Variable Frequency Drive.ppt

PSI Pump Systems Inc.
1(800)900-2220

1(800)900-2220
Variable Frequency Drives
Presented by
“Your Solutions Experts for
Pumps & Water Treatment”

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The Purpose
of
VFD’s

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The Purpose
1. Energy savings on most pump and fan applications.
2. Better process control and regulation.
3. Speeding up or slowing down a machine or process.
4. Inherent power-factor correction
5. Emergency bypass capability
6. Protection from overload currents
7. Safe Acceleration

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Electric & Power Basics
All VFD’s 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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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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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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Three-phase
motor operation.

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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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 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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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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The speed of the rotating electric field
within the induction motor.
Synchronous Speed = 120 x frequency
# of motor poles

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AC motor speed change can be accomplished
in three ways:
(1) Change the number of poles in the motor; this means
separate windings;
(2) 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
(3) Change the frequency of the power supplied to the
motor. This is the method of choice .

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Components of VFD’s
 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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 Vacuum tubes
 Thyristors (SCR’s)
 provided the standard method for
rectifying AC. Also referred to as a
diode.

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 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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 All VFD’s 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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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.

1(800)900-2220
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
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 VFD’s
 Solid-state AC VFD’s 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 VFD’s 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
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

1(800)900-2220
VFD Basics
Conclusion
 Significant energy savings
 Easy setup & programming
 Retrofits
 Space
 Better design
 Competitive edge

Variable Frequency Drive.ppt

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