frequency drive fundamentals

8/9/2019 Frequency Drive Fundamentals

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Why Adjustable Speed?

Energy savings

System control Reduced maintenance

Sound control


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Why Adjustable

Frequency Drives? Minimum maintenance

High efficiency

Easy retrofit

Remote mounting

Simple control

Bypass capability CAV Fan Clean Room


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How Does an Adjustable

Frequency Drive Operate?


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AC Motor Characteristics


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AC Drives Use

3-Phase Motors Necessary for large horsepowers

Lower current in each

wire Smooth operation

Capacitor-start circuitry notneeded


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Changing Frequency

Changes Motor Speed


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The Design of AC Drives

?


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Converts AC to DC and

then Back to AC

AC DC AC

Rectifier Inverter


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The Inverter Section

DC ACInverter


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Step 1

V1 V2

V1

V2


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Step 2

V1 V2

V1

V2


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Step 3

V1 V2

V1

V2


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Step 4

V1 V2

V1

V2


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Step 5

V1 V2

V1

V2


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Step 6

V1 V2

V1

V2


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Solid-State Switches

Diode

SCR (Thyristor)

IGBT (Insulated Gate BipolarTransistor)


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Diode


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0

Diode

Allows current to flow only in onedirection

The switching occurs at lowvoltages


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1

Diode

Used In

Diode-bridge rectifiers

Circuits to automatically controldrive currents and voltages


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SCR (Silicon Controlled

Rectifier Thyristor)


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Rectifier Thyristor) Like a diode, but the turn-on is

controllable

Easy to turn ON

Difficult to turn OFF

Slow

Very rugged


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Rectifier Thyristor)Used In

Phase-controlled rectifiers

Inverters

- Current source (high HP)

-Voltage source: 1

703

Series


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IGBT (Insulated-Gate

Bipolar Transistor)


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Bipolar Transistor) Easy to turn ON

Easy to turn OFF

Very fast

Rugged


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Bipolar Transistor)Used In

PWM Inverters: VLT Series


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The Inverter Controls the

Output Frequency

AC DC AC

Rectifier Inverter


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Constant V/Hz Ratio


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Motor Torque


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V/Hz for Constant Torque

Loads

drive output frequency

dr

iveoutputvoltage

motor speed

motortorque


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Variable Torque Loads

Dont Need Voltage Boost

otor s eed

requiredto

rque


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V/Hz for Variable Torque

Loads

drive output frequency

dr

iveoutputvoltage

motor speed

motortor

que

60% MaxTorque at 50%Speed


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Advantages of Variable

Torque V/Hz Increased motor efficiency

Reduced motor heating

Reduced motor noise


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Disadvantages of Variable

Torque V/Hz None, if the load is variable

torque

If the load is constant torque Increased motor current

Increased motor heating

Reduced motor torque

Inability to reach maximum speed


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Extended Frequency

OperationWhy not go beyond 50 Hz?


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Constant Horsepower

Extended Frequency

frequency (Hz)

olta

e(

)

50 Hz


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Running the Motor at an

Adjustable Speed Change frequency to change the

motors speed

Applied voltage must bechanged along with frequency

Care must be taken when

running above 50 Hz


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Pulse-WidthModulation

(PWM) Inverter


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PWM- Uncontrolled

Voltage to the Inverter

AC DC AC

Rectifier Inverter


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Motor Line Reactors


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Motor Line Reactors

Advantages

Increased rise

time Can be

retrofitted onto adrive

Disadvantages

Audible noise

Peak voltage atthe motor mayincrease

Requires aseparateenclosure


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Minimizing

Motor Insulation Stress Better motor insulation

Short wire length to the motor

Increased pulse rise time

Dont overheat motor (VVC)


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Minimizing

Motor Insulation Stress Better motor insulation

Short wire length to the motor

Increased pulse rise time

Dont overheat motor (VVC)

Minimize the number of output

pulses per second


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EMI Filter


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Audible Motor Noise

Caused by the PWM carrierfrequency

Depends on the motor, load anddrive


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ASFM

(Automatic Switch

ing FrequencyM

odulation)

High carrierfrequency at low

load for a quietmotor

Low carrierfrequency ath

igh

load for fulloutput

ASFM

0

2

4

6

8

10

12

14

16

18

20

0 10 20 30 40 50 60 70 80 90 100

Load (%)

CarrierFrequency(

kHz)


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Power Factor

General Definition

The Ratio Between thePower Supplied and the

Apparent Power

kWkVA


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Power Factor

Common Meaning

A measure of the phaseshift between the voltage

and current

cos N


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P = V x I

Power = Voltage x Current


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Ideal

Power Factor = 1


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Low Power Factor Reduces

the Power Delivered


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motor speed

powerfa

ctor

Diode Bridge Rectifier

Phase Shift Power Factor

0.95 orhigher


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Harmonic Distortion

Low frequency interference onthe power line

Caused by non-uniform currentflow

Caused by the input stage of the

drive


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Input Charges the DC Bus

Capacitor

Conventional PWM Drive


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Rectified AC Charges

the DC Busrectified AC

DC bus


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How Much Distortion?

Harmonic analysis can be usedto describe any repetitive wave

Describe the wave in terms of abase frequency and multiplefrequencies (harmonics)

Total harmonic distortion (THD) is

the square root of the sum of thesquares of the harmonicamplitudes


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Harmonic Analysis

Even harmonics

Harmonics whichare multiples of 3(150, 250, 350Hz...)

5

th

,7

th

, 11th

, 13

th

...(250, 350, 550,650 Hz...)

Loads withhalf-wave rectifiers very old technology

Single phase loads:computers, copiers,electronic ballasts...

Th

ree ph

asemachines orany ofthe above


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Voltage Distortion


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Voltage Distortion Affects

Other Equipment


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Effects of Voltage Distortion

Carrier current signals Simplex clocks Lighting and security systems

Sensitive electronic equipment Medical Communication Computer

Research Stand-by generators


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Voltage THD

Total Harmonic Distortion 3% Sensitive applications

Airports Hospitals

Telephone companies

5% General applications Office buildings Schools

10% Dedicated systems Factories


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Voltage Distortion

Depends On... The size of the drive

The design of the drive

The buildings power system

You cantguarantee a certainlevel of voltage distortion withoutknowing about the buildingspower system!


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Current Distortion


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Current Distortion Has

Limited Effects


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Effects of Current Distortion

Voltage distortion

Extra heating of power distributioncomponents that supply current to thenon-linear load

Unstable emergency powergenerators

Current distortion at the distributiontransformer of one facility may effectthe voltage at other facilities


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Current Distortion

Designed to protect other powerutility customers

Measured at the Point ofCommon Coupling (PCC) the electrical connecting point between

the utility distribution system and theusers electrical distribution system

It is notthe wiring to an individualdevice


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The Amount of Current

Distortion Depends On... Everything listed for voltage

distortion

PLUS The full load current draw of the

building

The size and short circuit currentrating of the buildings powerdistribution transformer


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Reducing Harmonic

Distortion DC link reactor

AC line reactors


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DC Link Reactors


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AC Line Reactors


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Comparison

DC Link Reactors

Reduces harmonic

distortion Built into the drive as

standard

Requires one or twocoils, can reduce the

size of the buscapacitor

AC Line Reactors

Reduces harmonic

distortion Extra cost option -

increases drive size

Requires three coils


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Comparison, continued

DC Link Reactors

Does not affect the

drives AC lineoperating range

Protects againstcurrent surges

Voltage snubbers in

drive protect againstvoltage surges

AC Line Reactors

Reduces the AC

voltage supplied tothe drive

Protects againstcurrent surges

Protects against

voltage surges


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Reducing Harmonic

Distortion DC link reactor

AC line reactors

Harmonic traps

12-pulse input

Active filters


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Flying Start Searches for thespeed of the motor

Matches the speedof the drive to the

speed of th

e motor Other options

Always start at minimumspeed

DC brake the motor to a

stop


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Connecting a Motor to a

Running Drive Causes a large current

draw

May trip the drive Examples

Non-interlocked outputdisconnect

Staging pumps

Motor selection on the fly

Switching number of motor poles


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Safety Circuitry

Current related Current limit

Protects the drive and motor from damage

Reduces drive speed if too much current is drawn

The drive can be set to trip off if it is in current limitfor more than 60 seconds

What is the correct current limit level?

150% is common for industrial drives

Only 110% is needed for HVAC drives

Too high of a current limit value can cause

damage to the motor or the driven equipment


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Safety Circuitry


Overload Protects the motor from damage

Uses an I2

t circuit to estimate motor temperature


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Safety Circuitry


Overload

Over current

Protects the drive components

Normally current limit keeps the current from gettingtoo high

Over current trip when a short circuit happens

Power must be cycled to reset (TRIP LOCK)


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Safety Circuitry


Overload

Over current

Ground fault Protects the drive components

Trips when the drive detects excessive motorleakage current

Power must be cycled to reset (TRIP LOCK)


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Safety Circuitry

Current related Voltage related

Over voltage Seldom caused by

a high power line Most often caused

by regenerationfrom the motor Attempting to

decelerate the motortoo quickly auto-

ramping eliminatesthis

The motor beingdriven by some otherpart of the system


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Safety Circuitry


Over voltage

Under voltage Prevents drive damage

Prevents motor damage

Some amount of ride through prevents nuisancetrips


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Safety Circuitry


Over voltage

Under voltage

Input phase loss Prevents damage to

The drives rectifiers

The drives DC bus capacitors

Many drives dont have this feature


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Other Considerations

Environment Enclosure type

IP20, NEMA 1finger proof


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IP2

0 IP54 / NEMA 12

dust proof


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De-Rating for High Ambient

0%

1 0%

20%

30%

40%

50%

60%

70%

80%

90%

1 00%

70 80 90 1 00 1 1 0 1 20 1 30 1 40

Maximum Ambient Temperature F

Output

Current

Example IP20, up to 45 kW VLT6000 HVAC

The average

daily ambienttemperature

must be 5 Cbelow the

maximumtemperature


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Ambient temperature Altitude


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Maximum Output at Rated

Ambient Temperature

0%

20 %

40 %

60 %

80 %

100%

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000

Altitude (f t)

PercentofFullOuptut

Current


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Maximum Ambient

Temperature for Full Output

-1 0

-9

-8

-7

-6

-5

-4

-3

-2

-1

0

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000

A ltit (f t)

A

mbi

tT

mperatre

hag

efr

mt

heRated

Vale(

)


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Situations to Avoid Output power factor correction

capacitors

Input power factor correction

capacitors Input disconnect for start/stop

Poor wiring practices


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Output Power Factor

Capacitors


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Input Power Factor

Correction

I t Di t f


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Input Disconnect for

Start/Stop

P Wi i P ti


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Poor Wiring Practices

Loose connections

Control, input and motor wiring

not separated Improper drive grounding

Do NOT daisy chain grounds

Do NOT rely on conduit ground

Improper wire shielding Should ground shields at only ONE end

frequency drive fundamentals

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