Fundamentals of VFDs and Refrigeration Applications

Fundamentals of VFDs and Refrigeration Applications

| Industrial Refrigeration Consortium Research and Technology Forum | May 13-14, 2009 | 2

Outline

� What is a VFD and how does it work

� Differences from across the line motor starting

� How a VFD is rated

� How to select a VFD


What is in a name?

� Adjustable Frequency Drive (AFD)

� Adjustable Speed Drive (ASD)

� Variable Frequency Drive (VFD)

� Variable Speed Drive (VSD)

� Drive

� Inverter


What is a VFD?

� An electronic controller that adjusts the speed of an electric motor by modulating the power being delivered.

� The key to successful operation is adjusting both the motor frequency and the motor voltage according to the requirements of the load.


What can a VFD do?

� Adjust the speed of a motor to match what is required by the system

� Provide a gentle ramp up and down in speed to eliminate surges mechanically, electrically, and hydraulically caused by starting a motor

� Monitor and protect the

motor and the load


What can’t a VFD do?

� Boost the output voltage significantly above the input voltage

� Increase the output power of the motor

� Magically solve all system problems


Why use a VFD

� Better regulation and control of the system

� Reduce system cycling

� Reduce maintenance cost

� Energy savings (almost all cases)


How does a VFD workRectifier

Converts 3 phase AC voltage to pulsating DC voltage

Inverter

Converts DC voltage to variable voltage, variable frequency AC

Intermediate Circuit

Stabilizes the pulsating DC voltage


Pulse Width Modulation - PWM


Differences from across the line starting

� Motor voltage waveform

� Motor current waveform

� Input current waveform


Motor Voltage Waveform

� The output voltage is not a smooth sine wave, but DC pulses

� The shape of the individual pulses in a function of VDF design and motor cable

� Waveform is described by

� Peak Voltage

� Voltage Rise Time

� Rate of Rise (dV/dt)

Rise time

Peak Voltage


Effect on the Motor

� The voltage waveform is more stressful on the motor than across the line operation

� When using a VFD the options for the motor include

� Use a motor designed for use on a VFD

� Use an output filter on the VFD


Motor for use with a VFD

� NEMA MG-1 is the standard that defines motors

� Part 30 describes General Purpose Motors

� Part 31 describes Definite Purpose motors for use with a VFD

Motor Type Peak Voltage Rise Time dV/dt Volts/µ sec

General Purpose 1000 >2 u sec 500

Inverter Duty 1600 >0.1 µ sec 16,000


Output Filter Effects on Motor Voltage

� VFD with no filtering

� VFD with dV/dt filter

� VFD with Sine filter


Motor Current Waveform

� The benefit of the PWM voltage switching is a smoother current waveform than previous technologies

� The closer the current is to sine wave the smoother the motor operates

� The closer the current is to sine wave the cooler the motor operates


Input current waveform

� The diodes in the rectifier section of the VFD only conduct current when the line voltage is greater than the DC bus voltage

� As a result current only flows at the peak of each voltage waveform

� Because the current is not sine wave the VFD is considered a “non linear”load

� This non linear current draw creates harmonics in the electrical system

Snapshot event at 1998-09-08 09:19:50,000

CHA Volts CHA Amps

09:19:50,000 09:19:50,005 09:19:50,010 09:19:50,015 09:19:50,020

Volts

-400

-300

-200

-100

0

100

200

300

400Amps

-300

-200

-100

0

100

200

300


1’st (fundamental)

11.th

7.th

13.th

5.th

What are harmonics

� Harmonics is decomposition of a signal into different (integer multiples of fundamental) frequencies

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1

1 5 9 13 17 21 25 29 33 37 41 45 49

Harmonic order

Ha

rmo

nic

am

plit

ud

e [p

u]


Harmonic distortion by nonlinear loads

� Current distortion is device level performance

� Voltage distortion is system level performance

Non-linear LoadNon-linear Load

Current DistortionCurrent Distortion Voltage DistortionVoltage DistortionSystem

Impedance

Disturbance to

other users

Disturbance to

other usersContribution to

system losses

Contribution to

system losses


Effects of Harmonics

� Current Distortion

� Adds to the total current flowing in the system

� Cables and transformers must be sized for this

� Current harmonics does NOT in itself generate interference with other equipment

� Leads to Voltage Distortion

� Voltage Distortion

� May lead to interference of other equipment

� Based on system information there are tools to predict system harmonics


Power Factor

� Without harmonics: Power Factor is the cosine of the phase angle between voltage and current

� With harmonics:harmonic current also decreases the power factor

� However: The cosine of the phase angle between voltage and current is almost 1 into a VFD

D = kVAH

(nonwork

producing)

S = kVA

P = kW (work producing)

Q = kVAR(nonwork

producing)

φ


VFD

M

Filter

THiD = 0

Cos (φ) = 0.85

PF=Cos(φ)

THiD = 45%

Cos (ϕ) = 0,98

THiD = 0%

Cos (ϕ) = 1

PF=1

21

1

THD

PF

+≈

Energy consumption and true power factor

Q = kVAR

(nonwork

producing)



D = kVAH

(nonwork

producing)



VFD efficiency based on load and speed

450kW

95

95.5

96

96.5

97

97.5

98

98.5

50% 75% 100%

% Speed

Eff in %

100% Load

90% Load

75% Load

50% Load


How a VFD is rated

� Required current to the motor

� Voltage rating

� Load Type

� Overload Requirements


Motor Basics

� Power = Torque * Speed

� Below base speed motors provide constant torque

� Above base speed motors

provide constant horsepower

0%

20%

40%

60%

80%

100%

120%

0HZ

10HZ

20HZ

30HZ

40HZ

50HZ

60HZ

70HZ

80HZ

90HZ

100HZ

110HZ

120HZ

FREQUENCY

T

O

R

Q

U

E


Load Types

� Variable Torque: The require torque increases as speed increases

� Most pumps

� Fans

� Constant Torque: The required torque is independent of the speed

� Compressors

� Some types of pumps

� Some types of blowers

� Constant Power: The required torque decreases as speed increases

� Cutting Tools


Constant Torque vs. Variable Torque

Output Frequency

Moto

r V

oltage

Constant Torque

Variable Torque

� For Constant Torque the ratio of motor voltage to motor current is constant

� For Variable Torque the motor voltage is reduced below what is required for constant torque

� By further reducing the motor voltage additional energy can be saved


Overload Characteristics

� The ratings are typical based on one minute at the overload rating every ten minutes of operation

� The other nine minutes of operation are assumed to be at rated output

� Required overload rating is a function of torque needed to start the load

0 2 4 6 8 10 12

Time (minutes)

Outp

ut Current

High Overload

Normal Overload


How to select a VFD

� Horsepower?

� Full Load Amps?

� CT vs. VT

� Temperature

� Altitude

� Current is what a VFD produces HP/kW is just a “guide”


Fan Applications

� Variable torque load type

� Low starting torque requirements

� Size based on normal

overload current ratings

� Centrifugal pumps are sized the same as fans


Screw Compressors

� Constant torque load type

� Starting torque is similar to running torque

� Size based on Normal

Overload current ratings

� Some manufactures size the motor based on the service factor, others do not


Piston Compressors

� Constant torque load type

� Starting torque depends on where in the stroke the piston stopped

� Size based on the high overload current ratings

� If running with three or fewer pistons talk to the VFD

supplier


Ambient Temperature Effect

� The Hotter the environment, the lower the current output


Altitude Effect

� Higher elevation means less air, less cooling

� Reduce surrounding temperature or reduce heat generation


Thank you for your time.

Any Questions?

Contact InformationHenry Ohme

Application Engineer

Danfoss Drives

Loves Park, Illinois

1-800-432-6367

[email protected]


Who is Danfoss?

� Danfoss was founded in 1933 in Denmark

� First company to mass produce VFDs starting in 1968

� From 0.25 to 1600 horsepower (up to 690 VAC)

� Number 4 globally

� Number 3 in Europe

� Up to 125 horsepower

� Number 1 globally

200820031998199319881983197819731968

Fundamentals of VFDs and Refrigeration Applications

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