User manual | Eaton/Cutler-Hammer Adjustable Frequency Drives Learning Module

101 BASICS SERIES
LEARNING MODULE 20:
ADJUSTABLE FREQUENCY DRIVES
Cutler-Hammer
ADJUSTABLE FREQUENCY DRIVES
WELCOME
Welcome to Module 20, which is about adjustable frequency drives. An adjustable
frequency drive is a device used to control the speed, torque, horsepower and
direction of an AC motor.
FIGURE 1. TYPICAL ADJUSTABLE FREQUENCY DRIVES
Like the other modules in this series, this one presents small, manageable sections
of new material followed by a series of questions about that material. Study the
material carefully then answer the questions without referring back to what you’ve
just read. You are the best judge of how well you grasp the material. Review the
material as often as you think necessary. The most important thing is establishing a
solid foundation to build on as you move from topic to topic and module to module.
A Note on Font
Styles
Key points are in bold.
Viewing the
Glossary
You may view definitions of glossary items by clicking on terms and words that are
underlined and italicized in the text. You may also browse the Glossary by clicking
on the Glossary bookmark in the left-hand margin.
Glossary items are italicized and underlined the first time they appear.
1
ADJUSTABLE FREQUENCY DRIVES
WHAT YOU
WILL LEARN
We’ll step through each of these topics in detail:
Section Title
•
Importance of Motor Speed Control
•
3
3
•
What Does an Adjustable Frequency Drive Do?
4
•
Why Use an Adjustable Frequency Drive?
5
•
Typical Adjustable Frequency Drive Applications?
7
•
Review 1
8
•
How Does it Work? – Motor Concepts
9
•
•
•
Synchronous Speed
•
Horsepower and Torque
How Does it Work? – Drive Concepts
9
10
11
•
Pulse Width Modulation
11
•
Power Factor
13
•
Impedance
13
•
Harmonics
14
How Does it Work? – Operator Interface Concepts
15
•
Operator Control
15
•
Control Signals
15
•
In Summary
16
•
Review 2
17
•
Concerns With Using Adjustable Frequency Drives
18
•
2
How Can Motor Speed Be Controlled?
Page Number
•
Heat Issues
18
•
Output Filtering
19
•
Harmonics
19
Helping the Customer
20
•
Factors to Consider
20
•
Application Checklist
21
•
Recommending a Product to the Customer
23
•
Normally Specified Features
24
•
Review 3
25
•
Glossary
26
•
Review Answers
28
ADJUSTABLE FREQUENCY DRIVES
IMPORTANCE
OF MOTOR
SPEED
CONTROL
Motor speed control is important in many applications. It allows for precise
control of the application. This allows the same equipment to be used for multiple
products or processes.
Here are some examples of the uses of motor speed control:
•
Stamping Machine – pressure exerted by the stamping head.
•
Cooling Tower – temperature of the cooling source, such as water.
•
Polishing Machine – speed of the rotating polishing head.
•
Commercial Bakery – speed at which conveyors move a batch through the
oven.
As you may recall from Module 16, Basics of Motors and Motor Control, all AC
motors have a synchronous speed. This is the speed at which the motor is
designed to run.
Suppose you have a motor connected to a conveyor belt application, and you want
to slow the conveyor speed. What can you do? The motor is built to run at only one
speed. The answer is you find a way to change the motor’s speed.
How Can Motor
Speed Be
Controlled?
Many methods for controlling the speed of a motor are in use today. The list that
follows is neither detailed nor complete, but will serve to give you an idea of the
variety of methods available.
•
•
Electrical Methods – The motor speed is actually changed electrically.
•
Adjustable Voltage – changing the DC input voltage changes the DC motor’s
speed.
•
Adjustable Frequency – changing the input frequency changes the AC
motor’s speed.
•
Eddy Current – changing the strength of the magnetic field changes the AC
motor’s speed.
Mechanical Methods – The motor speed stays constant, and is converted to
the desired speed, using gears, sheaves, clutches or other mechanical means.
3
ADJUSTABLE FREQUENCY DRIVES
WHAT DOES
AN
ADJUSTABLE
FREQUENCY
DRIVE DO?
The adjustable frequency drive is an electrical device used for controlling the speed
of a motor, by which we specifically mean an AC motor.
An adjustable frequency drive (or AF drive) is a device used to convert three
phase, 60Hz input power into an adjustable frequency and voltage source. This
source is then used to power a motor.
The AF drive allows for control of the motor’s speed by controlling the frequency of
the power fed to the motor. We will see how frequency affects motor speed later in
this module.
The AF drive system is very simple. It consists of only three components:
•
AC Motor – Usually, a NEMA Design B, squirrel cage induction, 3-phase motor
•
Motor Control Section (also called the Inverter section)
•
Operator Interface
OPERATOR
INTERFACE
INVERTER
OPERATOR INTERFACE AND INVERTER SECTION
AC MOTOR
FIGURE 2. TYPICAL ADJUSTABLE FREQUENCY DRIVE COMPONENTS
The operator control allows the operator to command the motor to function as
desired, through the use of motor control inputs and outputs.
The motor control section controls the motor’s speed by converting utility power
into adjustable frequency power.
The AC motor drives the device (fan, pump, etc.) by converting the electrical power
to mechanical power.
4
ADJUSTABLE FREQUENCY DRIVES
WHY USE AN
ADJUSTABLE
FREQUENCY
DRIVE?
There are a number of excellent reasons for choosing an AF drive over other
methods for motor speed control. These include:
•
Versatility
•
Energy Savings
•
Performance
•
Reliability
•
Size
•
Future
Let’s discuss each of these reasons in a little more detail.
Versatility
An AF drive is very versatile. It provides many expanded functions that other
motor speed control methods do not offer, such as self-diagnostics, current status
display, multiple-use programmability, and more precise speed control.
Energy Savings
Cost is a natural concern for many customers. Since the AF drive allows for precise
control of the amount of energy going to the motor, cost savings on energy can
be realized. It also helps reduce peak energy demand problems in the plant by
ramping up the power drawn by the motor.
Furthermore, the circuitry in the AF drive is designed to maximize the energy put
through the unit. In other words, very little of the costly electricity fed into an AF
drive is wasted as heat.
5
ADJUSTABLE FREQUENCY DRIVES
Performance
The operator interface is simple to operate. Any required changes to the drive
settings can be made quickly by the operator, and go into effect instantly.
The AF drive responds rapidly to changes in the motor’s load, maintaining the
requested speed, even when the load change is abrupt.
The tolerance for speed variance from the operator’s setpoint is very small as well,
assuring precise operation.
Reliability
AF drives are very reliable products. Because the unit is solid state, there are no
moving parts to fail, apart from the cooling fan. In fact, the mean time between
failures has been calculated at over 90,000 hours for some AF drives.
Because the drive controls are integrated into the drive “box,” no external control
system is required. This is one less thing to maintain.
Nuisance tripping is avoided by the built-in protective circuitry. And the trip
parameters are easily adjustable.
Size
The AF drive is lighter and smaller than most other methods of motor speed
control.
Future
Continuing improvements in digital logic and microprocessor technology will mean
more features at a lower cost.
Currently, the AF drive generates a certain amount of heat. Depending on the
application, this may need to be taken into consideration when choosing an
enclosure. As the power conversion circuitry in the AF drive continues to improve,
less and less of the electricity handled by the drive will be wasted as heat. This
means drive heat generation will become less of an issue.
6
ADJUSTABLE FREQUENCY DRIVES
TYPICAL
ADJUSTABLE
FREQUENCY
DRIVE APPLICATIONS
Here are some typical adjustable frequency drive applications:
•
Conveyors, belts, chains, screws, and bulk and packaged material handlers
•
Fans, blowers, compressors and pumps
•
Machine tools, grinders, lathes and stamping presses
•
Custom machinery, labelers, packaging machines, bottle washers, wire drawing,
textiles, etc.
•
Extruders
•
Process machinery, kilns, grinders, blenders and agitators.
IN THE WORKPLACE
Using AF drives on this
beverage bottling conveyor
allows the operator to run
different sections of the
conveyor at different speeds.
The bottles may be bunched
close together for filling, and
then spread out for labeling.
For this application, two motors
and two drives would be
required. One motor would run
the filling section at a given
speed, and a second motor
would run the labeling section
slightly faster, spreading the
bottles out.
AF DRIVE
MOTOR
AF DRIVE
MOTOR
BEVERAGE BOTTLING LINE
7
ADJUSTABLE FREQUENCY DRIVES
REVIEW 1
Answer the following questions without referring to the material just presented.
Begin the next section when you are confident that you understand what you’ve
already read.
1. An adjustable frequency drive is a device used to convert standard 3 phase,
60Hz input power into an adjustable _________ and _________ source.
2. In terms of reliability, the only moving part of the AF Drive that can fail is the
_________ ____.
3. Describe how an AF drive is used with an office building’s air-conditioning
system. Why is an AF drive useful for this sort of application?
4. Think of three applications (other than those mentioned in this section) that
would benefit from the use of an AF drive.
Application #1 ______________________________________________
Description of the benefit of using an AF drive with this application:
Application #2 ______________________________________________
Description of the benefit of using an AF drive with this application:
Application #3 ______________________________________________
Description of the benefit of using an AF drive with this application:
8
ADJUSTABLE FREQUENCY DRIVES
HOW DOES IT
WORK?
Motor Concepts
So far in this training module, we have discussed what an adjustable frequency
drive is, and why it is useful. What we have yet to do is discuss how an AF drive
works. We will devote this section to discussing precisely that. To do this, we need
to define a few concepts critical to understanding how an AF drive and a motor
work.
Let’s look first at some motor concepts:
Synchronous
Speed
•
Synchronous Speed
•
Horsepower and Torque
As you may recall from Module 16, Basics of Motors and Motor Control, a motor is
designed to run at what is called its synchronous speed.
Synchronous speed is determined by the number of poles that the motor has,
and the frequency of the current being supplied to it. The equation for
determining the synchronous speed of a motor is:
N = 120f/P
Where:
N=
the synchronous speed of the motor in revolutions per minute (RPM)
f=
the frequency of the current supplied to the motor in Hertz (Hz)
P=
the number of poles the motor has
The typical frequency supplied is 60 Hertz, and the “typical” motor has 4 poles.
Plugging these numbers into the equation, we find:
N = (120 x 60) / 4 = 1800
Now you can see why the “typical” motor runs at 1800 rpm.
You can also see clearly that changing the frequency of the current has a direct
effect on the motor’s speed. Suppose we wanted to run this “typical” motor at only
1200 rpm. Calculate the frequency setting for the AF drive by solving for f.
1200 = 120f/4
4800 = 120f
40 = f
So, you would have to adjust the frequency of the current to 40 Hz to run the motor
at 1200 rpm.
9
ADJUSTABLE FREQUENCY DRIVES
Horsepower and
Torque
Horsepower is the unit of measurement for the amount of work a motor can do.
All motors are rated according to the number of horsepower they can provide. This
is called, appropriately enough, the motor’s Horsepower Rating.
Torque is a measurement of rotational force – in this case, the rotational force
required to turn the input shaft of the machine the motor is driving.
By using a speed control device such as an AF drive, it is possible to get a motor to
provide a different amount of horsepower. Consider the formulas below:
HP = (T x N)/5252
T = 5252HP/N
N = 5252HP/T
Where:
HP = the horsepower provided by the motor
T=
the torque of the motor in foot-pounds
N=
the synchronous speed of the motor in rpm
Suppose you had a four-pole motor that runs at 1800 rpm on 60 Hertz. The motor is
rated at 5 horsepower. How much torque does this motor generate when it reaches
full speed?
Simply plug the numbers into the formula:
T = 5252HP/N
T = (5252 x 5) / 1800
This motor generates about 14.5 foot-pounds of torque.
IN THE WORKPLACE
The fan pictured here is used to
cool a motor. When the motor is
run at a higher speed, it requires
more cooling. This means the
fan needs to turn faster.
This can be accomplished with
an adjustable frequency drive.
Increasing the frequency of the
current sent to the fan increases
the rate at which the fan turns.
COOLING FAN
10
ADJUSTABLE FREQUENCY DRIVES
HOW DOES IT
WORK?
The most common control for an AC motor is the motor starter. The starter connects
the motor directly to the utility power supply, and operates the motor at its rated
speed continuously.
The AF drive, on the other hand, converts the utility power to adjustable
frequency and voltage. This allows for control of the motor’s speed.
Drive Concepts
Pulse Width
Modulation
Now, we will take a look at some concepts that are important to understanding how
the AF drive operates.
•
Pulse Width Modulation
•
Power Factor
•
Impedance
•
Harmonics
As we mentioned earlier, an AF drive is a device used to convert 3 phase, 60Hz
input power into an adjustable frequency and voltage source. However, we didn’t
explain how this is done.
LINE
INPUT
CURRENT
RECTIFIER
INVERTER
OUTPUT
TO
MOTOR
MOTOR
ADJUSTABLE FREQUENCY DRIVE
FIGURE 3. CURRENT PATH FROM LINE TO MOTOR
The drawing above illustrates the path the current takes through the AF drive to the
motor. When the line current first enters the AF drive, it passes into the rectifier,
which converts the incoming AC current to DC current.
Before being sent to the motor, the current enters the inverter. The inverter
converts the DC current into a “synthetic” AC current, which is then fed to the
motor. Let’s look at how the inverter does this.
11
ADJUSTABLE FREQUENCY DRIVES
Pulse Width
Modulation
(continued)
There are three common inverter types in use today. They are:
•
Pulse width modulation (PWM)
•
Current source inverter (CSI)
•
Variable voltage inverter (VVI)
Eaton/Cutler-Hammer AF drives use the first type of inverter, the pulse width
modulation (or PWM) inverter. (We will not take time here to explain how the other
two inverters work, as they are older technology, and no longer commonly used.)
AC current is represented by a sine wave. One full sine wave is called a cycle. The
frequency of the current is the number of cycles that pass a given point in one
second, and is termed Hertz (or Hz). Line current frequency in North America is 60
Hertz.
As shown in figure 4, the PWM inverter takes the DC current from the rectifier and
switches it on and off very quickly, approximating a sine wave.
AC LINE CURRENT WAVEFORM
“SYNTHETIC” AC PWM OUTPUT WAVEFORM
FIGURE 4. LINE CURENT VS. SYNTHETIC AC CURRENT SINE WAVES
By simply speeding up or slowing down the rate at which the current is switched on
and off, the AF drive can create a “synthetic” AC waveform with any frequency.
FIGURE 5. SYNTHETIC AC CURRENT CAN BE PRODUCED AT ANY FREQUENCY
12
ADJUSTABLE FREQUENCY DRIVES
Pulse Width
Modulation
(continued)
Power Factor
A device called an integrated gate bipolar transistor (IGBT) has been developed to
switch the current six times faster than the previous generation of AF drives. It
switches literally thousands of times each second. The results of using IGBT
technology include:
•
A refined synthetic AC sine wave which more closely matches the natural AC
sine wave
•
Less audible motor noise
•
Less heat generated due to the closer match with the “natural” sine wave
•
A smaller heat dissipation device (called a heat sink) is required within the AF
drive
Earlier in this module, we mentioned that energy (utility power) cost is a primary
concern of the customer. One attribute which affects the cost of power is a load’s
power factor. Utility companies often add a charge if the power factor is lower than
their specified level. The power factor is the ratio of the power in to the power out
of a device. Specifically, it is a ratio of the input kilowatts (KW) to the input kilovoltamperes (KVA) being drawn by a load.
Ideally, a power factor of 1.00 (called “unity”) is desired. But most electric loads are
not at unity power factor. In fact, power factor value can be positive or negative.
Capacitors are often used to correct system-wide power factors to near unity.
However, capacitors should be used with extreme caution on power systems with
AF drives. If they are not applied correctly, there is a high potential for capacitor
failure. Capacitors must never be used on an AF drive output.
The current being drawn by the drive (from the utility) contains harmonics, which we
will discuss on the next page. Harmonics cause the KVA to increase, leading to a
lower system power factor. The power factor determined by taking harmonics into
consideration is called total power factor.
The most typical power factor specified by most AF drive manufacturers is called
displacement power factor. Displacement power factor is the ratio of the input
KW to the input KVA, neglecting any harmonics.
Of the three inverter types, the pulse width modulation (PWM) type provides the
highest displacement power factor, about 0.95. Additionally, the PWM inverter has
the same displacement power factor at any AF drive frequency setting. This is
not true for the other two AF drive types (CSI and VVI).
The AF drive’s displacement power factor is not affected by the motor’s speed or
load. However, the total power factor is affected by these conditions.
13
ADJUSTABLE FREQUENCY DRIVES
Impedance
Impedance is the apparent opposition to the flow of current though an AC electrical
circuit, a force that hinders the flow of current.
AF drives have a minimum impedance requirement. This minimum impedance
protects the AF drive against damage from any brief line voltage surges.
You may need to add a line reactor – a device that works against the current flow –
to provide sufficient impedance.
AF drives also have a maximum impedance limit. If the impedance exceeds the
maximum allowed, it is possible that the AF drive’s full rated output may not be
delivered to the motor.
Harmonics
Harmonics are certain frequencies at which unwanted and unusable voltages and
currents are introduced into the system. Harmonics are produced by all non-linear
electrical loads. A non-linear load is a load that pulls current from the line in a nonlinear fashion, switching current on and off. The AF drive is a non-linear electrical
load, as are computers, printers, copy machines, uninterruptable power supplies,
and fluorescent lighting ballasts.
Harmonic frequencies are multiples of the fundamental frequency. In North
America, where the fundamental frequency is 60 Hz, harmonics would be at 120
Hz, 180 Hz, and so on. The system sees these additional waveforms as “noise” on
the line, which has a negative effect on the quality of the power.
Harmonics can cause equipment malfunction, data distortion, transformer and
motor insulation failure, and nuisance tripping of breakers.
We will look at what can be done about harmonics a little later in this module.
14
ADJUSTABLE FREQUENCY DRIVES
HOW DOES IT
WORK?
Operator
Interface
Concepts
Operator Control
Now, let’s look at some concepts relating to the operator interface.
•
Operator control
•
Remote speed control signal
The most basic requirements of operator control are the ability to start, stop, change
speeds and reverse directions of the AF drive. Operator command functions are
usually desired in both manual and automatic modes.
Higher control functions, such as feedback to the control station, are often desired.
Fully digital AF drives have all the standard control features mentioned above, plus
flexible features to meet the needs of specialized application needs, much like a
Programmable Logic Controller (PLC) would.
In addition to communicating easily with human operators, the AF drive should also
interface with computer-controlled automation systems. Inputs on the operator
control unit monitor system elements, such as motor temperature and critical
interlocks, and can shut down the AF drive if necessary. Inputs are also used to
switch the start and stop commands between the manual and automatic modes.
Control Signals
Control signals communicate information between components. They are grouped
as inputs (signals coming into the AF drive) and outputs (signals leaving the AF
drive). Inputs from the motor include current speed, direction, rate of acceleration,
overload warnings and so on. Outputs to the motor include requests to start, stop,
accelerate, maintain a preset speed, jog the load and so on.
Speed reference commands can be sent manually or automatically. Manual speed
input types include door-mounted speed potentiometers and digital keypads.
Automatic speed input types include customer-supplied instrument signals and
pneumatic sensors.
Input and output signals can be analog or digital. Every manufacturer in the industry
offers three standard analog signals: current, voltage and pneumatic. However,
digital signals are preferred, as they are more reliable and exacting. Only a few
manufacturers (including Eaton/Cutler-Hammer) can provide digital signals.
15
ADJUSTABLE FREQUENCY DRIVES
IN SUMMARY
As you can see, adjustable frequency drives are fairly complex in their function. And
explaining their function has required us to look at a lot of underlying engineering
concepts.
Let’s take a moment to sum up what we have covered in this section.
An AC induction motor is designed to run at a synchronous speed. This is
determined by the number of poles that the motor has, and the frequency of the
current being supplied to it. Changing the frequency of the current changes the
motor’s speed.
Horsepower is the unit used for measuring the amount of work a motor can do. By
using an AF drive, a motor can provide a variable amount of horsepower.
When the line current first enters the AF drive, it passes into the rectifier, which
converts the incoming AC current to DC current. It then enters the inverter. The
inverter converts the DC current into a “synthetic” AC current, by switching it
on and off very quickly, approximating an AC sine wave. By simply speeding up or
slowing down the rate at which the current is switched on and off, the AF drive can
create a “synthetic” AC current with any desired frequency.
Total power factor includes the effects of harmonics, while displacement
power factor excludes harmonics.
Impedance is a force that restricts the flow of AC current. AF drives have a
minimum and a maximum impedance requirement. Keeping the impedance
between the minimum and maximum:
•
Reduces the peak charging currents allowed into the rectifier
•
Protects the AF drive against damage from any brief line voltage surges
•
Allows the AF drive’s full rated output to be delivered to the motor
Harmonics are frequencies at which unwanted and unusable voltages and
currents are introduced into the system. The system sees these additional
waveforms as “noise” on the line, which has a negative effect on the quality of the
power. Harmonics can cause equipment malfunction, data distortion, transformer
and motor insulation failure, and nuisance tripping of breakers.
The operator interface controls allow the worker to start, stop and change the speed
of the AF drive. Higher control functions, such as feedback to the control station,
are often desired.
Control signals can be sent manually or automatically, and can be produced by
analog or digital means. Digital control signals are preferred, as they are more
reliable and exacting.
16
ADJUSTABLE FREQUENCY DRIVES
REVIEW 2
Answer the following questions without referring to the material just presented.
Begin the next section when you are confident that you understand what you’ve
already read.
1. The synchronous speed of a motor is determined by the number of ______ that
the motor has, and the _________ of the current being supplied to it.
2. The equation for determining synchronous speed is: N = 120 x F / P. Suppose
you had an AC motor with 8 poles running at 60 Hz. What would be the motor’s
synchronous speed?
_________ RPM
3. Suppose your application called for running the motor described above at 600
rpm. The AF drive attached to the motor would need to be reset to provide how
many Hertz?
_________ Hz
4. Suppose you had a 3 horsepower, 4 pole motor running at 60 Hz. How much
torque is the motor capable of producing?
_________ foot-pounds of torque.
5. The device in the AF drive which converts the incoming AC line current to DC is
the ___________.
6. Name 2 of the benefits of using IGBTs in an AF drive.
___________________________
___________________________
7. The ratio of the AF drive’s KW input to KVA input is called its __________
___________.
8. Keeping the impedance between the minimum and maximum allows for these
three desirable effects:
•
______________________________________________________
•
______________________________________________________
•
______________________________________________________
17
ADJUSTABLE FREQUENCY DRIVES
CONCERNS
WITH USING
ADJUSTABLE
FREQUENCY
DRIVES
We have looked at how adjustable frequency drives work, and why they are so
useful. We have also discussed their advantages over other speed regulation
methods. However, do not assume that the AF drive is foolproof. There are a
number of issues you need to consider. These are:
•
Heat Issues
•
Output Filtering
•
Harmonics
If these issues are ignored while planning the implementation, problems can arise in
application.
Heat Issues
As we have discussed previously, an AF drive generates a certain amount of heat.
This heat generation is taken into consideration in sizing the drive’s cooling fan,
particularly if the AF drive is in an enclosed space.
Likewise, you need to be aware of heat issues in the customer’s application. All the
other components in the system also generate heat that must be dissipated. And
the ambient temperature of the operating environment needs to be considered as
well.
A motor will run at a higher temperature when operating on an adjustable frequency
drive than it will operating on line current. Why is this?
•
Slowing the speed of the motor results in an equal slowing of the motor’s
cooling fan. Heat dissipation may be hampered when running the motor below
its designed speed.
•
As we mentioned earlier, harmonics waste power. This power is lost as heat. AF
drives are affected by harmonics, so more heat is generated and must be
dissipated.
For these reasons, look closely at the customer’s application to be sure that heat
problems won’t cause insulation failure, or, worse, burn up the motor. You may
need to recommend a larger motor and/or a larger enclosure, or a separate cooling
system for the customer’s application.
18
ADJUSTABLE FREQUENCY DRIVES
Output Filtering
When the motor is located far away from the AF drive, a protection issue arises.
(This distance varies by drive model. Consult your catalog for specific information.)
The long cable connecting the drive to the motor could potentially produce frequent,
repetitive voltage surges. These surges could eventually cause insulation failure
and destroy the motor.
When the cable length for the application exceeds the acceptable length in the
product catalog, you will need to add an output filter (also called a dv/dt filter) to the
circuit. The output filter will catch voltage surges before they can damage the motor.
Harmonics
As we mentioned, harmonics are produced by all non-linear electrical loads,
including AF drives. What can be done about harmonics to protect a customer’s
equipment and computer systems?
•
A trap filter can be added to the system to catch each harmonic frequency and
take it to ground. The equipment is protected from damage.
The disadvantage to using this method is that the trap filter can serve as a
“magnet” for all harmonics on the electrical grid. This means that your
customer’s other machines – or even a neighbor’s machines – can use (and
burn up) your customer’s trap filter.
•
A line reactor can be added to lessen the problem. This is a device which adds
impedance to the line to keep the impedance between the minimum and
maximum allowed values. Risk of equipment damage is minimized.
•
A “Clean Power” rectifier can be used. This is rather expensive, but where the
other two methods “clean up” the harmonics, this method prevents harmonics
from ever occurring.
19
ADJUSTABLE FREQUENCY DRIVES
HELPING THE
CUSTOMER
When helping a customer select an adjustable frequency drive for an application,
there are a large number of factors that you must take into consideration. Let’s look
at each one briefly.
Factors to
Consider
•
Consider the application itself
High-torque, large inertia, quick ramp-up applications are not recommended for
use with an AF drive.
•
Load type, power and torque
How large is the load? How much torque is required to get it moving? Is it a
constant torque or variable torque application?
•
Motor type, full load amps and horsepower
What type of motor is it? What is the motor’s full load amps and horsepower?
Get the brand and model number of the motor so you know exactly what you
are dealing with.
•
Speed range needed
What range of motor speeds does the customer want available to him?
•
Speed regulation needed
How precise does the speed setting need to be?
•
Control response
How quickly should the control respond to changes in the load, or changes in
operational settings?
•
Efficiency
Higher efficiency means lower energy costs.
•
Overload capacity
What class (10, 20) of overload protection is required for the application?
•
Reliability
We can assume the customer would like the unit to fail as little as possible, of
course. How critical is it to avoid nuisance tripping? How important is it to be
able to adjust the trip settings?
•
Physical size of the unit
How much physical space is available at the application site for a motor speed
control unit?
•
Audible noise
Does the customer have any concerns related to audible noise produced by the
motor or speed control unit?
•
Harmonics
Does the customer have any concerns regarding harmonics on the line? Take
into consideration the entire system of which the drive is a part.
20
ADJUSTABLE FREQUENCY DRIVES
Factors to
Consider
(continued)
Application
Checklist
•
Enclosure type
Does the customer have any special needs relating to the application’s
environment? Choose a NEMA enclosure type to match.
•
Location
Is the distance between the motor and the AF drive a problem? Check in the
product catalog. An output filter may be necessary.
•
Cost
Customers want the least expensive product that will meet all the application’s
requirements. As you explain the cost of a particular unit, be sure to put it in
terms of the total cost to the customer. What may seem like a high initial cost
may in fact save the customer a lot of money over the life of the unit. Energy
cost savings, reduced downtime because of on-board diagnostics, and the like
can be presented as returns on the investment in the device.
Here is a good checklist to use when helping a customer decide which type of AF
Drive is best for the application.
MOTOR
New ____
Existing ___
Hp ____
Base Speed _____
Voltage _____
FLA ____
LRA ___
NEMA Design ____ Gearbox/Pulley Ratio _____
Hz _____
Service Factor _____
LOAD
Application: ________________________________________________________
Load Type: Constant Torque _____ Variable Torque ____Constant HP _____
Load inertia reflected to motor: _____________ lb.ft2
Required breakaway torque from motor: ___________ ft-lb.
Running load on motor: ___________ ft-lb.
Peak torque (above 100% running): __________ ft-lb.
Shortest/Longest accel time: _____ / _____ sec. up to ____ Hz from stop
Shortest/Longest decel time: _____ / _____ sec. down to ____ Hz from max. speed
Operating speed range: ______ Hz to _____ Hz
Time for motor/load to coast to stop: _____ seconds.
21
ADJUSTABLE FREQUENCY DRIVES
Application
Checklist
continued
AF drive
Source of start/stop commands: I/O terminals ____ Keypad ____ Other ______
Source of speed adjustment: ________________________________
Other operating requirements: _______________________________
Will the motor ever be spinning when the AF Drive is started? Yes___
No ____
How far apart are the motor and the AF Drive? _________ feet
Is the load considered high-inertia? Yes___
Is the load considered hard to start?
No ____
Yes___
Options desired: Multiple operators ______
No ____
Output Signals ______
Power Options _________________________________
Other ________________________________________
Other requirements or conditions: ________________________
POWER SUPPLY
Supply transformer: ______ KVA, or short circuit current at drive input _____ amps
Total Horsepower of all drives connected to supply transformer/feeder _______ Hp
Is a drive transformer of line reactor desired? (May be required) ________
Any harmonic requirements? Y/N
% Voltage THD ____ % Current THD ____
Total non-drive load connected to the drive’s feeder: ________ amps
SERVICE
Start-Up Assistance: ____________
Customer Training:__________
Preventive Maintenance: _________
Spare Parts: ________
ADDITIONAL ISSUES – Answer Yes or No
_____ Will the AF Drive operate multiple motors?
_____ Will the power supply source ever be switched with the AF Drive running?
_____ Is starting or stopping time critical?
_____ Are there any peak torques or impact loads?
_____ Will user-supplied contactors be used on the input or output of the AF Drive?
22
ADJUSTABLE FREQUENCY DRIVES
Recommending
a Product to the
Customer
Using all the information gathered here, consult your product catalogs to make a
recommendation to the customer.
It is possible that you may have to suggest more than one product, as you may not
find a perfect match for all the application’s requirements.
IN THE WORKPLACE
The centrifugal pump on a
chilled water system, shown
here, is an ideal candidate for
use with an adjustable frequency
drive.
As the pump turns faster, it
moves more water, and it
requires more torque.
This application also requires
water flow through the pump to
be regulated.
You will need to work with the
customer to select an adjustable
frequency drive for an
application such as this, taking
into consideration the factors
mentioned above.
TYPICAL PUMP ON A CHILLED WATER SYSTEM
23
ADJUSTABLE FREQUENCY DRIVES
NORMALLY
SPECIFIED
FEATURES
There are four features that the customer will generally specify when looking into an
adjustable frequency drive. These are:
•
Bypass
•
Line Reactor
•
Control Signal
•
Output Filter
Let’s take a brief look at each feature.
Bypass
Adding a bypass to the AF drive allows the motor to be transferred via a set of
contactors to the utility line, to allow maintenance to be done on the drive.
There are two bypass types available:
Line Reactor
•
Manually operated
•
Automatically transferred
The customer may specify a line reactor for the application.
Get all the pertinent information from the customer and from the product catalog.
Then, refer to the section on impedance (earlier in this module) to determine
whether a line reactor is needed for the customer’s application.
Control Signals
The customer may specify a control signal type or types for the application.
Determine the customer’s requirements and decide whether analog or digital inputs
and outputs are more appropriate.
Output Filter
When the cable length for an application exceeds the acceptable length in the
product catalog, you will need to add an output filter (also called a “dv/dt” filter) to
the circuit. The output filter will reduce the voltage transients before they can
damage the motor.
The customer may specify the need for an output filter if the cable length is
excessive, but may not be aware of this requirement. In any case, you should verify
whether an output filter is needed for the customer’s application.
24
ADJUSTABLE FREQUENCY DRIVES
REVIEW 3
Answer the following questions without referring to the material just presented.
1. What two motor heat issues crop up when running a motor with an AF drive?
______________________________________________________________
______________________________________________________________
2. The output filter works to protect the motor by:
______________________________________________________________.
3. Name four of the factors that you must take into consideration when choosing
an AF drive to suit a customer’s application.
_________________________
_________________________
_________________________
_________________________
4. Name two methods for dealing with harmonics.
_________________________
_________________________
25
ADJUSTABLE FREQUENCY DRIVES
GLOSSARY
26
AC
Alternating current.
Adjustable
Frequency Drive
An electrical device used for controlling the speed of an AC
motor. Also called AF Drive.
Bypass
A device added to an AF drive which allows the motor
power to be transferred via a set of contactors. This is
useful in case of a drive failure, or if maintenance is being
done on the drive.
Capacitor
A device that consists of two conductors separated by an
insulator. It is used to store electricity.
Displacement
Power Factor
The ratio of input KW to input KVA of the AF drive,
excluding harmonics.
Filter
A component for smoothing the DC current in an AF drive.
Frequency
The number of cycles per second made by an alternating
electrical current. The unit of measure is the Hertz.
Fundamental
Frequency
The frequency of line current. In North America, this is 60
Hertz.
Harmonics
Certain frequencies; specifically, multiples of the
fundamental frequency, at which unwanted and unusable
voltages and currents are introduced into the system.
Horsepower
Unit of measurement used for describing the amount of
work a motor can do.
Impedance
The apparent opposition to the flow of current though an
AC electrical circuit; a force that literally impedes the flow of
current.
Integrated Gate
Bipolar Transistor
A device used to switch the DC current in the inverter
section of an AF drive.
Inverter
The section of the AF drive which converts DC current into
a “synthetic” AC current, which is then fed to the motor.
Line Reactor
A device which adds impedance to the line to keep the
impedance between the minimum and maximum allowed
values for a particular piece of electric equipment, such as
an AF drive.
ADJUSTABLE FREQUENCY DRIVES
Output Filter
When the cable length for the application exceeds the
acceptable length, adding this device to the circuit protects
motor from voltage surges. Also called a DV/DT Filter.
Phase
The relationship between the current and voltage in an AC
circuit, with respect to their angular displacement.
Power Factor
A blanket term, used to describe various measurements of
the percentage of the input current available after passing
through the system. See Total Power Factor, Displacement
Power Factor.
Pulse Width
Modulation
A type of inverter which takes the DC current from the
rectifier and switches it on and off very quickly,
approximating a sine wave.
Rectifier
A device which converts the incoming AC current to DC.
Remote Speed
Control Signal
This signal is used to control the speed of the drive
remotely, from a control panel not located directly on the
drive.
Synchronous
Speed
A motor’s design speed, determined by the number of poles
that the motor has, and the frequency of the current being
supplied to it.
Three-Phase
A type of AC power where three AC currents are introduced
at 120° variances.
Torque
A measurement of rotational force.
Total Power
Factor
The ratio of the input KW to input KVA of the AF drive,
including harmonics.
Voltage
Electrical potential difference. The unit of measure is the
Volt.
Voltage Transient
Technical term for an unexpected and undesirable voltage
surge in a circuit.
27
ADJUSTABLE FREQUENCY DRIVES
REVIEW 1
ANSWERS
1. frequency, voltage
2. cooling fan
3. many possible answers
4. many possible answers
REVIEW 2
ANSWERS
1. poles, frequency
2. 900 RPM
3. 40 Hz
4. 8.75 foot-pounds of torque
5. rectifier
6. Ane two of the following:
Refined synthetic AC sine wave, more closely matching natural AC sine wave
Less audible motor noise
Less heat generated due to the closer match with the “natural” sine wave
Smaller heat sink required
7. power factor
8. Reduces the peak charging currents allowed into the rectifier, protects the AF
drive against damage from any brief line voltage surges, and allows the AF
drive’s full rated output to be delivered to the motor
28
ADJUSTABLE FREQUENCY DRIVES
REVIEW 3
ANSWERS
1. Answer should basically say: “1. Slowing the speed of the motor results in an
equal slowing of the motor’s cooling fan. 2. Harmonics waste power. This power
is lost as heat. AF drives are affected by harmonics, so more heat is generated
and must be dissipated.”
2. reducing voltage transients before they can damage the motor
3. Ane four of the following:
Load type, power and torque
Motor type, full load amps and horsepower
Speed range needed
Speed regulation needed
Control response
Efficiency
Overload capacity
Reliability
Physical size of the unit
Audible noise
Harmonics
Location
Cost
4. Ane two of the following:
Trap filter
Line reactor
Clean power rectifier
29
Cutler-Hammer
Milwaukee, Wisconsin U.S.A.
Publication No. TR.08.03.T.E
February 1999
Printed in U.S.A. (GSP)
101 Basics Series and 201 Advanced Series are trademarks of Cutler-Hammer University, Cutler-Hammer and Eaton Corp.
©1999, Eaton Corp.