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© ABBMonth DD, YYYY | Slide 1

ASHRAE Rocky Mountain Chapter VFD Fundamentals

April 16, 2010Jeff Miller -

2010

What is a Drive / VFD/ AFD?

0

230

460

Volts

Hertz30 60

460 V

60 Hz= 7.67

VHz

230 V

60 Hz= 3.83

VHz

If 230 VAC Power Line:

230 V Motor

460 V

Motor

What is a Drive?

Motor

L1

L2

L3

C

L

Input Converter(Diode Bridge)

Output Inverter(IGBT’s)

DC Bus(Filter)+

_+_

+

_

+

_

+ +

_ _

What is a Drive?

A variable frequency drive converts incoming 60 Hz utilitypower into DC, then converts to a simulated variable voltage, variable frequency output

VFD Fundamentals

60 Hz Power

Electrical Energy

ABB

Zero - 120 Hz

To Motor

VFD

RECTIFIER(AC - DC)

INVERTER(DC - AC)

AC DC AC

VFD

Zero - 120 Hz60 Hz

RECTIFIER

PositiveDC Bus

NegativeDC Bus

+

-

INVERTER

RECTIFIER

PositiveDC Bus

NegativeDC Bus

+

-

INVERTER

Area Under The Square-Wave PulsesApproximates The Area Under A Sine Wave

Frequency

Volta

ge

RECTIFIER

PositiveDC Bus

NegativeDC Bus

+

-

INVERTER

How Often You Switch From PositivePulses To Negative Pulses DeterminesThe Frequency Of The Waveform

Frequency

Volta

ge

Frequency = 30Hz

Frequency = 60Hz

RECTIFIER

PositiveDC Bus

NegativeDC Bus

+

-

INVERTER

Motor

Non-Linear Loads?• Loads which draw non-sinusoidal current from

the line:– Non-incandescent lighting– Computers– Uninterruptible power supplies– Telecommunications equipment– Copy machines– Battery chargers– Electronic variable speed drives – Any load with a solid state AC to DC power

converter

Typical AC Drive Configuration

M460VAC 3-phase

650VDC

Simulated AC

(PWM)

All AC Drives rectify AC to DC, then convert to simulated AC (PWM) to provide the motor Variable voltage and Frequency. The AC to DC conversion generates harmonics.

Harmonics — Definitions

• Non-linear loads draw current in a non-sinusoidal or distorted manner

• Harmonics or harmonic content is a mathematical concept implemented to allow quantification and simplified analysis of non-linear waveforms

• Harmonics are typically present in both network currents and network voltages

• Non-linear current draw creates non-linear voltage as it flows through the electrical network– Current harmonics Voltage harmonics

Harmonic Frequencies

Fundamental5th Harmonic7th harmonic11th Harmonic13th Harmonic17th Harmonic19th Harmonic

60 Hz300 Hz420 Hz 660 Hz780 Hz

1020 Hz1140 Hz

The Theory: Fundamental, 5th and 7th Harmonics

f1 60= w1 2p f1×= i1 t( ) 1 cos w1 t×( )×=

f5 300= w5 2p f5×= i5 t( ) 0.32 cos w5 t× p-( )×=

f7 420= w7 2p f7×= i7 t( ) 0.09 cos w7 t× p-( )×=

0 0.005 0.01 0.015 0.02 0.025 0.031

0.5

0

0.5

1

i1 t( )

i5 t( )

i7 t( )

t

Fundamental

5th

7th

iT t( ) i1 t( ) i5 t( )+ i7 t( )+=

0 0.005 0.01 0.015 0.02 0.025 0.031.5

1

0.5

0

0.5

1

1.5

iT t( )

t

Components

Summation

Harmonic Content, 6- Pulse DrivePWM Drive Harmonic Input Spectrum

5th

7th

Fundamental

11th13th

– Harmonic Current Distortion —• Added heating in transformers and

cables, reduces available capacity

• May stimulate a resonance condition with Power Factor Correction Capacitors

–Excessive voltage–Overheating of PF correction capacitors–Tripping of PF protection equipmentVoltage Distortion interfering w/ sensitive

equipment. Largest Concern!

Harmonics — Why worry?

– Harmonics produced by an individual load are only important to the extent that they represent a significant portion of the total connected load

– Linear loads help reduce system harmonic levels – TDD equals the THD of the nonlinear load multiplied by

the ratio of nonlinear load to the demand load:

Harmonics — A System Issue!

DLNLTHDTDD NL ×=Where

TDD = TDD of the systemTHDNL = THD of the nonlinear loadsNL = kVA of nonlinear loadDL = kVA of demand load

(nonlinear + linear)

Harmonics — By the Numbers

Table 10.2Low-Voltage System Classification and Distortion Limits

SpecialApplications

GeneralSystem

DedicatedSystem

Notch Depth 10% 20% 50%

THD (Voltage) 3% 5% 10%

Notch Area, mVs 16,400 22,800 36,500

Note: Notch area for other than 480 V systems should be multiplied by V / 480.

IEEE 519 - 1992

Harmonics — By the Numbers (cont.)

Table 10.3Current Distortion Limits for General Distribution Systems

ISC / IL <11 11£h<17 17£h<23 23£h<35 35£h TDD<20 4.0 2.0 1.5 0.6 0.3 5.0

20<50 7.0 3.5 2.5 1.0 0.5 8.0

50<100 10.0 4.5 4.0 1.5 0.7 12.0

100<1000 12.0 5.5 5.0 2.0 1.0 15.0

Note: All harmonic current levels are in percent with fundamental current IL as the base.

IEEE 519 - 1992

Harmonics — Attenuation Options• Reactors (Chokes)• Passive Filters

– Harmonic Trap– Hybrid

• High Pulse Count Rectification

• Active Filters– Drive Front End– Stand Alone

Reactors (Chokes)• Simplest and least

expensive harmonic reduction technique

• May be included in base drive package

• Often meet harmonic needs provided drive load is a small portion of total connected load

• May be implemented with AC line reactors or with DC link reactors– AC line reactors provide

better input protection– DC link reactors provide

load insensitive drive output voltage

– Both types provide similar harmonic benefits

• “Swinging” choke design provides enhanced light load harmonic performance

Reactors, AC Line or DC LinkAC LineReactor

M

DC LinkReactor

M

• Different design techniques

• Equal harmonic reduction for same normalized% reactance

• Typical full load THD (current) at drive input terminals28% 46%

Hybrid Filter• Installs in series with

drive input• May feed multiple

drives• Improves power factor

(may go leading)• Typical full load THD

(current) at filter input terminals5% 8%

• Relatively unaffected by line imbalance

High Pulse Count Rectification• Typical configurations are either 12 pulse or 18 pulse• Phase shifting transformer is required• Additional drive input bridge(s) is needed• Typical full load THD (current) at transformer

primary 8% 12% (12 pulse), 4% 6% (18 pulse)• Performance severely reduced by line imbalance

(voltage or phase)• Excellent choice if step-down transformer is already

required

High Pulse Count Rectification (cont.)

• 6 pulse rectifier

Transformer and cabling simple

Current very distortedIthd typically 45% with 3% reactor

DC/AC

DC/AC

Transformer and cabling complicated

Current distortedIthd 8% to 12% (depending on network impedance)



DC/AC

Transformer and cabling complicated

Current wave form goodIthd 4% to 6% (depending on network impedance)

Active Filter Front End with LCL Filter

LCL Filter (Sine Filter) removes high frequencies >1 kHz. (Current and voltage)

Full output voltage is available with 80% input voltage(400VIn = 480VOut)

Full regenerative capability No transformer required Not affected by line imbalance

LCL filter

Line inverter(rectifier)

Motor inverter Motor

L L

C

M

Remember!Even an 80% THD nonlinear load with a will result in only 8% TDD if the nonlinear load is 10% and the linear load is 90%.

(80%•(10%/(10%+90%))=8%)

Harmonic Reduction Summary

Effectiveness of Harmonic Mitigation Techniques (Assuming 100% Nonlinear Loading, ISC / IL = 60)

Technique THD (Current)

Harmonic Reduction

No mitigation (reference level) 72% 3% line reactors (or equivalent DC link reactor) 39% 45.8% 5% line reactors (or equivalent DC link reactor) 33% 54.2% 5% line reactors + 5th harmonic trap filter 12% 83.3% 12 pulse input rectifier with 5% impedance transformer 10% 86.1% Hybrid filter 7% 90.3% 18 pulse input rectifier with 5% impedance transformer 5% 93.1% 12 pulse input rectifier with 5% impedance transformer + 11th harmonic trap filter 4% 94.4%

Active harmonic filter 3.5% 95.1%

Summary – Practical Advice• With a main distribution transformer, 20-30% of its load

on non-linear loads will typically comply with IEEE 519-1992

• Voltage distortion causes interference with sensitive equipment, not current distortion!

• 5% reactors address 90+% of typical applications. They also provide protection against line transients and keep input currents low to avoid oversizing power wiring to comply with NEC.

• Make VFD vendor perform a harmonic distortion calculation with the submittals.

PEAK: 1,040 volts

Peak Voltage all at 50’ of cable

Peak Voltage has many Contributing Factors

Inverter Rated Motors Help Minimize the Issue

Less dV/dT minimizes; problems with RFI/EMI Motor Insulation & Bearing Current

Drive Peak Voltage

1 1040

2 1110

3 1180

4 1290

5 1350

6 2454

Recommendations

• Keep cable length short as possible• Use a NEMA MG1, Part 31 motor (not

“inverter duty” or “inverter ready”• Ensure that grounding is sound

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