Variable Frequency Drives

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ASHRAE NB PEI Chapter May 12, 2009
Variable Frequency Drives Motors
in the HVAC Industry
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HVAC Variable Frequency Drives
Pumps Fans Towers
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Control HP Save Energy
Used to be the main topic
Centrifugal Loads

• Flow µ Speed
• HP µ Speed3
• At 50 Speed, HP 12.5

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Affinity Laws

• Flow vs Speed
• Q1/Q2 S1/S2
• Head vs Speed
• H1/H2 S12/S22
• HP vs Speed
• bhp1/bhp2 S13/S23

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Typical HVAC VFD Applications

• Fans
• AHU (Supply Air Fans)
• Return Air
• Exhaust
• Cooling Towers
• Evaporative Condensers
• Pumps
• Chilled Water
• Condenser

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Energy Savings
Speed 1
Head
Head
Power
Speed 2
Power
Throttling (riding the curve)
VFD (varying the speed)
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Variable Frequency Drives

• Over 25 years of HVAC applications
• Fundamental principal remains unchanged
• Convert line voltage frequency to controllable
  values and apply to induction motors
• For a 4 pole motor
• 60Hz 1800 rpm
• 30Hz 900 rpm

The preferred choice to achieve variable flow
control
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Variable Frequency Drives

• Numerous advancements over the 25 yrs
• Power devices evolved
• Thyristor (SCR) GTO Bipolar Transistor
  to present day IGBT devices
• IGBT (Insulated Gate Bipolar Transistor)
• Faster Switching / Higher performance
• Lower losses / Higher efficiency
• Smaller packaging
• Robust / Increased reliability

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Variable Frequency Drives

• Numerous advancements over the 25 yrs
• Microprocessor power
• Better control and accuracy
• Enhanced firmware
• High speed current limit Tripless
• Regen Avoidance
• Built-in PLC Function
• PID control loop
• Network communications
• BACnet MSTP, BACnet IP, FLN, N2, LonWorks,
• Modbus RTU, Modbus TCP/IP
• Today VFDs are rated for 100kA interrupt capacity
  

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• Keypad
• Store Multiple Parameter Sets
• Upload / Download Parameters
• Download Parameters without powering up the VFD

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High Speed USB port to capture oscilloscope
type data traces
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Typical PWM VFD
DC Bus
Speed µ Hz
Torque µ V/Hz
Input Line
Filter Capacitor
AC Motor
PWM Output
Inverter
Rectifier
575/3/60
0 - 575V 0 - 60Hz
AC to DC
DC to AC
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Motors

• Manufactured to Nema MG1 standard
• Part 30
• General Purpose not generally associated with
  VFD applications
• Insulation rated for 1000V / 2 msec rise time
  (majority of quality motors rated for withstand
  voltage of 1200V to 1400V)

Note that specs such as Class B or F relate to
temperature classes not voltage withstand rating.
Similarly efficiency ratings also do not relate
to voltage withstand ratings.
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Motors

• Manufactured to Nema MG1 standard
• Part 31
• Definite Purpose Inverter Duty
• Insulation rated for 1600V / 0.1 msec rise time

Note that specs such as Class B or F relate to
temperature classes not voltage withstand rating.
Similarly efficiency ratings also do not relate
to voltage withstand ratings.
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Motors

• Manufactured to Nema MG1 standard
• Part 31
• Definite Purpose Inverter Duty
• Insulation rated for 1600V / 0.1 msec rise time
• Newer motor designs rated for 2000V
• Speed Turndown range
• CT 101
• VT 1001
• 5 Year Warranty

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Motors

• Comments and Recommendations
• The voltage withstand rating relates to the
  voltage waveform applied to the motor windings
• When connected to the line a normal sine wave has
  a peak voltage of approx 800V
• PWM output with voltage reflection due to fast
  rise time (0.1 msec) and long cable can approach
  two times the peak voltage (1600V).
• Recommend using a Nema MG1 Part 31 motor for new
  installations
• For retrofit installations use an output LRC
  dv/dt filter
• Some installations with short cables can use a
  load reactor.

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Component VFD vs Packaged System

• Stand Alone Component VFD
• VFD mounted and individually connected to
  external components (in their own enclosures)
  such as
• Disconnects or Circuit Breakers
• Fuses
• Reactors and/or filters
• Bypass components
• Packaged System
• VFD and components mounted and wired in a Nema 1
  wall or floor mount enclosure

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Component VFD vs Packaged System

• A Packaged System provides the following
  benefits
• More cost effective vs Field mounting and
  interconnecting
• Less space required
• Avoids finger pointing re warranty issues
• Reduces material management on site

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System Components
Fused Disconnect
Bypass
Load Reactor or LRC filter
VFD
O/L
Line Reactor or Harmonic Filter
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Harmonics

• Many devices are defined as non-linear and
  draw current from the line in such a manner as
  to induce current harmonics. Examples are
• Adjustable Speed Drives (AC DC)
• UPS
• Switch-mode power supplies (computers)
• Electronic ballasts
• Phase converters

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What Are Harmonics

• Basic power source is 575/3/60
• Base or fundamental frequency is 60Hz -
  Sinusoidal
• We will see that VFD line current is a
  distorted waveform
• A repeating, distorted waveform can be expressed
  as the Fundamental multiples of the fundamental

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Loads Without Harmonics

• Most common loads do not cause harmonics e.g.
• Incandescent light bulbs
• Baseboard heaters
• Motors
• These are known as Linear Loads
• The current they draw follows the voltage

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Loads With Harmonics

• These are known as Non-Linear Loads
• The current they draw does not follow the
  voltage

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Harmonics
Voltage
Current
Non-Linear Load Fund. Current Harmonics
Linear Load Fund. Current
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Harmonics
Non-Linear Load Reduced Harmonic Content
Non-Linear Load High Harmonic Content
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What are Harmonics?
Fundamental (60Hz)

3rd Harmonic (180Hz)
2nd Harmonic (120Hz)
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Harmonics

• Fundamental 5th Harmonic 7th Harmonic

Fundamental
5th Harmonic
7th Harmonic
Note Lowest harmonic in 6 pulse VFDs is the 5th
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Effects of Harmonics

• Previous slides referred to Current Distortion
• Current Distortion results in Voltage Distortion
• Potential for overheating of Transformers, Motors
• Harmonic resonance
• High neutral currents (1 ph - 3rd harm.)

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IEEE 519

• Originally introduced in 1981 to place limits of
  Voltage Distortion on the power utility at a
  point of common coupling between users at the
  Utility level.
• The 1992 revision added limits of Current
  Distortion relative to the system in which
  non-linear loads are utilized.

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6 12 Pulse Configuration


-
-
6 Pulse
12 Pulse
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IEEE 519

• A simplified summary of the 1992 revision can be
  expressed as
• A small non-linear load installed in a
  large or
  stiff system will not result in excessive
  harmonic distortion.
• As the ratio of non-linear loads vs system
  capacity increases, mitigation of harmonic levels
  may be necessary.

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IEEE 519 - 1992

• current (fundamental frequency component) at PCC.

• Current distortions that result in a dc offset,
  e.g., half-wave converters, are not allowed.

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IEEE 519 PCC
Xfmr 1
xxMVA 69kV / 13.8kV
PCC-1
xxkVA 13.8kV / 480V
Xfmr 2
PCC-2
X
VFD
M
M
M
M
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IEEE 519 - PCC

• From the previous slide
• The intent of IEEE 519 is not to define the
  PCC as point X but rather as points PCC 1 or 2
  depending on the Utility metering point. (TDD
  applied here)
• If Xfmr 1 has a large MVA value the ISC will be
  large as well leading to a greater ISC/IL value
  thus a higher allowable TDD.

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IEEE 519 - PCC

• From the previous slide
• The practice suggested is to reduce the harmonic
  distortion at point X but measure the results
  at the PCC which is the intent of IEEE 519
• Specifying that TDD be met at point X can lead
  to expensive solutions and may not be possible.

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IEEE 519 - PCC

• From the previous slide
• If calculated levels of both Current and Voltage
  distortion are required at the quotation stage
  the following must be made available
• A single line diagram
• Transformer details to calculate Isc
• Load details to calculate IL
• A specified PCC

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Harmonic Solutions

• If the IEEE 519 levels at the PCC cannot be met,
  then harmonic mitigation must be employed.

• Reduce Harmonic Levels by
• Adding Line Reactors
• 12 , 18 .Pulse Configuration
• Add Tuned Filter Traps
• Add Passive Broadband Filters
• Add Active Filters
• Add Active Front End

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Harmonic Solutions

• The various solutions will reduce current
  distortion values in varying degrees

• A 6 pulse VFD without any filtering will
  typically have a current distortion level of
  approximately 75
• Adding line reactors will reduce this to 35-40
  at a low cost
• Mid level filtering or 12 pulse systems can
  achieve levels of 9-15 at increased cost

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Harmonic Solutions

• High level filtering or 18 pulse systems can
  reduce the current distortion level down to
  approximately 5 at a further increase in cost

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Summary

• The solutions have limitations
• Tolerance of Voltage unbalance
• Leading power factors at light load
• Compare efficiency
• Compare size

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Note physical size of Harmonic Filters relative
to Drive Panels beside them
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Bypass Packagewith Input andOutputReactors
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Summary

• There are various solutions
• Specify performance not the method