Secondary effects on output side of VSD and mitigation methods

1. Secondary effects on output side of VSD and mitigation methods

2. The secondary effects of VSD’s Supply grid VSD Motor cable Motor    Excess energy High - frequency radiated Insulation stress  Mains line high - due to and conducted emissions through partial frequency  regenerative Leakage current discharges conducted  braking Bearing stress emissions  Heat ( pitting through  Harmonics  High - frequency electrical  Leakage current radiated discharges )  emissions Acoustic switching noise

3. Topic Outline • How Variable Frequency Drives (VFDs) cause du/dt • VFD Output Voltage and Current • Motor Cable Affects Pulse Shape (ringing and double pulsing) • Voltage Waveform Comparison • IEC vs. NEMA Rise-Time Calculations • The effects on the motor • Motor Windings • Motor Insulation and enhancement • Insulation Damage • Partial Discharge • Effects of Common-mode voltages (leakage currents) • Bearing failures • Output filters and performance of Danfoss filters • Output Reactors • Output du/dt Filter • Sinusoidal Filter • Motor Termination Unit

4. How VFDs Cause du/dt • VFD Output Voltage and Current • Motor Cable Affects Pulse Shape • Voltage Waveform Comparison • IEC vs. NEMA Rise-Time Calculations

5. VFD output voltage and current The switching of the inverter IGBTs produces variable width pulses The motor sees an approximated voltage sine wave

6. (1/ LC ) m/s Motor cable affects pulse shape • Short rise-times cause pulse distortion as they propagate along the length of the motor cable • The cable can be represented as a string of series/parallel inductors and capacitors A pulse travels at a speed equal to

7. Voltage Pulse Leaving VFD • Each pulse represents 1 “edge” in the PWM waveform • Pulse enters drive-end of cable @ t=0 and rises to Ud in time tr

8. After One Cable Propagation • Time = tr + tp • Pulse travels along cable to motor and is reflected back because motor’s high frequency impedance is higher than that of the cable • Result: Voltage rises 2 times greater than peak

9. After Two Cable Propagations • Time = 2tr + 2tp • Reflected pulse returns to drive • Result is a negative current pulse which is changed into a negative voltage pulse as it travels back to the motor

10. After Three Cable Propagations • Time = 2tr + 3tp • The 2nd reflection returning from drive in reverse polarity is reflected and doubled at the motor • Counteracts original motor voltage increase • If 2tp is less than tr, the voltage never reaches 2Ud • With longer motor cables, reflection arrives too late to reduce peak voltage

11. Example of Waveform at Motor Cable length = 42 m Motor peak voltage is a function of cable length and rise time

12. Voltage Waveform Comparison Based on 460 VAC test supply Cable Length = 0.5 m *no overshoot Cable Length = 4.0 m *someovershoot Cable Length = 42.0 m *almost 100% overshoot [changed scope setup]

13. Motor terminal overvoltages Voltage ringing overshoot occurs at the motor terminals due to pulse reflection phenomena in the long motor cable. Simulation showing the inverter output voltage and the motor terminal voltage with a 200m shielded cable.

14. Overvoltages higher than 2Vdc Double pulsing Simulation showing the inverter output voltage and the motor terminal voltage with a 200m shielded cable and a double pulsing.

15. Installations with long motor cables Long motor cables have both internal and external effects: External effects: • motor insulation stress (increase possibility of double pulsing) can be eliminated by using sine-wave filters • leakage current can not be eliminated by sine-wave filters, only by filters with DC link connection. To reduce it is possible to use unshielded cables Internal effects: • heating of the frequency converter because of current ringing in the motor cable can be eliminated by using sine-wave filters • saturation of the RFI filter because of the high leakage current can be avoided by extra common-mode inductance – either on the input, or at the output by using a filter with DC link connection

16. IEC vs. NEMA • IEC defined by: IEC60034-17 1998 • IEC calculations result in approximately twice the value of NEMA calculations

17. IEC vs. NEMA • NEMA defined by: MGI part 30:1998

18. The Effects of du/dt on the motor • Motor Windings • Motor Insulation • Enhanced Motor Insulation • Insulation Damage • Failure Mechanism – Partial Discharge

19. Motor Windings • Two types of winding (low voltage motors): • Random wound: turns of round section wire are randomly located in the coil forming process (low power) • Form wound: preformed coils are layered up uniformly (higher power)

20. Motor Insulation Elements of random and form wound insulation systems: • Phase to ground insulation – slot liner and closure • Phase to phase insulation – slot separator and end-winding • Inter-turn insulation – slot and end-winding • Impregnating varnish – slot and end-winding Typical slot cross section area for Random winding and Form winding

21. Motor Insulation • Class F or H provides mechanical strength and electrical insulation and resistance to environmental contamination Partially wound stator core with random winding Partially wound stator core with form winding

22. Enhanced Motor Insulation • Reinforcement of slot liners, slot closures, slot separators, inter-phase barriers, end-winding bracing and possibly special winding wire Completed random winding

23. Insulation damage Possible Causes for Insulation Damage: • Breakdown between coil and stator core Normally not a problem when slot liners are used • Phase to phase failure in the slots or end windings Normally not a problem when inter-phase barriers are used or if the motor is form wound • Inter-turn failure between adjacent conductors in the stator winding Most probable cause of insulation failure due to non-uniform distribution of voltage along the stator winding, associated with short rise times of incident voltage pulses as generated by VFDs

24. Insulation damage • Voltage over-shoot stresses the insulation between motor windings Propagation of a voltage pulse through motor windings

25. Motor insulation breakdown If the overvoltages are severe they can eventually cause the failure of the motor insulation. Following aggravating factors are usually associated with the insulation failure: • old motors with poor insulation (retrofit) • applications with intensive regenerative braking that causes the rise of the DC-link voltage • aggressive environments (heat, humidity, chemical atmosphere)

26. Partial Discharge The peak value of the applied voltage is lower than the actual breakdown voltage of the insulation system The local electric field intensity that is created in a void or cavity is sufficient to exceed the breakdown strength in air (Partial Discharge Inception Voltage) Effects: • Motor insulation system degrades, causing premature aging, when continuously subject to partial discharge • Insulation material gets thinner at discharge points until breakdown occurs To ensure no motor insulation degradation: The applied voltage needs to be less than the partial discharge inception voltage

27. Common-mode voltage generation In a pulsewidth-modulated voltage-source inverter (PWM-VSI) the common-mode voltage is always either +/- Vdc/6 (during an active vector) or +/- Vdc/2 (during a zero-vector).

28. Secondary effects (PWM and dv/dt)

29. The leakage current path

30. Shaft voltage and bearing currents Capacitive coupling caused by the common-mode voltage

31. Shaft voltage and bearing currents Inductive coupling caused by the high dv/dt

32. Bearing failure The shaft voltage causes electrical discharges in the bearing. Eventually the bearing fails because of electrical discharge machining (EDM) Aggravating factors: • Rotor eccentricity • Eccentric load, for example a belt drive • Poor motor and load grounding • Insulated/not grounded load (for example a fan) • Dry atmosphere and applications where electrostatic charges can easily build-up, for example in the textile industry

33. Output filters • Output Reactors • Motor Termination Unit • du/dt Filter • Sinusoidal Filter

34. Output Reactors • Used to reduce du/dt • Can extend the duration of over-shoot if incorrectly selected (double pulsing) • Reduces efficiency (0.5%) Rise Time = 5 s Peak Voltage = 792 V du/dt = 158 V/s

35. Motor Termination Unit • Series resistive/capacitive filters • As the capacitor charges, the current through the circuit reduces – losses in resistor limited to the rising edge duration • Efficiency losses: 0.5 – 1.0% • Not a popular device Peak Voltage = 800 V

36. du/dt Filter Advantages: • Protects the motor against voltage peaks and high du/dt values hence prolongs the motor lifetime • Allows the use of motors which are not specifically designed for converter operation, for example in retrofit applications Application areas: • The typical application areas for dv/dt filters are: • Applications with frequent regenerative braking • Motors that are not rated for frequency converter operation and fed through very short motor cables (less than 15 meters) • Motors placed in aggressive environments or running at high temperatures • Installations using old motors (retrofit) or general purpose motors according to IEC 60034-17

37. du/dt Filter

38. Output du/dt Filter Output voltage and current

39. Output Sine-wave Filter Advantages: • Protects the motor against voltage peaks hence prolongs the lifetime • Reduces the losses in the motor • Eliminates acoustic switching noise from the motor • Reduces semiconductor losses in the drive with long motor cables • Decreases electromagnetic emissions from motor cables by eliminating high frequency ringing in the cable • Reduces electromagnetic interference from unshielded motor cables • Reduces the bearing current thus prolonging the lifetime of the motor

40. Output Sine-wave Filter The typical applications of sine-wave filters are: • Applications where the acoustic switching noise from the motor has to be eliminated • Retrofit installations with old motors with poor insulation • Applications with frequent regenerative braking and motors that are not rated for frequency converter operation • Applications where the motor is placed in aggressive environments or running at high temperatures • Applications with motor cables above 100 meters up to 200 meters. The use of motor cables longer than 200 meters depends on the specific application. (No influence on EMC performance) • Applications where service interval on the motor has to be increased

41. Output Sine-wave Filter Output voltage and current

42. Output Sine-wave Filter Relative Sound pressure measurements with and without filter

43. Filter drawings and info http://dd.danfoss.net/DD-CAT_ProductsServices_Products/PowerOptions/index.htm

44. 2008 2008