Secondary effects on output side of VSD and

mitigation methods

The secondary effects of VSD’s

Mains line high-frequency conducted emissions

Harmonics Leakage current

Excess energy due to regenerative braking

Heat High-frequency

radiated emissions

High-frequency radiated and conducted emissions

Leakage current

Insulation stress through partial discharges

Bearing stress (pitting through electrical discharges)

Acoustic switching noise

Supply grid VSD Motor cable Motor

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

How VFDs Cause du/dt

• VFD Output Voltage and Current• Motor Cable Affects Pulse Shape• Voltage Waveform Comparison• IEC vs. NEMA Rise-Time Calculations

VFD output voltage and current

The switching of the inverter IGBTs produces variable width pulses

The motor sees an approximated voltage sine wave

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

(1/ LC ) m/sA pulse travels at a speed equal to

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

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

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

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

Example of Waveform at Motor

Cable length = 42 m

Motor peak voltage is a function of cable length and rise time

Voltage Waveform Comparison

Cable Length = 0.5 m

*no overshoot

Cable Length = 4.0 m

*some overshoot

Cable Length = 42.0 m

*almost 100% overshoot

[changed scope setup]

Based on 460 VAC test supply

Motor terminal overvoltagesVoltage 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.

Overvoltages higher than 2Vdc

Simulation showing the inverter output voltage and the motor terminal voltage with a 200m shielded cable and a double pulsing.

Double pulsing

Installations with long motor cablesLong 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

IEC vs. NEMA

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

NEMA calculations

IEC vs. NEMA

• NEMA defined by: MGI part 30:1998

The Effects of du/dt on the motor

• Motor Windings• Motor Insulation• Enhanced Motor Insulation• Insulation Damage• Failure Mechanism – Partial Discharge

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)

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

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

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

Insulation damage

Possible Causes for Insulation Damage:

1. Breakdown between coil and stator core

Normally not a problem when slot liners are used

2. 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

3. 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

Insulation damage

• Voltage over-shoot stresses the insulation between motor windings

Propagation of a voltage pulse through motor windings

Motor insulation breakdownIf 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)

Partial Discharge

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

1. The peak value of the applied voltage is lower than the actual breakdown voltage of the insulation system

2. 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)

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).

Secondary effects (PWM and dv/dt)

The leakage current path

C

C

MLongCable

Vdc/2

Vdc/2

L

L

(o)

(+)

(-)

Rectifier Inverter

Mains line RFI filter

Load

Heatsink

Shaft voltage and bearing currents

CM

Csr

Csf

CrfED

Stator winding

Motor frame

Shaft

Ground

ZrgZfg

Load

Capacitive coupling caused by the common-mode voltage

Shaft voltage and bearing currents

Csf

ED

Stator winding

Motor frame

ED

Magnetic coupling

Crf1

Crf2

Shaft and frame impedance

Inductive coupling caused by the high dv/dt

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

Output filters

– Output Reactors– Motor Termination Unit– du/dt Filter– Sinusoidal Filter

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

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

du/dt FilterAdvantages:• 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

L

L

L

M3~

PE

98

97

96

VLT Filter

W1

V1

U1

W2

V2

U2

CC

C

C

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

du/dt Filterdv/dt limit curves

150m

150m dv/dt filter

50m

50m dv/dt filter15m

15m dv/dt filter

0

0,2

0,4

0,6

0,8

1

1,2

1,4

1,6

1,8

0 0,5 1 1,5 2 2,5 3

tr [us]

Up

ea

k [

V]

IEC60034-25 A

IEC60034-17

Output du/dt Filter

Output voltage and current

Output Sine-wave FilterAdvantages:• 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

L

L

L

M3~

PE

98

97

96

VLT Filter

W1

V1

U1

W2

V2

U2

CC

C

C

Output Sine-wave FilterThe 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

L

L

L

M3~

PE

98

97

96

VLT Filter

W1

V1

U1

W2

V2

U2

CC

C

C

Output Sine-wave Filter

Output voltage and current

Output Sine-wave Filter

Relative Sound pressure measurements with and without filter

Filter drawings and infohttp://dd.danfoss.net/DD-CAT_ProductsServices_Products/PowerOptions/index.htm

Performance

Output Filter with DC-link

Connection

dv/dt Filte

rs

Sine-wave Filters

Output Filters

for Unshiel

ded Cables

Physical size (relative) 160% 60% 100% 30%

Cost (relative) 200% 40% 100% 30%

Losses (relative) 150% 50% 100% 10%

dv/dt according to NAMUR NE38 & IEC60034 X X X  

Nominal cable length unlimited 50m 300m TBD 

EMV compatible with unshielded cables ++   + ++

Limitations of leaking current  ++      

HF noise emissions from the motor cable ++ - - (+) ++

Motor insulations stress reduction ++ + ++  

Motor bearing stress reduction ++ - + +

Multiple motors running in parallel ++   +  

Reduced acoustic switching noise from motor ++ - +  

20082008