Long Cable Motor Protection 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)