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PDHeng ineer . com Course E-3014

Variable Frequency Drives

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Variable Frequency Drives

Robert J. Scoff, PE, PA, OH, TN, & MS

Copyright November 2010, Robert J. Scoff

Copyright November 2010 Robert J. Scoff 1

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Table of Contents Page

1. Introduction 42. Definitions 4

2.1 Voltage 4-5

2.2 DC Voltage 52.3 AC Voltage 5

2.4 Current 52.5 Resistance 52.6 Ohms Law 5-6

2.7 Energy and Power 6

2.8 RMS 6-7

2.9 Capacitors and Capacitance 72.10 Inductors and Inductance 7

2.11 Diodes 7-8

2.12 Single Phase Full Wave Bridge Rectifier 82.13 Three Phase Full Wave Bridge Rectifier 9-11

2.14 Bi Polar Transistors 11

2.15 Field Effect Transistors 122.16 Insulated Gate Bi Polar Transistors (IGBT) 12-13

2.17 Power Control With One IGBT 13

2.18 Full Wave Single Phase Bridge DC to AC Converter 13-152.19 Full Wave Three Phase Bridge DC to AC Converter 15-16

2.20 Contactors 16-17

3. Inverters How They Work 17

3.1 Block Diagram 173.1.1 The Input Full Wave Bridge Rectifier 17

3.1.2 The Surge Suppressor Resistor/Contactor 18

3.1.3 The Filtering Capacitor 18

3.1.4 The Three Phase Full Wave IGBT Output Circuit 19-203.1.5 Internal Power Supply of an Inverter 20-21

3.1.6 Inverter Control Circuit 213.1.7 Inputs and Outputs 21-22

3.1.8 The Microprocessor 22

3.1.9 What VFDs Look Like on the Outside 23

3.1.10 Some Other Considerations 23-244. Vector Drives 24-25

5. Changes to the Input Circuit 25

6. Conclusion 25


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List of Illustrations Page

2.1 Simple Circuit Showing Ohms Law 52.2 Sine Wave With Peak Voltage Value of 1 7

2.3 Symbol for Solid State Diode Showing Allowed Direction of Current Flow 7

2.4 Diagram Showing the Effect of Reversing a Diode in a Simple One Resistor Circuit 82.5 Half Wave Rectifier Showing Waveforms of Input and Output Voltages 8

2.6 Diagram Showing the Operation of a Single Phase Full Wave Bridge Rectifier 82.7 Another Way to See a Single Phase Full Wave Bridge Rectifier 92.8 A Three Phase Full Wave Bridge Rectifier 9

2.9 Waveforms of the Three Voltages In a Three Phase Source 10

2.10 Graph Showing Full Wave Rectified Waveforms of the Three Phases of Circuit of Figure 2.8 10

2.11 Graph Showing Actual Full Wave Three Phase Bridge Output 112.12 Diagram Showing Operation of a Bi Polar Transistor 11

2.13 Symbol for JFET or Junction Field Effect Transistor 12

2.14 Symbol for Power FET or Insulated Gate FET 122.15 Symbol for Insulated Gate Bi Polar Transistor (IGBT) 12

2.16 IGBT Sown Amplifying a Voltage Pulse 13

2.17 Full Wave Single Phase DC to AC Converter 142.18 Input and Output Voltages for the Circuit of Figure 2.17 15

2.19 Three Phase Full Wave DC to AC Converter 15

2.20 Output of the Three Phase Full Wave DC to AC Converter 162.21 Symbol for a Contactor or Power Relay 17

3.1 Block Diagram of a Typical Inverter 17

3.2 480 VAC RMS Sine Wave Showing Peak Value of 680 Volts 17

3.3 Filtering and Protection Circuit Found In Many Inverters 183.4 Typical Single Phase PWM Signal 19

3.5 Sine Wave Approximation to a PWM Signal 19

3.6 Pulse Wave Modulated Waveform of a Single Phase of a Three Phase Voltage and

The Line Current for a Wye Connected Load 203.7 Block Diagram of the Typical Power Supply for Many Inverters 21

3.8 Potentiometer Used for Speed Control 223.9 Block Diagram for a Typical Variable Frequency Drive System 23

3.10 Circuit Showing Addition of Regen Resistor to Absorb Energy from Overhauling load 24


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1. Introduction

In todays industrial and commercial world, variable frequency drives (also known as VFDs, Frequency

Drives, AC motor controllers, and Inverters) are becoming a more and more important method of speed

control. They are used for all but the most demanding speed control applications. Large rolling mills are

among the few places where DC motors and controls are still used. One of the reasons for this is that ACmotors and their associated controls are much more reliable than the DC motors and the associated controls

that to a large extent they have replaced. According to the Rockwell Automation website, they make VFDsto control to 30,000 horsepower. I put the comma in there just so there would be no confusion. Thewebsite really says 30,000 horsepower.

Lets give a brief history of VFDs. The first inverters were made in the 1960s. They had a rather limited

application due to the small size and reliability of the solid state devices of the day. When higher powertransistors became widely available in the 1980s, larger inverters were made and many more applications

opened up. Reliability and Mean Time Between Failures (MTBF) was still a problem. All of these earlier

devices used linear amplifiers and controls for their basic operation. Small potentiometers and dip switcheswere used to set their operating characteristics. In the 1990s digital controls began to be used more and

more in Inverters. Solid state devices were also developed that allowed higher voltage and current ratings.

This made it possible for inverters to be used on larger motors. Micro processors have also made theInverter a much more versatile device. In the last decade or so they have become much more flexible and

reliable. For many applications, the Inverter can be removed from a packing box, wired to a motor, and

turned on and operated without additional set up. Of course, for some applications, additional effort isneeded to program and tune a drive to the application.

The motors that are usually controlled by VFDs are induction motors. A three phase induction motor is

one of the simplest power conversion devices ever made. It has one moving part. Of course, if the motorhas ball bearings, and we call ball bearings moving parts, then an induction motor with ball bearings does

have more than one moving part. In any case, they are very simple, and hence very reliable. They have a

winding on the stator, or part that stands still, and a winding on the rotor, or the part that turns. When

voltage is applied to the stator, a voltage is induced (Hence induction motor) in the stator coil. This causesa current to flow in both the stator and the rotor. The design of the motor is such that the magnetic fields of

the two currents act against each other to cause a force on the rotor and make it rotate. The designers ofthese motors have done an excellent job in making motors with a very high efficiency and power factor.

Efficiencies of over 90 % and power factors of over 80 % are common at full load. Some larger motors

have power factors of up to 90 % when fully loaded. However, lightly loaded AC induction motors typically

have low efficiency and low power factor.

To really understand VFDs, a review of some of the concepts that apply to inverters will be given. Also, a

description of some of the components that are a part of most Inverters will be given.

2. Definitions

2.1 Voltage

Voltage is similar to the hydraulic pressure that exists in water and hydraulic oil systems. This pressure canexist without anything happening. Consider a hydraulic oil pump turning and generating pressure. Nothing

will happen unless or until a valve is turned on to allow the oil pressure to move something, such as a

hydraulic cylinder. Electrical pressure, or voltage, is similar. This voltage, or pressure, can exist and not doanything. Consider a simple battery. It has voltage across the terminals, but nothing happens unless the


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battery is put in your i-pod, or whatever else you desire to make work. One important thing to notice here is

that both terminals of the battery, or voltage source, have to be connected. This will be explained when we

get to current.

2.2 DC voltage

DC voltages have pressure in only one direction. Batteries are examples of DC voltage sources.

2.3 AC Voltage

AC voltages periodically change direction. Most of the electrical power generated in the world is in the form

of a sine wave. This is because the circular design of electrical generators naturally generates a sine wave. It

would be difficult to do anything else. The voltage changes polarity in a periodic fashion, going positive andnegative, just like a sine wave generated by a mathematical expression.

2.4 Current

We can think ofcurrent as the flow of electricity. Even though you cant see it like water flowing, it does

flow nevertheless. There is a difference between turning on a water valve and seeing the water flow out of a

pipe and electricity flowing. Electricity has to return to something, such as the source. Water can just flowout of the pipe and down the drain. Even though you cant see electricity flow, under the right conditions it

can be felt.

2.5 Resistance

There is a property of materials called electrical resistance. When a material called a conductor is placedacross a voltage source, current can flow. This resistance limits the current flow. Resistors are mentioned

here because some of the circuits of an inverter use resistors. The unit of resistance is the Ohm.

2.6 Ohms Law

Ohms Law is perhaps the most important principle in the electrical field. It is very simply stated as:

E = I * R

Equation 2.1

This simple equation has given more beginning electrical students more trouble over the years than almostanything else. Here is a simple circuit illustrating Ohms Law.

E

I

R

Figure 2.1 Simple Circuit Showing Ohms Law


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Notice in Figure 2.1, that the voltage arrow points toward the positive end of the voltage source and the

current arrow points in the direction of conventional current flow, or from plus to minus. If we always keep

that in mind, understanding how electrical stuff works becomes easier.

2.7 Energy and Power

Almost everything that works in the electrical field concerns energy and power. Energy is the ability to do

work. Consider a pound brick, held 1 foot off of the ground. That brick has a potential energy of about 1joule or ft-pounds. Its not doing anything, just being held there in the air. This is called potential energy.

Now if this brick falls to the ground in one second, it has power during that one second. As a matter of fact,the power of the brick during that one second is one watt. The definition of a watt is a joule per second.

Another way of saying this is that power is the rate of doing work. Power is a very active thing. Now

looking at figure 2.1, we can say that resistor R is dissipating power, or getting hot. The relationship forpower (and this is the second important equation in the electrical world) is:

P = E * I

Equation 2.2

With these two expressions, E = I * R and P = E * I, many of the electrical problems that the world presents

can be solved.

2.8 RMS

In understanding how Inverters work, it is important to at least be introduced to the concept of RMS voltages

and currents. RMS comes from Root Mean Square, and it is sometimes called the effective value. To getRMS, what we do is take the square root of the mean of the squares of whatever voltage or current that we

are concerned with. When we talk about the 120 VAC that is in our homes, the 120 volts is the RMS valueof the waveform. If a DC voltage of a certain value is placed across a resistor (causing it to dissipate heat) it

gives off heat. If an equal RMS voltage is placed across the same resistor, it gives off the same amount ofheat. For practical sine waves, the following equation is all we need to know:

VRMS = 0.707 * Vpeak

Equation 2.3

For the waveform shown in Figure 2.2, the peak value is 1, and the RMS value is 0.707 volts. That meansthat a one ohm resistor placed across a voltage with that waveform will have 0.707 amps RMS flowing

through it, and dissipate 0.5 watts. Show that this is true using E = I * R and P = E * I

T

0

T

[Vpeak * f (t)]2dtRMS = Volts

Equation 2.3A Mathematical Expression for

Determining the RMS Value of Any Waveform

I have shown the expression for finding the RMS value of any waveform. An easy way to check this is to let

Vpeakbe equal to 1, f(t) = sin (2 f) t, the time for one cycle be 2 , which makes f = 1/(2 ), and T = 2 . Ifyou do that, RMS will equal 1 /2 or 0.707.


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Figure 2.4 Diagram Showing the Effect of Reversing a Diode in a Simple One Resistor Circuit

If the DC voltage source in Figure 2.4 is replaced with an AC source an interesting thing happens. Theoutput voltage turns into a pulsating DC voltage as shown in figure 2.5. It is a DC output voltage because it

never goes below 0. It does have a lot of ripple. This circuit is called a half wave rectifier. Notice that the

peak output voltage is shown as equal to the peak value of the input voltage. This can be done since the VFD,

the forward voltage drop of the diode is small compared to the peak voltage of VAC for the typical inverter.

Figure 2.5 Half Wave Rectifier Showing Waveform of Input and Output Voltages

2.12 Single Phase Full Wave Bridge Rectifier

A full wave bridge rectifier uses 4 diodes in what is called a Bridge Circuit. It turns both the positive half

of a sine wave and the negative half of that same sine wave into positive voltages. Look at Figure 2.6 to see

how a Single Phase Full Wave Bridge Rectifier works.

RVAC

+-

Figure 2.6 Drawing Showing the Operation of a Single Phase Full Wave Rectifier

Notice that when the input waveform is positive (in red), the current flows through the resistor from right to

left. Also notice that when the input waveform is negative (in blue), the current also flows through theresistor from right to left. Since the current is always flowing through the resistor in the same direction, the

polarity of the output voltage is always the same. I have shown the right hand side of the resistor as positive.

And with this particular circuit, drawn as it is drawn, it will always be positive.


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2.13 Three Phase Full Wave Bridge Rectifier

One way to look at a three phase full wave bridge rectifier is to consider the three phase voltage to be threesingle phase voltages. The three phase full wave bridge rectifier only contains six diodes. It works because

each phase uses four of the six diodes, but not the same four. The way to see this is to look at another

diagram. I will first take the single phase full wave bridge rectifier and draw it a little differently, as shown

in Figure 2.7.

Figure 2.7 Another Way to See a Single Phase Full Wave Bridge Rectifier

Look at Figure 2.7 carefully and you will see that it is the same as Figure 2.6 with the diodes arranged a little

differently. The input voltage is still a sine wave and the output is still a time varying DC voltage. The next

thing that we are going to look at is a full wave three phase bridge rectifier.

Figure 2.8 A Three Phase Full Wave Bridge Rectifier

The way to think about this circuit is to look at the three voltage sources as independent of each other. This

is much easier to see if a Delta three phase source is used. Voltage source 1 is only connected to diodes

A,B,D, and E. Voltage source 2 is only connected to diodes A, C, D, and F. Voltage source 3 is onlyconnected to diodes B, C, E, and F. What we really have is three single phase circuits. The only thing

needed to make this circuit make sense now is to look at voltage sources 1, 2, and 3 independently of each

other. Figure 2.9 shows the waveforms of these three voltages.


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2.15 Field Effect Transistors

Field effect transistors come in two basic varieties, Junction FETs and Power FETs. The symbol for ajunction field effect transistor, or JFET is shown below in Figure 2.13.

Figure 2.13 Symbol for JFET or Junction Field Effect Transistor

Generally JFETs have not had a high enough current rating to be used for motor control, and are presented

here for information purposes only. They are devices where an input voltage on the gate controls a current

flow through the Drain Source channel. The connection of the gate to the drain source channel is actuallya reverse biased diode. This means that the input impedance is very high. A disadvantage of the JFET for

Inverter applications is that the Drain Source resistance is relatively high. This results in high power losses.

The Power FET on the other hand has an insulated gate. The symbol for a Power FET is shown in Figure2.14.

Figure 2.14 Symbol for Power FET or Insulated Gate FET

Power FETs have a high enough current rating to be used for motor control, but most of the present state ofthe art VFDs use another type of transistor called the Insulated Gate Bi Polar Transistor (IGBT).

2.16 Insulated Gate Bi Polar Transistors (IGBT)

IGBTs work almost like regular Bi Polar transistors, except that the gate is insulated from the rest of the

transistor, as in power FETs, and a voltage controls current flow through the transistor. Figure 2.15 showsa symbol for an Insulated Gate Bi Polar Transistor.

B

C

E

Figure 2.15 Symbol for IGBT


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The IGBT is the device used on most of the low and medium power inverters made today. They have a high

enough switching speed to work with todays technologies, and a high collector emitter breakdown voltage.

They also have a high enough current rating, and relatively low power losses when used in switchingapplications such as Inverters.

2.17 Power control with one IGBT

An IGBT can be used to control the current flow in a load. To show this we will use a resistor load andshow an input voltage controlling an output current that will show up as a voltage drop across the resistor.

The IGBT still controls current, even though we see a voltage across the resistor, R. Look at Figure 2.16to see this.

B

C

E

+ VDC

0

1

0

VDC

R

ICE

Figure 2.16 IBGT Shown Amplifying a Voltage Pulse

Something to notice here is that the input voltage pulse has very little current because the base is an open

circuit. The resistor, R, however has a current flow limited only by VDC, VCE and R, and the current rating ofthe transistor. This current can be a hundred amps or more for current day transistors. The voltage can be

1000 volts or more. It thus is capable of controlling a lot of power. Remember P = E * I. We could almost

use the circuit of Figure 2.16 to run a small DC motor. The reason that the IGBT transistors have low losses

is that when the transistor is on, the voltage drop across it is almost 0, and when it is off the current

through it is almost 0. Since P = E * I, if one of the two parts of the expression is almost 0, the product is

also almost 0. This makes the power controlling devices smaller, because they dont have to dissipate a lot

of power compared to the power that they are controlling.

2.18 Full Wave Single Phase Bridge DC to AC Converter

Next, lets make a full wave bridge circuit using 4 IGBT transistors, and turn a DC voltage into a single

phase AC voltage. For simplicity, we will generate a square wave. Lets look at Figure 2.17


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B

C

E

B

C

E

B

C

E

B

C

E-

+

1 2

3 4

R

Vout

VDC

Figure 2.17 Full Wave Single Phase DC to AC Converter

Notice the similarity between circuits 2.7 and 2.17. In Figure 2.7, we are converting AC to DC. In

Figure 2.17, we are converting DC into AC. Now lets see how we can make this circuit work. The VDCis some value of voltage, called the bus voltage. The numbers within the small circles, 1, 2, 3, and 4 are

connected to the bases of their respective transistors and also to a control circuit that does nothing other that

make the 4 terminals 0 volts, which turns the transistor off, or some positive voltage high enough to turn thetransistor on, which makes the transistor a short circuit or conductor of current.

If the transistor leads numbered 1 and 4 are turned on, current will flow downward through transistor 1,

downward through the resistor R, and downward through the transistor 4, as shown in blue. This will cause

a voltage to appear across R with the top of R being positive.

If transistor leads numbered 2 and 3 are turned on, current will flow downward through transistor 2, upward

through resistor R, and downward through transistor 3, as shown in red. This will cause a voltage to appear

across R, with the bottom of R being positive, which makes the top negative. Lets look at Figure 2.18 to seeinput and out put voltages for the above scenario.

Note that if we turn on transistors 1 and 3 at the same time, a short circuit results. This will either blow afuse, or destroy the transistors, maybe both. The same thing is true if we turn transistors 2 and 4 at the same

time. The programmers who actually make the above circuit work have to be very careful of the timing of

the control signals. When 6 transistors get involved, as in a full wave bridge, even more care has to beexercised.


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Figure 2.18 Input and Output Voltages for Circuit of Figure 2.17

What has happened is that an AC output voltage has been generated from a DC input voltage. Of course,the output is a square wave, but there are ways to make the output an approximate sine wave. What I wanted

to show here was the concept and how it can really work. The control circuit that turns the transistors off

and on is usually derived from a micro processor controller. This makes it easier to keep track of the timescale so that the frequency of the square wave that we generated can be changed. If we do change the

frequency of the output voltage we have made a variable frequency generator. This is pretty close to a

Variable Frequency Drive.

2.19 Full Wave Three Phase Bridge DC to AC Converter

B

C

E

B

C

E

B

C

E

B

C

E

B

C

E

B

C

E

1 2 3

4 5 6

RA

RB

RC

VDC

Figure 2.19 Three Phase Full Wave DC to AC converter


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To understand the operation of the three phase DC to AC converter, we only need to look at 4 transistors at

a time, connected to one load at a time. I used a Delta load because it is easier to see this concept than with a

Wye load. It can be shown that any Wye load has an equivalent Delta load, and vice versa. First observethat RA is connected to transistor numbers 1, 2, 4, and 5. Lets let the four transistors be turned off and on

just like the single phase example in Figure 2.17. A square wave output across RA, like Figure 2.18, would

result. The next thing to notice is the angles at which the other two loads (RB & RC) are drawn. RB is drawn

at 120o, and RC is drawn at 240

o. We can say that one electrical cycle takes 360

o, and use transistors

number 1, 3, 4, and 6 to also generate a square wave, but have that square wave start at 120 electricaldegrees. That voltage will be across RB. Then, use transistor numbers 2, 3, 5, and 6 to generate another

square wave, except that square wave will start at 240o. Without further explanation, Figure 2.20 will show

the three phase square wave output voltage of the DC to AC converter.

Figure 2.20 Output of the Three Phase DC to AC Converter

I have offset the black waveform up a small amount and the red waveform down a small amount so that theplaces where the waveforms are the same voltage at the same time would not cover each other up. Notice

that the blue waveform, representing the voltage across RA starts at 0 degrees, is positive for 180 degrees,

then goes negative for 180 degrees. The red waveform, representing the voltage across RB is negative for thefirst 120 degrees, goes positive and then stays there for 180 degrees, before going negative for the rest of the

cycle. The black waveform, representing the voltage across RC, is positive for the first 60 degrees, goes

negative for 180 degrees, and then go positive at 240 degrees for the remaining 120 degrees. This is verysimilar to Figure 2.9. Figure 2.9 is a three phase sine wave, where Figure 2.20 is a three phase square wave.

If the voltage and current were high enough, this would run a three phase induction motor.

2.20 Contactors

A contactor is a relay rated for higher currents. It consists of a coil and one or more contacts. When

voltage is applied to the coil of a contactor, its contact changes state. Figure 2.16 shows a symbol for acontactor. Notice that the coil and the contact are both identified by the same symbol, in this case CR.


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Figure 2.21 Symbol for a Contactor or Power Relay

3. Inverters - How They Work

Figure 3.1 Block Diagram of a Typical Inverter

3.1 Block Diagram

Figure 3.1 shows the four major power parts of a typical inverter. They are the input full wave bridgerectifier, the surge suppressor resistor/contactor, the filter capacitor, and the full wave bridge IGBT transistor

output.

3.1.1 The Input Full Wave Bridge Rectifier

Now that we have looked at the parts of an inverter, lets see how they all work together. Look at Figure 3.1and follow this through. First, the three phase full wave bridge takes the three phase input voltage and full

wave rectifies it. If the input voltage happens to be 480 VAC - RMS, the DC voltage out of the three phase

rectifier will be about 680 VDC. This is because the peak voltage of a RMS rated sine wave is the RMSvoltage times the square root of two. Refer to Figure 3.2 to see this.

480 VAC RMS Sine Wave

-800

-600

-400

-200

0

200

400

600

800

0 50 100 150 200 250 300 350 400

Electrical Degrees

Volts

Figure 3.2 480 VAC RMS Sine Wave Showing Peak Value of 680 VDC


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Refer to Figures 2.10 and 2.11 to see how a three phase voltage would give a pretty good DC output voltage

with a peak value of close to 680 VDC. Most inverters have a warning label saying DANGER DO NOTTOUCH HIGH VOLTAGE. This is the reason. That 680 volts is really there on a 480 volt input

inverter. For a 240 VAC input inverter, this voltage is only about 340 VDC. That is still high enough, and

with enough charge behind it, to hurt. The inverters are designed so that the high voltage will dissipate in 2

or 3 minutes after power is removed. In any case, read the manual or the warning label to see how much

time is necessary to allow the charge to dissipate.

3.1.2 The Surge Suppressor Resistor/Contactor

Referring to Figure 3.1 again, notice that there is a contactor, CR, whose coil is across the 480 VAC input

line. Most inverters have a variation of this circuit. Its function is to simply short the resistor, R, shortly

after power is turned on to the inverter. This limits the initial current trying to charge capacitor, C, whenpower is applied. Without this time delay circuit (it really is a time delay that depends upon the inertia of the

relay parts) the inrush current trying to charge capacitor C could cause the full wave diode bridge to burn

out. After the relay contact closes, the output of the full wave diode bridge is connected to the capacitor.

The capacitor smoothes out most of the ripple that exists in the DC waveform. Sometimes an inductor isplaced in series with the resistor to give better filtering. The circuit would look like Figure 3.3.

L

CR

R

Figure 3.3 Filtering and Protection Circuit Found in Many Inverters

A very common problem that occurs in inverters is that the coil of the relay, CR, burns out. Shortly

afterward, the resistor, R, releases some smoke and quits working. Now the inverter simply stops. No

further damage to the inverter occurs. However, there might be some process damage, especially if theInverter is running an important motor. The DC voltage across the diode bridge will be there, but the DC

voltage across the capacitor will not be there. Use a voltmeter to test for this voltage. In emergencies, when

the right parts are not available, a contactor and an approximately properly sized resistor can be mounted inor near the inverter and connected to the right terminals. This will get the inverter up and running again.

3.1.3 The Filter Capacitor

The purpose ofthe filter capacitor is to make the DC bus voltage more nearly a non varying DC voltage. It

acts like a shock absorber, taking the variations or ripple out of the waveform shown in Figure 2.11. Without

the filter capacitor, the ripple would be about 100 volts on a 480 VAC system. High voltage on thewaveform would be 480 times the square root of 2 or 678 volts (most times it is approximated to 680 volts).

The low voltage on the waveform would be 0.866 times the high voltage, or 587 volts. To get the 0.866

multiplier, look at the waveforms in Figure 2.11 and see that each half wave bump in the waveform is 60degrees wide. One half of 60 degrees is 30 degrees, and so the bumps are 30 degrees on each side of the

maximum voltage. This makes the angle from the zero crossing to the peak 60 degrees. The sine of 60

degrees is 0.866, which is the multiplier that was used. This gives 91 volts of ripple. Good filtering wouldbring that down to less than 10 volts of ripple. A nice DC voltage at this point in the circuit makes it

possible for the output circuit to generate a better output voltage. This capacitor is also the part that holds the

charge when the inverter is turned off.


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3.1.4 The Three Phase Full Wave IGBT Output Circuit

This is the part of the inverter that actually makes the three phase output. Up to now all that we talked aboutwas a square wave output from the inverter. At one time that is what actually happened. State of the art

inverters use a technology called Pulse Width Modulation. This technology is able to be applied because

the IGBT transistors have the right characteristics to make it work. These include fairly fast switching

speeds (able to run at 20,000 Hertz), high current handling capability, and high voltage ratings. Lets take a

look at a PWM (Pulse Width Modulation) signal to get an idea of what it is and how it can generate anapproximate sine wave of current to feed the motor being controlled. Figure 3.4 is a picture of a typical

single phase PWM signal.

Figure 3.4 Typical Single Phase PWM Signal

The red markers are there only to indicate the center of each pulse. This shows that the frequency of thepulses doesnt change. The width, however, does. Since the average value of a waveform is the area under

the waveform divided by the time of the waveform, we can see that the average value of the PWM signal

shown above is smaller at the ends and higher in the middle. By proper choosing of the width of the pulses,

a sine wave can be approximated. Notice that the pulse is there, even though its width can be very small, andmay even approach zero width. If the above PWM signal is a sine wave it only gives the positive half of the

sine wave. The signal in Figure 3.4 can be generated by turning transistors 1 and 4 off and on at the right

times. To get the negative half of the waveform, it is only necessary to turn transistors 2 and 3 off and on atthe right times. Note that when transistors 1 and 4 are generating the positive half of the waveform,

transistors 2 and 3 are off. And when transistors 2 and 3 are generating the pulses of the negative half of the

waveform, transistors 1 and 4 are off.

Figure 3.5 Sine Wave Approximation to a PWM Signal

Figure 3.5 shows a sine wave approximation to a single phase PWM signal. The PWM carrier frequency

(the base frequency of the waveform) would be much higher in practice than the example shown in Figures3.4 and 3.5. Typical inverters use carrier frequencies of 2000 Hertz to 16000 Hertz. At the standard

minimum frequency of 2000 Hertz PWM carrier frequency, a full sine wave at 60 cycles would consist of

about 33 variable width pulses, or 16 & variable width pulses per half wave.


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Some real advantages to using PWM as the voltage being fed to a motor include the following:

1. When the IGBT transistors are off they dissipate little power because the current through them isclose to zero. Remember P = E * I.

2. When the IGBT transistors are on they dissipate little power because the voltage drop across them

is close to zero. Remember P = E * I.

3. The approximate sine wave of voltage gives an approximation of a sine wave of current to the

motor, and current is what gives the motor torque.

4. The induction motor is designed to run with a sine wave of current. 5. It is easy to change the frequency of the PWM signal to enable the AC induction motor to run at a

variable RPM (revolutions per minute).

6. At low RPM, it is necessary to lower the voltage to the motor. PWM enables this to be possibleby making the pluses narrower. This has the effect of making the average voltage less.

7. As the frequency of the PWM signal increases, the motor voltage can be increased at the same

time. This is called volts per hertz control.

Up to now we have concentrated on single phase power output. The pulse width modulation idea can be

extended to include the six transistors of a three phase full wave IGBT bridge circuit. The on and off timesof the six transistors is determined by a computer algorithm. But the same principles apply. We just need to

work with the transistors 4 at a time to generate a three phase voltage which will run the three phase motor.Because of the fact that the three phases of the three phase voltage waveform occur simultaneously (refer to

Figure 2.9), a single phase of the PWM output looks a little strange. The voltage and current waveformsshown in Figure 3.6 were copied from the public domain on the internet. I have personally measured that

waveform on an operating inverter, and it really does look like that. The line to neutral voltage is shown

instead of the line to line voltage because a three phase induction motor is usually wound as a Wyeconfiguration.

Figure 3.6 Pulse Width Modulated Waveform of a Single Phase Line to

Neutral Voltage and the Line Current for a Wye Connected Load

3.1.5 The Internal Power Supply of an Inverter

There are two parts of the inverter that we havent mentioned yet. The first is a power supply to run theelectronics that are necessary to control the output stage. This power supply is usually a special circuit called

a DC to DC converter. It uses the DC bus voltage (680 VDC in a 480 VAC unit) to run an oscillator

connected to a transformer. The oscillator runs at a relatively high frequency, 10,000 Hertz or higher. The

high frequency allows the transformer to be smaller. The secondary of the transformer is a low voltage,


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which is rectified and filtered and regulated to run the microprocessor and other electronics that are needed

to control the inverter. A block diagram of this circuit is shown in Figure 3.7.

When these power supplies fail, and they sometimes do, the usual symptom is that the screen that many

inverters have does not light up. And the inverter doesnt work at all. At one time it was possible to repair

these power supplies. But, with smaller and smaller components in the newer inverters, its usually easier to

just replace the device or the power supply module if there is one.

Figure 3.7 Block Diagram of the Typical Power Supply for Many Inverters.

3.1.6 The Control Circuit

The last part of the inverter is the control circuit that generates the pulses that turn the IGBT transistors off

and on. We are now going to tackle that.

The control circuit has inputs and outputs. The inputs include start and stop pushbuttons, a reversing switch,

a way to set the speed, and some way to program the device. The outputs include the signals to the sixtransistors and signal lights and a display. Lets look at some of these, one at a time.

3.1.7 Inputs and Outputs

A. StartThis is usually a pushbutton or selector switch. It sends a signal to the inverter control

board (a small special purpose micro processor) to generate the pulses to make a PWMvoltage to drive a motor. Push buttons and selector switches are sometimes called digital

inputs. They can be external or part of the keyboard on the VFD.

B. StopThis is like the start pushbutton or it can be part of the selector switch. It is a digital input

whose function is to get the inverter to slow down and stop.

C. ReverseThis can be a push button or selector switch whose function is to cause a motor to reverse

direction. This is done entirely with the electronics of the Inverter. We havent explainedphase sequence. But if we look at Figure 2.9, we can see that the phase sequence is Red,

Blue, Black. Think of when the voltage goes through zero in the positive direction. If the

three leads of a three phase induction motor are connected to the red, blue, black voltage

terminals, the motor would run in a certain direction. If we interchanged any two of the threeleads on the motor, it would run in the opposite direction. With an inverter, we dont have to

move wires around. It is only necessary to use the program of the Inverter to generate the

opposite phase sequence. The reverse push button causes the inverter to slow down, stop,


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and then start moving in the opposite direction. Think of it as going from red, blue, black

sequence to red, black, blue sequence.

D. Speed PotentiometerThe speed input is an analog input. That means it is a continuously variable voltage or

current. The potentiometer is a resistor with a variable tap. That means that it can be used as

a voltage divider to give a variable signal to the Inverter to tell it how fast or slow to go.

Figure 3.5 shows a typical speed control input.

Figure 3.8 Potentiometer Used for Speed Control

+10

VDC

0-10

VDCR

E. Run, Stop, Fault, and Directional LightsMany inverters have lights to indicate operational conditions. These are usually stopped,

running, reverse and fault.

F. Tachometer InputMost inverters have an input for an external tachometer or encoder. The function of the

tachometer or encoder is to give a signal to the drive telling it how fast the motor is going. Atachometer has a DC output, while an encoder has an AC output. Both devices are able to tell

direction of rotation and speed. Encoders are more accurate. Then the Inverter generates an

output signal to make the motor go whatever speed the potentiometer is set for.

G. Programming KeypadThere are many special functions that can be programmed on almost every inverter made

today. These include, but are not limited to acceleration and de-acceleration time, current

limit, speed limit, carrier frequency setting, fly-catching, and many others. The only wayto understand them is to work with a specific Inverter and follow the instruction manual for

your specific inverter.

3.1.8 The Micro-Processor

The microprocessor or control board of an inverter is a small special purpose computer. It has input signalscoming from the various input devices, and output signals going to the 6 IGBT transistors and the keyboard

and lights. The outputs are relatively easy to control. However, the 6 IGBT transistors each have to be

controlled in such a fashion to generate 3 sine waves of voltage that are 120 degrees out of phase with each

other. Something to notice is that there are always some transistors that are a part of two circuits at the sametime. If the timing isnt perfect, a short circuit could result in the 6 transistor circuit.


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3.1.9 What VFDs Look Like on the Outside

Now that weve looked what goes on inside of a VFD, lets look at what it takes to connect one and make itoperate. Figure 3.6 shows a block diagram of a typical variable frequency drive connected to a motor with

tachometer feedback. The tachometer feedback is not necessary for many applications. Also, the tachometer

can be either a DC tachometer or an encoder. An encoder gives a variable frequency square wave to tell how

fast the motor is moving, instead of a variable DC voltage. For precision speed control, an encoder is better.

The digital inputs are the start stop circuit and a reverse switch. The speed control input is connected to a

potentiometer. The display can be programmed to show several different operating conditions. Theseinclude output frequency, output voltage, output current, DC bus voltage, and several others including fault

conditions.

+10

VDC

T

3 Phase Power

to Motor

Tachometer Feedback

Signal

Motor

VFD

Controller

. ...

Display

StartStop

Run Stop FaultRev

Rev

3 Phase

Input

Power

Speed

Pot

Figure 3.9 Block Diagram of a Typical Variable Frequency Drive System

3.1.10 Some Other Considerations

One of the things that can happen to an induction motor when it is operating is that the load can attempt topush the motor (this is sometimes called an overhauling load). In this case the motor starts operating like

a generator. Before VFDs this was not a big problem. The motor just acted like a generator, and put

energy back into the incoming AC line. If the line was soft (poor voltage regulation), the voltage tended torise, and could, in fact, cause other electronic equipment to shut down. If the line had good voltage

regulation, it just absorbed the extra energy, and things kept running as before. With VFDs an entirely new

problem occurs. When the load becomes overhauling, the inverter will absorb the extra energy that the

motor, now acting like a generator, puts out. A problem occurs because the DC bus voltage will rise.Thats where the extra energy goes. If you look at the block diagram, theres really no place else for it to go.

Figure 3.10 shows the solution developed for this problem.


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OverhaulingMotor

G

Regen

Resistor

C 1 2 3

4 5 6

Figure 3.10 Circuit Showing Addition of Regen Resistor to

Absorb Energy from an Overhauling load

Now when the load overhauls the motor and makes it act like a generator, the voltage on the DC bus rises,

but a voltage detector reads this voltage and turns the transistor, G, on enough to take the energy out of the

DC bus and pull the DC bus voltage down. Notice that the transistor, G, is shown as a Bi Polar Transistor.

IGBTs may also be used here. What is needed is a linear device that can be neither fully off nor fully on.

Of course, when the motor is not overhauling, the transistor, G, is fully off, because we do not want totake energy from the DC bus when it is not necessary. In practice, the drive is programmed to not allow the

DC bus voltage to go above 720 VDC for a 480 VAC input voltage.

Another problem that occurs with AC drives that did not occur before AC drives were used is catching arunning load. An old across the line starter would just turn on and force the motor to do its bidding. Thiswasnt always good for the system, but it usually worked. Figure 3.10 can be used again to illustrate fly

catching. What we have to do is pretend that the motor is being kept running by an overhauling load, but the

inverter is turned off. If we turn the inverter on at that time, there could be a large difference in the

frequency that the drive is putting out (remember that the inverter starts at a low frequency) and thefrequency that the motor, acting like a generator is putting out. This causes an over current or over bus

voltage trip. The solution is to detect the frequency that the overhauling motor is running at, and make thecontrol start at that frequency. This is called fly catching. This is sometimes useful in starting pumps thatare started under a head pressure.

Sometimes it is desirable to change the carrier frequency of the PWM signal. Most inverters allow this tobe done. The reason for changing the PWM frequency is that a system resonance might occur. Also, the

motor might be making a louder than normal noise. These problems are sometimes alleviated by changingthe carrier frequency. The usual frequency range is 2000 to 16000 Hertz.

4. Vector Drives

From the physical standpoint, there is very little difference between regular Inverters and Vector Drives. Allof the same components are used, but the programming is significantly different. The micro processor in a

vector drive needs to do a lot more than the processor of a regular inverter. Also, encoder feedback isrequired to fully utilize a Vector Drives capabilities. When using a Vector Drive some of the motor

characteristics are needed to be known and used as input data to the Vector Drive. With this in mind, lets

see what a Vector Drive does differently, and how it does it.

A standard Inverter using Pulse Width Modulation (PWM) takes a speed reference signal and generates

signals to the 6 transistor output circuit to cause the inverter to produce an output voltage and frequency


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determined by the speed reference. The motor will not actually go at that speed because a standard induction

motor will go a little slower than the frequency of the power supplied to the motor. For instance, the

synchronous speed of a 4 pole induction motor is 1800 revolutions per minute (RPM). At no load, the motorwill actually run a little slower than that, say maybe 1790 RPM. As the motor is loaded, the speed will

decrease even more, until at full load the speed might drop to 1750 RPM. So, a standard inverter might feed

60 cycles per second to an induction motor, and the speed could vary from 1790 to 1750 RPM as the load

changed. This is not much of a problem for many applications, but there are loads that require more accurate

speed regulation. Tachometer feedback will help this, but very slow speed operation is still not possible withstandard inverters.

To overcome this limitation, the vector drive was introduced. The Vector Drive separates the current

flowing to the motor into two components. One of the components is called the magnetizing current, and the

other component is called the torque producing component. The total current is then the vector sum of thetwo currents. This is where the name Vector Drive comes from. The control circuit is programmed to

always produce a PWM signal that causes the magnetizing current to flow all of the time. When a load is

added to the motor the control circuit detects a change and produces a torque producing current. The

feedback to the Vector Drive includes both speed and motor current. With these two feedbacks, it is possibleto have very low speed operation comparable to DC Drives. Because the magnetizing current is always

flowing it is possible for an AC motor controlled by a vector drive to hold position at zero speed. This is

something that could only be done previously by DC motors and servo drives.

5. Changes to the Input Circuit

A standard full wave diode bridge input circuit has a fairly good power factor, but it generates harmonic

currents that can cause difficulties. The power factor is said to be about 0.97. The harmonic currents that are

generated in operation include the 5th

, 7th

, 11th

, 13th

, and so on. There is a way of both improving powerfactor and minimizing harmonic currents by making a change to the input circuit. Instead of using six diodes

in a full wave passive bridge, six Insulated Gate Bi Polar (IGBT) transistors are used in a full wave activebridge. In other words, the input circuit can now be operated in such a way to allow the input current to

follow the input voltage almost exactly. When that happens, the power factor is made close to unity and the

harmonic currents are minimized. This again increases the complexity of the software built into the device,

but only slightly increases the hardware needed.

6. Conclusion

This course is intended to give a general idea of how AC Variable Speed Controllers work. Having an

understanding of what they really do will be a great help in applying them to whatever process that you, as

engineers, have presented to you. Now that you understand how they work, you will find applications forthem that the manufacturers have not thought of yet. Good luck and have fun with them.

Robert J. Scoff, PE

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