Dave Mellis

PSI Pump Systems Inc.

1(800)900-2220


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Variable Frequency DrivesPresented by



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The Purposeof

VFD’s


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The Purpose1. 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 correction5. Emergency bypass capability6. Protection from overload currents7. Safe Acceleration


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Electric & Power BasicsAll 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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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 BasicsSinusoidal 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 IssuesStarting 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 TheoryThe 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 TheoryAC 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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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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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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VFD Basics

A VFD in a block diagram.


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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 three-phase, full-wave-diode bridge.


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Components of VFD’s

Rectifier is that special type of converter that converts AC to DC.


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VFD BasicsDC 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 BasicsSimplified 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 BasicsInput 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 BasicsPower-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 BasicsA 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 BasicsTypes 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 BasicsThe 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 ControlThe area within each pulse is the power delivered to the motor in volt-microseconds.


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VFD BasicsHow 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 BasicsFunctions & Features

Set-up Parameters The Control Method Acceleration or Accel-Ramp

Rate Automatic Restart Stopping Method Automatic Signals


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VFD BasicsFunctions & 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 BasicsConclusion

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

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Dave Mellis