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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SCR (Silicon Controlled
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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SCR (Silicon Controlled
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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IGBT (Insulated-Gate
Bipolar Transistor) Easy to turn ON
Easy to turn OFF
Very fast
Rugged
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IGBT (Insulated-Gate
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
Current related Current limit
Overload Protects the motor from damage
Uses an I2
t circuit to estimate motor temperature
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Safety Circuitry
Current related Current limit
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
Current related Current limit
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
Current related Voltage related
Over voltage
Under voltage Prevents drive damage
Prevents motor damage
Some amount of ride through prevents nuisancetrips
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Safety Circuitry
Current related Voltage related
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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Other Considerations
Environment Enclosure type
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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Other Considerations
Environment Enclosure type
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
top related