or older thyristor-based drives. The allowable rise time is relatively slow, since some of these motors may not be able to handle the faster rise times of Silicon IGBT drives.

Inverter-Rated Motors

NEMA MG1 Part 31 specifies allowable rise times and peak voltages for inverter-rated motors that are directly connected to Silicon IGBT-based drives. Section 31.4.4.2 specifies allowable rise times that are much shorter than the Part 30 standard. This allows for higher dv/dt than for general purpose motors. The peak voltage also changes based on the system voltage, with different limits for absolute peak voltage, and peak voltage for partial-discharge-free operation.

Are your system waveforms NEMA MG 1 compliant? Head over to the Voltage Waveform Simulation Page on this site to see how the fast rise times of drive waveforms affect overshoot and ringing at the motor terminals. The simulation allows you to test how filters may bring voltage waveforms into compliance with your specifications.

Dv/dt can be a problem when motors are connected to motor drives. Figure 1 shows a typical electric motor and motor drive (also called a VFD). A cable connects the drive to the motor. In some situations, a filter is installed at the output of the drive.

VFDs for electric motors use pulse width modulation (PWM) to synthesize electrical waveforms of various frequencies to control the motor. These voltage waveforms look almost like square waves. When zooming in on the leading edge of the voltage pulse, the rise time of the pulse becomes apparent. The change in voltage ‘dv’ and the change in time ‘dt’ can be obtained to determine the dv/dt. Figure 2 shows a schematic of a voltage waveform from a VFD and a zoomed in view of the leading edge.

High dv/dt due to short rise times may lead to voltage spikes and ringing on the leading edge of the voltage pulse. This can also be seen in figure 2 with the peak of the voltage spike labeled as Vpeak. This ringing is a source of high-frequency EMI and the voltage spikes may damage the motor windings if the peak voltage is too high.

Mitigation of dv/dt

If the dv/dt or the peak voltage is too high for the application, some of the mitigation methods listed below may be useful:

Use a filter to lower the dv/dt and damp the ringing to lower the peak voltage from the voltage spike.

Modify the drive to turn on the semiconductor devices more slowly in order to lower the dv/dt.

Specify a motor that is tolerant of the dv/dt and peak voltage from the drive.

Want to see these principles in action? Check out the Motor Voltage Waveform Simulation WebApp to see how different filters can change the waveforms from the motor drive.