Variable Frequency Drives

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Variable Frequency Drives

• Presented by
• PSI Pump Systems Inc.
• Your Solutions Experts for
• Pumps Water Treatment

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The Purposeof VFDs
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The Purpose

• Energy savings on most pump and fan applications.
  
• Better process control and regulation.
• Speeding up or slowing down a machine or process.
• Inherent power-factor correction
• Emergency bypass capability
• Protection from overload currents
• Safe Acceleration

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Electric Power Basics

• All VFDs must
• Run a machine or process at a desired speed.
• Produce adequate torque to handle the load.
• Use power efficiently to produce the necessary
  torque at a given speed.
• Effectively monitor the application or process.

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Electric Power Basics

• The typical waveform consists of the frequency
  portion (time based) of the wave and the
  amplitude portion (the magnitude). This wave is
  actually in sine-wave form, commonly referred to
  as the fundamental.

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Electric Power Basics

• Sinusoidal Waveform With Frequency Amplitude
  Components

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AC-Induction-Motor Theory
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AC Induction Motor Theory
AC Induction Motor Squirrel Cage Design
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AC Induction Motor Theory
Three-phase motor operation.
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AC Induction Motor Theory

• How a motor shaft rotates
• Torque is produced as the induction motor
  generates flux in its rotating field.
• This flux must remain constant to produce
  full-load torque.

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AC Induction Motor Theory

• As shaft torque load increases, the slip
  increases and more flux lines cut the rotor
  windings, which in turn increases rotor current,
  which increases the rotor magnetic field and
  consequently the rotor torque.

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AC Induction Motor Theory
Typical speed versus torque curve for a NEMA
design B motor.
Synchronous Speed
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AC Induction Motor Issues

• Starting Induction Motors
• The starting current is very high, between 3 to
  8 times the full load current. Depending on the
  size of the motor, this can result in voltage
  sags in the power system.
• The full torque is applied instantly at
  starting and the mechanical shock can eventually
  damage the drive system, particularly with
  materials handling equipment, such as
  conveyors.
• In spite of the high starting current,
  for some applications the starting torque may be
  relatively low, only 1.0 to 2.5 times full load
  torque.

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AC Induction Motor Theory
The speed of the rotating electric field within
the induction motor. Synchronous Speed 120
x frequency of motor poles

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AC Induction Motor Theory

• AC motor speed change can be accomplished in
  three ways
• (1) Change the number of poles in the motor this
  means separate windings
• Change the slip characteristics of the motor
  this is done with varying resistors, such as is
  done with a wound-rotor motor or by varying the
  stator voltage or
• Change the frequency of the power supplied to the
  motor. This is the method of choice .

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Components of VFDs

• Power electronics is that field of electronics
  which covers the conversion of electrical energy
  from one form to another for high power
  applications.

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Components of VFDs

• Vacuum tubes
• Thyristors (SCRs)
• provided the standard method for rectifying
  AC. Also referred to as a diode.

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Components of VFDs

• Transistors provide fast switching capability
  for a relatively low cost.
• The general types of transistors are
• The bipolar transistor
• The gate turn off transistor (GTO)
• The field-effect transistor (FET)
• The insulated gate-field-effect transistor
  (IGFET)
• The insulated gate-bipolar transistor (IGBT).

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VFD Basics
A VFD in a block diagram.
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Components of VFDs

• All VFDs need a power section that
  converts AC power into DC power.
• This is called the converter bridge.
• Sometimes the front end of the VFD, the
  converter is commonly a three- phase,
  full-wave-diode bridge.

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Components of VFDs
Rectifier is that special type of converter that
converts AC to DC.
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VFD Basics

• DC Bus
• The DC bus is the true link between the
  converter and inverter sections of the drive.
  Any ripple must be smoothed out before any
  transistor switches on. If not, this
  distortion will show up in the output to the
  motor. The DC bus voltage and current can be
  viewed through the bus terminals.

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VFD Basics
Simplified Circuit showing DC bus components
The DC link is an important section of the drive
as it provides much of the monitoring and
protection for the drive motor circuit. It
contains the base-drive fusing and pre-charge
capacitor network, which assures steady voltage
DC voltage levels prior to the inverter bridge
and allows a path for over-voltage dissipation.
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VFD Basics

• Input Waveforms
• The voltage that is not stepped down is
  rectified through the diodes, and a DC bus
  voltage should be present.
• The DC and DC terminals will typically read
  approximately 325 volts DC on a 230 volts AC
  supplied drive and 650 volts DC on a 460 volts
  AC supplied drive.
• This waveform, when viewed, is straight
  DC, possibly with some rippling effect from the
  AC input.

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VFD Basics

• Power-Module
• The inverter section is made up primarily of
  modules that are each made up of a transistor
  and diode in combination with each other which
  inverts the DC energy back to AC.

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VFD Basics

• By switching the inverter-transistor devices on
  and off many times per half cycle, a
  pseudosinusoidal current waveform is
  approximated.

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VFD Basics
Simplified Inverter Section of a VFD
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VFD Basics

• The DC waveform looks more like an AC waveform
  but the voltage waveform is much different.
• The power semi-conductors in the inverter
  section act as switches, switches of the DC
  bus, and therefore, are pulsing the motor
  with some voltage.

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VFD Basics

• A typical square wave takes its shape on the
  square-wave look due to this switching function (
  which explains the sharp rise to peak) rather
  than a rotational, changing state of amplitude.
• This frequency and amplitude pattern is sometimes
  called the carrier frequency of a PWM drive.

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VFD Basics
Pulse-width-modulated voltage and current
waveforms.
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VFD Basics

• The VFD changes the DC energy into three
  channels of AC energy that an AC induction motor
  can use to function properly.
• Inverters are classified as voltage- source,
  current-source of variable- voltage types. This
  has to do with the form of DC that the inverter
  receives from the DC bus.

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VFD Basics

• Types of VFDs
• Solid-state AC VFDs can be named for their
  use, by their DC bus/inverter voltage or current
  source, by their waveform (PWM or PAM), by the
  type of power device used in their inverter
  section, or by their performance
  characteristics.

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VFD Basics

• The main objective of the VFD is to vary the
  speed of the motor while providing the closest
  approximation to a sine wave for current (while
  pulsing DC voltage to the motor).

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VFD Basics
Volts-per-Hertz Control
The area within each pulse is the power delivered
to the motor in volt-microseconds.
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VFD Basics

• How VFDs Operate
• The PWMs drive ability to maintain the AC
  levels through all types of load conditions at
  given speeds is the factor which separates one
  drive manufacturer from the other.

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VFD Basics
Sample PWM-equivalent circuit with waveforms.
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VFD Basics

• Advantages
• Sometimes getting a little more speed out of the
  motor can increase the flow just enough to
  satisfy the demand.
• When there is capacity in the motor and the VFD
  can be programmed to do this, a new, larger
  motor does not have to be purchased and
  installed.
• The VFD can run 10 to 20 higher in speed and
  make up for lost capacity in a flow-and-demand
  type of system.
• However, many motors as built are not
  balanced for these speeds.

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VFD Basics

• Functions Features
• Set-up Parameters
• The Control Method
• Acceleration or Accel-Ramp Rate
• Automatic Restart
• Stopping Method
• Automatic Signals

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VFD Basics

• Functions Features
• Jump, Skip or Critical Frequencies
• Fault Logs and On-Board
• Diagnostics
• Power Loss Ride Through
• Slip Compensation
• Catch a Rotating Motor, Speed
• Search or Pick-Up a Spinning Load

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VFD Basics

• Conclusion
• Significant energy savings
• Easy setup programming
• Retrofits
• Space
• Better design
• Competitive edge