VFD Theory

www.emcsolutions.com

Specializing in Motors, Drives, Generators, and Electrical Asset Management

Craig Hartman, P.E.

VFD Theory



When was the microprocessor invented?

a) 1969

b) 1942

c) 1921

d) 1977Intel 4004, the first general-purpose,

commercial microprocessor



Answer: a - 1969

The microprocessor was invented by Ted Hoff, at Intel, for Busicom, a

Japanese calculator manufacturer. It was first advertised in November of

1971 and cost thousands of dollars. It is fascinating to note that the first

manned landing on the moon was also in 1969. Apollo 11 launched from

Kennedy Space Center on July 16, 1969. Neil Armstrong became the

first man to walk on the moon, stepping on its surface on July 20th.

Twelve people have landed on the moon.

VFD Theory: 4

ENIACElectronic Numerical Integrator And Computer

The first Turing-complete general-purpose electronic computer built to calculate artillery firing tables for the US Army’s Ballistic Research

Lab. It was a classified military project known as Project PX. Completed on Feb 14, 1946, it utilized

17,468 vacuum tubes, 7,200 crystal diodes, 1,500 relays, 70,000 resistors, 10,000 capacitors, and 5 million hand soldered joints. It

weighed 30 tons, occupied 1800 square feet of floor space, and used 160kW of electricity. It was also used in the development of the

hydrogen bomb.

VFD Theory: 5

The First Super-Computer Hard Drive

In September, 1956, IBM launched the 305 RAMAC, the first “Super-Computer” with a hard disk drive (HDD). The HDD weighed over a ton

and stored a whopping 5MB of data. This photo shows the “portable” version.

By comparison, a 16GB flash drive holds about 3200 times as much data.

VFD Theory: 6

How to identify a meth lab

VFD Theory: 7

6-pulse Converter

ABC

AC to DCConverter

480 V

VFD Theory: 8

Add DC bus

ABC

AC to DCConverter

DCBus

+

-

+650Vdc

0Vdc

480 V

VFD Theory: 9

Pre-Charge Circuit

ABC

On power-up, the capacitor charges through the pre-charge resistor. Once the dc bus reaches rated voltage, the

contactor (or transistor) bypasses the resistor.

+

-

VFD Theory: 10

Add Inverter

ABC

AC to DCConverter

DCBus

DC to ACInverter

M+

-

480 V

+650Vdc

0Vdc

VFD Theory: 11

PWM Output to Motor

480 V60 Hz

650V

VFD Theory: 12

PWM Output to Motor

480 V60 Hz

240 V30 Hz

650V

650V

VFD Theory: 13

PWM Output to Motor

480 V60 Hz

240 V30 Hz

650V

650V

650V

80 V10 Hz



PWM Waveforms

VFD Theory: 15

Pulse Width Modulation(Pseudo Sine Wave)

Carrier Frequency = 1kHzLight Load

VFD Theory: 16

Pulse Width Modulation


Carrier Frequency = 1kHzFull Load

VFD Theory: 17




VFD Theory: 18




VFD Theory: 19

PWM frequency

Advantages of high carrier frequency

• Lower acoustic noise level

VFD Theory: 20

PWM frequency

Advantages of high carrier frequency

• Lower acoustic noise level

Disadvantages of high carrier frequency

• Lower VFD efficiency• Increased VFD heating• Shortens VFD life• Increased motor heating• Higher overshoot peak voltages• Harder on motor insulation• Exacerbates peak voltage concerns• Increased radiated electro-magnetic noise• Increased ground “leakage” currents in motor

cable and motor capacitances



VFD Construction

VFD Theory: 22

VFD Internals

Cooling Fan

AC to DC Inverter

(2 transistors per module)

DC Bus

AC to DC Converter

(2 diodes per module)

AC Power InputPower Output

to Motor

Heat Sink

DC Bus

Capacitors

Variable Frequency Drivecirca 1980’s

VFD Theory: 23

VFD Internals

Transistor

Snubber

Capacitor

discharge

resistors

Capacitor

pre-charge

circuit


Control

Board

VFD Theory: 24

AC to DC

Converter Filter

DC to AC

Inverter

MDC Filter

Capacitor

VFDNo Harmonic Mitigation

TH Free-wheeling

diodes

VFD Theory: 25

Internals

USB programming port

Serial CommsRS485

Modbus RTU

Option Slots (3))


VFD Theory: 26

VFD(with line reactor)

6-Pulse

VFD

Line

Reactor

3-Contactor

Bypass

VFD Theory: 27

VFD(2-reactor harmonic filter)

6-Pulse

VFD

Harmonic Filter

Capacitor

Harmonic Filter

Reactors

3-Contactor

Bypass

VFD Theory: 28

VFD

VFD Device Island

VFD Theory: 29

Why isn’t this VFD working?

VFD Theory: 30

Why isn’t this VFD working?

VFD Theory: 31

VFD

VFD Theory: 32

Dual VFD’s(harmonic filter with dc-link and ac rectors)

6-Pulse

VFD

Dc-link

reactor

Harmonic filter

capacitors

Harmonic filter

reactors

dV/dt

filter

VFD Theory: 33

VFD(3-reactor harmonic filter with dc link reactor)

6-Pulse

VFD

Dc-link

reactor

Harmonic filter

capacitors

Harmonic filter

reactors

dV/dt

filter

VFD Theory: 34

VFD’s in custom assembly

VFD Theory: 35

VFD’s in custom assembly

VFD Theory: 36

VFD’s in Motor Control Center

VFD Theory: 37

VFD’s in Motor Control Center

VFD Theory: 38

NEMA 3R Custom Enclosure (front)

VFD Theory: 39

NEMA 3R Custom Enclosure (internal)

VFD Theory: 40

NEMA 3R Custom Enclosure (rear)

VFD heat sink extends out

the rear of the enclosure

VFD Theory: 41

NEMA 3R Custom Enclosure (rear)

VFD heat sink extends out

the rear of the enclosure

VFD Theory: 42

NEMA 3R Custom Enclosure (installed)

VFD Theory: 43

Constant Torque vs Variable Torque

• Variable Torque (VT) refers to centrifugal pump and fan

applications. Their load torque “varies” in proportion to the

square of motor speed.

• Constant Torque (CT) refers to most other applications. Their

load torque is based on friction losses which are relatively

“constant” at all speeds

• These terms can be confusing and misleading. With a fixed

speed setting, over time, fan and pump loads fluctuate less than

many other applications

• The fundamental difference between VT and CT is the

VFD overload rating

– VT ratings typically range from 0% to 20% overload

for one minute

– CT ratings are typically 50% overload for one

minute

VFD Theory: 44

Speed Range

VFD Theory: 45

Control Methods

F700A500A700A700/V500

• Speed Control

• Torque Control• Position Control

VFD Theory: 46

Regenerative Braking Resistor

Converter DC Link Inverter

L1

L2

L3C

on

tro

l L

og

ic

M

Braking

Resistor

Chopping

Transistor

Regenerative Power FlowRegenerative Power Flow

VFD Theory: 47

Power-Down Braking

Keeps the motor under control even if supply power is lost.

Improved safety for critical applicationse.g. centrifuge, machine tool

VFD Theory: 48

Optimum Excitation Control

� Greater energy savings at constant speed or during acceleration/deceleration� Internal calculations are used to sense the motor current and apply the ideal output voltage for the load condition.



Motor Peak Voltages & dV/dt

VFD Theory: 50

dV/dt

Reflective Wave Phenomenon

Voltage wave reflection is a function of the voltage rise time (dV/dt) and the

length of the motor cables. Impedance mismatches cause voltage pulses to be

reflected back in the direction from which they arrive. As these reflected waves

encounter incoming waves, their values add, causing higher peak voltage. As

wire length or carrier frequency increases, the overshoot peak voltage

also increases. This causes motor insulation degradation and failure.

Resonant Circuit Phenomenon

Electrical systems of every nature have a natural frequency. When system

components have a resonant frequency that matches the natural resonant

frequency of the system, peak voltages can quickly exceed standard reflective

wave overshoots.

VFD Theory: 51

Voltage Stress

ALL MOTORS(NEMA MG1 – Part 30)

VFD MOTORS(NEMA MG1 – Part 31)

Vrated ≤ 600 Volts

Vpeak ≤ 1kV 3.1 * Vrated

Rise time ≥ 2µs 0.1µs

Vrated > 600 Volts

Vpeak ≤ 2.04 * Vrated 2.04 * Vrated

Rise time ≥ 1µs 1µs

Vrated is the line-to-line voltageVpeak is a single amplitude zero-to-peak line-to-line voltage.For 480V: Recommend voltage spikes be limited to 1000V and dV/dt to 1000V/µs

VFD Theory: 52

Typical Voltage Response at Motor TerminalsNEMA MG-1

Voltage

Time

100%Steady-state voltage

Vpeak

90%

10%

∆tRise time

∆V dV

dt

∆V

∆t=

VFD Theory: 53

Common mode voltage

Courtesy TCI

Common mode voltage occurs when the voltages on the three output lines of a drive do not sum instantaneously to zero. dV/dt filters slow down the rate of change of PWM

switching as seen by the load. This reduction in the rate of change results in increased capacitive coupling impedance between bearings and bearing races. This increase in

impedance, in turn, reduces the damaging Common Mode currents.

VFD Theory: 54

dV/dt

Courtesy TCI

VFD Theory: 55

dV/dt

Courtesy TCI

VFD Theory: 56

Is there another way?

dV/dt

VFD Theory: 57

Mitsubishi Soft PWM

dV/dt(proprietary solution)

• Changes acoustic motor noise from a metallic tone into an unoffending complex tone.

• Protects motors from dV/dt for longer lead lengths.

PWM

Frequency

1 hp

and below

2hp 3 hp

and above

2 kHz 300 meters 500 meters 500 meters

3-15 kHz 200 meters 300 meters 500 meters

Note: Assumes NEMA MG1-Part 31VFD-rated motor insulation

VFD Theory: 58

Soft PWM Noise Reduction

VFD Theory: 59

dV/dt filter

dV/dt filter• Reduces voltage spikes to below 1000 Volts• Slows down PWM dV/dt by a factor of 3• Reduces common mode currents by approximately 30%• Protects both the motor and the cable insulation

Recommendations• Consider when VFD-to-motor cable length exceeds 100 ft• Option: Leave room in VFD cabinet and install only if required• Stay within filter manufacturer’s recommendations for carrier frequency• Use proprietary solution

Note: Keep the VFD to motor voltage drop to 2% or less

HintIf motor runs fine on sine wave power, but trips on VFD, then

have the motor surge tested by a qualified motor rewind shop.



VFD’s & Motor Bearings

Copyright 2010 – Electro Static Technology-ITW – Patented Technology – All Rights ReservedCopyright 2010 – Electro Static Technology-ITW – Patented Technology – All Rights Reserved

=460 VAC

60Hz

Electric Motor DesignElectric Motor Design

Most electric induction motors were designed for operation on 3 phase sign wave power – either 50 or 60 Hz.

The input power was balanced in frequency, phase (120 degrees apart) and in amplitude.

Common mode voltage – the sum of the 3 phases would always equal zero volts.

61


62

+

Electric Motor Operation by VFDElectric Motor Operation by VFD

When operated by VFD, the power to the motor is a series of pulses instead of a smooth sign wave.

The input power is never balanced because the voltage is either 0 volts, positive, or negative with rapid switching between pulses.

The Three phases of voltage pulses ensures that the common mode voltage is never equal to zero and instead is a “square wave” or “6 step” voltage.

=


What effect does this have on the bearings?

Voltage builds up until it exceeds the insulation level of the bearing’s oil film layer –the “break-down” voltage of the bearing.

The voltage then arcs through the bearing creating an electrical discharge machining (EDM) pit.

Thousands of pits per second may be created and over time the ball rolling over the disturbed surface can cause “fluting damage”.

63


Shaft Voltage Readings

A number of

different wave

forms may be

present…

64


Bearing Discharge Voltage Pattern

Voltage Increase & drop

Signifying Current flow

through Bearings

Bearing Discharge~50 nano sec.Creates EDM pitting

65

Copyright 2010 – Electro Static Technology-ITW – Patented Technology – All Rights Reserved

Prevent Bearing Fluting Damage with

AEGIS™ Bearing Protection Ring

66

Copyright 2010 – Electro Static Technology-ITW – Patented Technology – All Rights Reserved

Motor Bearing Damage from Electrical CurrentsMotor Bearing Damage from Electrical CurrentsElectrical Discharge Machining (EDM)

Bearing Pitting Damage

Bearing Fluting Damage

Electron Microscope

(SEM) Image

EDM Pitting

1000x Magnified

EDM Pit

67

VFD Theory: 68

Mitigation Techniques

VFD Waveform

MitigationSine Wave Filter Expense, produces heat

Bearing Insulating

Sleeve

Expense, does not protect driven equipment, capacitive

coupling may allow currents to pass through insulation,

contamination

Ceramic Bearings Expense, does not protect driven equipment

Conductive Grease(Not in current use)

Conductive particles would increase mechanical wear,

rendering lubricants ineffective.

Grounding Brush Expense, wear, contaminants, oxidation, maintenance

Shaft Grounding Ring Expense (lower than others above), contamination

Bearing

Insulation

Alternate

Discharge Paths

Sine-wave filter Ceramic Bearing Ceramic Insulated Bearing


New Conductive Microfiber Shaft Grounding Technology

Uses several methods to transfer

electrical currents*

*IEEE paper, September 2007: Design Aspects of Conductive Microfiber Rings for Shaft Grounding Purposes, by Dr. Annette Muetze et. Al.

Direct Contact Conduction

Electrical Contact without

mechanical contact by field

emission

69


Micro Fiber Shaft Grounding Ring

• Discharges shaft voltages to ground

– Easy to install– Maintenance free– Improves VFD motor bearing reliability– Best ROI - Small investment

• Addresses the root cause of the problem (shaft currents)

– Diverts the shaft current away from the motor bearings

– Protects motor with highly reliable solution

• Longest lasting protection

– Wear rate less than 1 mill per 10,000 hours operation

– Lasts for over 200,000 hours operation– 2 million direction reversals - Zero fiber fatigue or broken

fibers

70


AEGIS SGRBracket Mounted Shaft Grounding Ring

Fits most motor end brackets

Drill and tap for mounting brackets and small screws

Ring slides over shaft

Easy Installation

Applications

• OEM installations

• All VFD driven Motors

• HVAC

• Industrial Process

• OEM installations

• All VFD driven Motors

• HVAC

• Industrial Process


Standard Mounting BracketsShaft diameters: 0.311” to 6.02” (8mm to 153mm)

Ships with mounting brackets, screws and washers

Quick and easy installation to most surfaces

Bolt Through MountingShaft diameters: 0.311” to 6.02” (8mm to 153mm)

M3 x 14 socket head cap screws and lock washers

2 mounting holes up to shaft size 99mm

4 mounting holes for larger sizes

Press Fit MountingShaft diameters: 0.311” to 6.02” (8mm to 153mm)

Clean dry 0.102mm press fit

Custom sizes available

Split RingShaft diameters: 0.311” to 6.02” (8mm to 153mm)

4 to 6 mounting brackets, screws and washers

Installs without decoupling motor

NEMA-IEC Mounting KitsShaft diameters: see chart for standard kits

Custom kits available for other shaft diameters

Clears any slinger, shaft shoulder or protrusion


AEGIS SGRConductive Epoxy Mounting

Split ring or solid ring

Installs with Conductive Epoxy to clean metal surface

No drilling holes or tapping for screws

Best solution for field installation

Applications

• Pump motors

• Fan Motors

• Mechanical Rooms

• Coupled Equipment

• Pump motors

• Fan Motors

• Mechanical Rooms

• Coupled Equipment


AEGIS CS015Colloidal Silver Shaft Coating

Enhances shaft surface conductivity –lowers residual shaft voltage

Can be used in all AEGIS Bearing Protection Ring installations

Required for vertical motors and roller bearings

Helps prevent oxidation

Applications

• For shaft grounding ring installations in harsh areas

• Included in all iPROs

• For shaft grounding ring installations in harsh areas

• Included in all iPROs


Shaft Grounding Ring Bearing Current

Mitigation motors to 100 HP

Stator

RotorShaft

V

F

D

Ground

75


Large Low and Medium Voltage Motors Large Low and Medium Voltage Motors

over 100 HP

Stator

RotorShaft

Ground

V

F

D

Insulatedbearing on

ODE

AEGIS Shaft Grounding Ring on DE

76

TM GE Automations Systems

MMV ASD & Systems SchoolTM GE Automations Systems

MMV ASD & Systems SchoolASD Fundamentals & MV Drive Evolution

We drive industrySlide #77Copyright TM GE Automation Systems March 2011

IEGT Voltage Controlled Gate Driver Equipment

IEGT4.5kV-4kA

IEGT Gate Drive Board

IEGT = Injection

Enhanced Gate

Transistor





Building Block for MV PWM Drives

• NPC: Neutral Point Clamped Configuration

• Multiple supply voltage levels allows good waveforms

• Compatible with IGBT, GCT, IEGT Devices

E

E

Q1

Q2

Q3

Q4

E

E

Q1

Q2

Q3

Q4

E

E

Q1

Q2

Q3

Q4

+E 0V -E





Complete 3 Level Circuit, Neutral Point Clamped

E

E

Q1

Q2

Q3

Q4

E

E

Q1

Q2

Q3

Q4

E

E

Q1

Q2

Q3

Q4

+E 0V -E

M

3 Level phase output voltage

3 Level Inverter

Output voltage of 3 Level Inverter

3 Level inverter is

� 2 times higher output voltage

� 2 times larger capacity

� Twice as clean waveform

3 Level+E

0V

-E

Line to Line

3 / 5 levels

Including zero





Progress of Inverter

Circuits to High Capacity

3

2 Level Inverter

3 Level Inverter

5 Level Inverter

Low voltage application

460V, 690V

Large capacity

3kV - 15MVA 6kV～～～～7kV, 8MVA～～～～120MVA

High voltage, large capacity, clean waveform





MV IGBT Drive with Integral Transformer

INCOMING

POWER

BYPASS

CONTACTOR

[option]

TRANSFORMER &

DC CONVERTER

INVERTER

SECTION

DRIVE

CONTROL





Line up of Large Capacity Inverters6.0 – 7 kV, 5 level output

5 Level IEGT Inverter

Capacity: 6kV - 20MVA, up to 20MVA x 4 = 80 MVA

5 Level IEGT Inverter

Capacity: 7kV - 30MVA

up to 30MVA x 4 = 120 MVA

5 Level IGBT Inverter

Capacity: 6kV - 8MVA

VFD Theory: 83

VFD Custom Assembly Shop

VFD Theory: 84

VFD Repair Area

VFD Theory: 85

VFD Inventory



Thank You!Questions?

Documents

VFD Theory