Silicon Carbide Motor Protection Issues
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.
References
[1] Wolfspeed, 1.2kV, “80 mΩ Silicon Carbide Six-Pack (Three Phase) Module”, CCS020M12CM2 Datasheet, Available: https://www.wolfspeed.com/downloads/dl/file/id/187/product/103/ccs020m12cm2.pdf [Accessed July 2018].
[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)