NR 145 Refrigeration Fundamentals, Components and Air Conditioning Fundamentals2007 (2)
Fundamentals of VFDs and Refrigeration Applications
Transcript of 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
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What is in a name?
� Adjustable Frequency Drive (AFD)
� Adjustable Speed Drive (ASD)
� Variable Frequency Drive (VFD)
� Variable Speed Drive (VSD)
� Drive
� Inverter
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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.
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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
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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
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Why use a VFD
� Better regulation and control of the system
� Reduce system cycling
� Reduce maintenance cost
� Energy savings (almost all cases)
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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
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Pulse Width Modulation - PWM
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Differences from across the line starting
� Motor voltage waveform
� Motor current waveform
� Input current waveform
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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
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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
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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
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Output Filter Effects on Motor Voltage
� VFD with no filtering
� VFD with dV/dt filter
� VFD with Sine filter
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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
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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
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400Amps
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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]
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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
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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
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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
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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
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1
THD
PF
+≈
Energy consumption and true power factor
Q = kVAR
(nonwork
producing)
P = kW (work producing)
P = kW (work producing)
D = kVAH
(nonwork
producing)
P = kW (work producing)
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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
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How a VFD is rated
� Required current to the motor
� Voltage rating
� Load Type
� Overload Requirements
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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
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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
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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
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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
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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”
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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
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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
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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
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Ambient Temperature Effect
� The Hotter the environment, the lower the current output
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Altitude Effect
� Higher elevation means less air, less cooling
� Reduce surrounding temperature or reduce heat generation
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Thank you for your time.
Any Questions?
Contact InformationHenry Ohme
Application Engineer
Danfoss Drives
Loves Park, Illinois
1-800-432-6367
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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
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