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PSI Pump Systems Inc.
Variable Frequency Drives
Presented by
PSI Pump Systems Inc.
“Your
Solutions Experts for
Pumps & Water Treatment”
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PSI Pump Systems Inc.
The Purpose
of
VFD’s
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PSI Pump Systems Inc.
The Purpose
1.
2.
3.
4.
5.
6.
7.
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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PSI Pump Systems Inc.
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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PSI Pump Systems Inc.
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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PSI Pump Systems Inc.
Electric & Power Basics
Sinusoidal Waveform With Frequency &
Amplitude Components
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PSI Pump Systems Inc.
AC-Induction-Motor Theory
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PSI Pump Systems Inc.
AC Induction Motor Theory
AC Induction Motor – Squirrel Cage Design
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PSI Pump Systems Inc.
AC Induction Motor Theory
Three-phase
motor operation.
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PSI Pump Systems Inc.
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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PSI Pump Systems Inc.
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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PSI Pump Systems Inc.
AC Induction Motor Theory
Typical speed
versus torque
curve for a
NEMA design B
motor.
% Synchronous Speed
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PSI Pump Systems Inc.
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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PSI Pump Systems Inc.
AC Induction Motor Theory
The speed of the rotating electric field
within the induction motor.
Synchronous Speed =
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120 x frequency
# of motor poles
PSI Pump Systems Inc.
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;
(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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PSI Pump Systems Inc.
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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PSI Pump Systems Inc.
Components of VFD’s



Vacuum tubes
Thyristors (SCR’s)
provided the standard method for
rectifying AC. Also referred to as a
diode.
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PSI Pump Systems Inc.
Components of VFD’s


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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PSI Pump Systems Inc.
VFD Basics
A VFD in a
block
diagram.
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PSI Pump Systems Inc.
Components of VFD’s



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 threephase, full-wave-diode bridge.
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PSI Pump Systems Inc.
Components of VFD’s
Rectifier is that special type of converter that
converts AC to DC.
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PSI Pump Systems Inc.
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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PSI Pump Systems Inc.
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 basedrive 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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PSI Pump Systems Inc.
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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PSI Pump Systems Inc.
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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PSI Pump Systems Inc.
VFD Basics
Simplified
Inverter
Section of a
VFD
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PSI Pump Systems Inc.
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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PSI Pump Systems Inc.
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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PSI Pump Systems Inc.
VFD Basics
Pulse-width-modulated voltage and
current waveforms.
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PSI Pump Systems Inc.
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 voltagesource, current-source of variablevoltage types. This has to do with
the form of DC that the inverter
receives from the DC bus.
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PSI Pump Systems Inc.
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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PSI Pump Systems Inc.
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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PSI Pump Systems Inc.
VFD Basics
Volts-perHertz
Control
The area within
each pulse is
the power
delivered to
the motor in
voltmicroseconds.
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PSI Pump Systems Inc.
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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PSI Pump Systems Inc.
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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PSI Pump Systems Inc.
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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PSI Pump Systems Inc.
VFD Basics
Conclusion






Significant energy savings
Easy setup & programming
Retrofits
Space
Better design
Competitive edge
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PSI Pump Systems Inc.