101 basics series starters and contactors

101 basics series starters and contactors
101 BASICS SERIES
LEARNING MODULE 19:
STARTERS AND CONTACTORS
Cutler-Hammer
STARTERS
WELCOME
Welcome to Module 19, which is about starters, devices that control the use of
electrical power to equipment, usually a motor.
As the name implies, starters “start” motors. They can also stop them, reverse them,
accelerate them, and protect them.
Starters are made from two building blocks, contactors and overload protection:
•
Contactors control the electric current to the motor. Their function is to
repeatedly establish and interrupt an electrical power circuit.
•
Overload Protection protects motors from drawing too much current and
overheating, from literally “burning out.”
=
STARTER
+
CONTROLLER
OVERLOAD PROTECTION
FIGURE 1. A STARTER IS MADE UP OF A CONTROLLER
(MOST OFTEN A CONTACTOR) AND OVERLOAD PROTECTION
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.
Glossary terms are underlined and italicized the first time they appear.
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.
1
STARTERS
WHAT YOU
WILL LEARN
We’ll start by talking about the building blocks of a starter: the contactor and
overload protection. We will then conclude with a discussion on starters. Here are
the topics that we will cover:
Section Title
•
3
•
Knife Blade Switch
3
•
Manual Controller
4
•
Magnetic Contactor
5
•
Contactor Components
6
•
How the Contactor Operates
6
•
Contact Life
8
•
Review 1
•
Overload Protection
9
10
•
How Motors Work
11
•
What is an Overload?
12
•
Overload Relay
13
•
The Eutectic Overload Relay
13
•
The Bimetallic Overload Relay
15
•
The Solid State Overload Relay
16
•
Tripping
17
•
Overload Relays in Action
17
•
Review 2
18
•
The Starter
19
•
2
The Contactor
Page Number
•
Manual Starter
20
•
Magnetic Motor Starter
21
•
Starter Circuitry
22
•
Features of Motor Starters
25
•
Types of Motor Starters
25
•
Standards and Ratings
26
Helping the Customer
27
•
Interviewing the Customer
27
•
NEMA or IEC?
28
•
Checking the Motor Nameplate
28
•
Modifications
29
•
Review 3
30
•
Glossary
31
•
Review Answers
34
STARTERS
THE
CONTACTOR
A contactor can stand on its own as a power control device, or as part of a starter.
Contactors are used in applications ranging from the light switch to the most
complex, automated industrial equipment. Contactors are used by electrical
equipment that is frequently turned off and on (opening and closing the circuit),
such as lights, heaters, and motors.
Whatever the application, the function of the contactor is always the same: to make
and break all power supply lines running to a load. Or, as defined by NEMA, to
repeatedly establish and interrupt an electrical power circuit.
Knife Blade
Switches
The first device used to stop and start electric motors was a simple knife blade
switch. This was a lever that would drop a strip of metal onto a contact to make the
electric circuit. In the late 1800’s, “throwing the switch” meant exactly that –
someone had to stand next to the knife blade switch and lever it into the closed
position.
When industry began to demand more powerful electric motors, the knife blade
switch quickly became obsolete and was no longer used. Why?
TO POWER SOURCE
KNIFE
BLADE
SWITCH
MOTOR
FIGURE 2. A KNIFE BLADE SWITCH USED TO CONTROL A MOTOR
Engineers discovered that the contacts quickly wore out because humans could not
open and close the switch fast enough to prevent arcing. Arcing, a condition
where high voltage leaps across the open space as the contacts closed in or
pulled away from the switch, corroded the soft copper switches with pits. Dirt and
moisture compounded the problem.
More importantly, as motors became larger, the currents to operate them also had
to become larger, creating a serious safety concern. It was physically dangerous
to handle the switch. Willing knife blade switch operators became harder and
harder to find.
3
STARTERS
Knife Blade
Switches
(continued)
Manual
Controller
Mechanical improvements were made, but with their dangerous operation and short
contactor life, knife blade switches remained at a design dead-end. The knife blade
switch certainly wasn’t the best solution, but from it, engineers learned what issues
needed to be addressed:
•
speed of operation
•
contactor life
•
protection for the motor
•
protection for the person who operates the switch (afforded by either remote or
automatic operation)
The manual controller was the next step up the evolutionary ladder, offering
several important new features:
•
The unit is encased, not exposed
•
Double break contacts are used, instead of single break
•
The unit is physically smaller
•
The unit is much safer to operate
Double-break contacts open the circuit in two places simultaneously. Dividing
the connection over two sets of contacts allows you to work with more current in a
smaller space than you get with a single-break contact. In addition, the mechanical
linkage more quickly and consistently opens and closes the circuit, sparing the
metal from some of the arcing experienced under knife blade switches.
MOVABLE CONTACTS
STATIONARY CONTACTS
CONTACTS CLOSED
CONTACTS OPEN
FIGURE 3. DOUBLE-BREAK CONTACTOR
4
STARTERS
With a manual controller, the operator presses a button or moves a switch that is
integral to the electrical equipment being run. In other words, the button or switch is
physically attached to the controller itself, and is not operated remotely.
When an operator activates a manual controller, the power circuit engages,
carrying the electricity to the load.
The manual contactor was a big improvement over the knife blade switch.
Variations of manual contactors are still in use today.
MANUAL MOTOR CONTROLLER
ENCLOSED MANUAL MOTOR CONTROLLER
FIGURE 4. TWO TYPICAL MANUAL CONTACTORS STILL IN USE TODAY
Magnetic
Contactor
Engineers eventually made a breakthrough with the magnetic contactor.
A magnetic contactor is operated electromechanically without manual
intervention. This means that the contactor can be operated remotely, without the
need for putting a person in a potentially dangerous location. Magnetic contactors
use a small control current to open and close the circuit.
NOTE: From this point forward, the term “contactor” will refer only to an AC
magnetic contactor.
5
STARTERS
Contactor
Components
If you sat down and took apart a contactor as shown in Figure 5, you would find the
following components: an electromagnet (E-frame), an armature, a coil, a spring,
and two sets of contacts, one movable set and one stationary set.
MOVABLE CONTACTS
STATIONARY CONTACTS
ARMATURE
SPRING
COIL
E-FRAME
FIGURE 5. MAGNETIC CONTACTOR COMPONENTS
How the
Contactor
Operates
So, how exactly does the contactor open and close? The E-Frame, when energized
by the coil, becomes an electromagnet. The armature, a companion to the Eframe, is connected to a set of contacts. The armature is moveable but is held by a
spring.
When the coil is energized, the moveable contacts are pulled toward the
stationary contacts, because the armature is pulled toward the E-frame. Once the
two sets of contacts meet, power can flow through the contactor to the load.
When the coil is de-energized, the magnetic field is broken, and the spring forces
the two sets of contacts apart.
In Figure 6 on the next page, we step through the process again, using pictures to
help you understand.
Contactors are used when no overload protection is necessary, and at lower levels
of electrical current. Applications include lighting circuits, heaters, and transformers.
In summary, contactors operate electromechanically and use a small control
current to open and close the circuit. (We will discuss control current in more
detail in the section on starters.) The electromechanics do the work, not the human
hand, as in a knife blade switch or a manual controller.
IN THE WORKPLACE
Pushbuttons and selector
switches, like the ones on this
control panel, are used in
hundreds of manufacturing
industries.
Each button and switch is
connected to a contactor, for use
in making or breaking an
electrical circuit remotely.
6
STARTERS
1. ELECTRICITY IS APPLIED TO THE TERMINALS OF
THE COIL. CURRENT FLOWS INTO THE COIL,
CREATING A MAGNETIC FIELD.
2. THE COIL, IN TURN, MAGNETIZES THE E-FRAME,
TURNING IT INTO AN ELECTROMAGNET.
3. THE ELECTROMAGNET DRAWS THE ARMATURE
TOWARD IT…
4. ...PULLING THE CONTACTS TOGETHER.
5. POWER FLOWS THROUGH THE CONTACTOR TO THE LOAD.
FIGURE 6. HOW A MAGNETIC CONTACTOR OPERATES
7
STARTERS
Contact Life:
Arcing
A major customer concern is the life expectancy of a contactor. It has been
said that, “The worst thing you can do to a car is start it.” The same is true for
contacts. The more frequently the contacts are opened and closed, the shorter the
life of the contactor.
As contacts open and close, an electrical arc is created between them. The
arcs produce additional heat, which, if continued, can damage the contact surfaces.
ARC
CONTACTS
CORROSION
AND PITTING
OXIDE
CONTACTS
FIGURE 7. ARCING CAUSES CORROSION, PITTING AND OXIDE BUILD-UP
Eventually, the contacts become blackened with burn marks and pitting made by
the electrical arcs. This is not a reason for contact replacement. In fact, this black
deposit (oxide) helps them to make a better “seat” to conduct the electricity.
However, contacts do need to be replaced once the surface is badly corroded or
worn away.
Contact Life:
Bounce
Applying some logic, you can conclude that the faster the contact closes, the
sooner the arc is extinguished, and the longer the life expectancy of the
contact. But, modern contactors have been designed to close so quickly and with
such energy that the contacts slam against each other and rebound, causing a
bouncing action. This is referred to as contact bounce. When the contact bounces
away, a secondary arc is created. The contacts slam together again and again,
each time the bouncing and arcing become less and less.
So, in addition to closing the contacts as fast as possible, you also want the
contacts to bounce as little as possible, to reduce secondary arcing and wear.
8
STARTERS
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. Explain the two problems with knife blade switches that caused engineers to
seek other solutions.
____________________________________________________________
____________________________________________________________
____________________________________________________________
2. What is the function of a contactor?
____________________________________________________________
____________________________________________________________
____________________________________________________________
3. Explain the difference between a manual contactor and a magnetic contactor.
____________________________________________________________
____________________________________________________________
____________________________________________________________
4. Explain the two main concerns with contact life.
____________________________________________________________
____________________________________________________________
____________________________________________________________
9
STARTERS
OVERLOAD
PROTECTION
Now that you have a good understanding of what a contactor is and how it works,
let’s move on to a discussion of overload protection. As we mentioned at the
beginning of this module, overload protection prevents an electric motor from
drawing too much current, overheating, and literally “burning out.”
Like a contactor, overload protection is a building block of starters. Remember the
starter equation:
=
STARTER
+
CONTROLLER
OVERLOAD PROTECTION
FIGURE 6. A STARTER IS MADE UP OF A CONTROLLER
(MOST OFTEN A CONTACTOR) AND OVERLOAD PROTECTION
Let’s begin this section by learning how a motor works, and why overload protection
is needed. Then we will move on to the different types of overload protection.
10
STARTERS
Part of understanding overload protection is understanding how motors work. A
motor goes through three stages during normal operation: resting, starting,
and operating under load.
MOTOR CURRENT DRAWN
How Motors
Work
STARTING
INRUSH
CURRENT
MOTOR
RUNNING
CURRENT
MOTOR
AT REST
TIME
NO CURRENT
FLOW
L1 L2 L3
INRUSH CURRENT
6 TO 8 TIMES RUNNING
CURRENT
L1 L2 L3
RESTING
STARTING
L1 L2 L3
NORMAL
RUNNING
CURRENT
OPERATING
UNDER LOAD
FIGURE 9. MOTOR OPERATION STAGES
A motor at rest requires no current because the circuit is open.
But once the circuit is closed, the motor starts drawing a tremendous inrush
current; as much as 6-8 times its running current.
Here is the problem: this large inrush current can cause immediate tripping of
the circuit breaker. A fuse or circuit breaker sized to handle the normal running
load of the motor will open the circuit during startup.
You might think that sizing the fuse or circuit breaker for the spike in current draw
would solve the problem. But if you did this, once the motor was running, only the
most extreme overload would open the circuit. Smaller overloads would not trip the
breakers, and the motor would burn out.
L1 L2 L3
L1 L2 L3
FUSES ARE LARGE
ENOUGH TO ALLOW
INRUSH STARTING
CURRENT
FUSES DO NOT
BLOW
OVERLOADED
MOTOR
FIGURE 10. THE PROBLEM WITH OVERSIZED FUSES
11
STARTERS
What Is An
Overload?
So, what is an overload? The term literally means that too much load has been
placed on the motor. A motor is designed to run at a certain speed, called its
synchronous speed. If the load on the motor increases, the motor draws more
current to continue running at its synchronous speed.
It is quite possible to put so much load on a motor that it will draw more and more
current without being able to reach synchronous speed. If this happens for a long
enough period of time, the motor can melt its insulation and burn out. This condition
is called an overload.
In fact, the motor could stop turning altogether (called a locked rotor) under a large
enough load. This is another example of an overload condition. Even though the
motor shaft is unable to turn, the motor continues to draw current, attempting to
reach its synchronous speed.
While the running motor may not draw enough current to blow the fuses or trip
circuit breakers, it can produce sufficient heat to burn up the motor. This heat,
generated by excessive current in the windings, causes the insulation to fail and the
motor to burn out. We use the term locked rotor amps to describe when the
motor is in this state and is drawing the maximum amount of current.
So, because of the way a motor works, an overload protection device is required
that does not open the circuit while the motor is starting, but opens the circuit
if the motor gets overloaded and the fuses do not blow.
12
STARTERS
Overload Relay
The overload relay is the device used in starters for motor overload protection. It
limits the amount of current drawn to protect the motor from overheating.
An overload relay consists of:
•
A current sensing unit (connected in the line to the motor).
•
A mechanism to break the circuit, either directly or indirectly.
To meet motor protection needs, overload relays have a time delay to allow
harmless temporary overloads without breaking the circuit. They also have a trip
capability to open the control circuit if mildly dangerous currents (that could result in
motor damage) continue over a period of time. All overload relays also have some
means of resetting the circuit once the overload is removed.
IN THE WORKPLACE
The blower motor on this
furnace uses an overload relay
to protect the motor when the
blower turns on and current
inrush begins.
A circuit breaker could not
provide both running and inrush
protection.
The inrush continues until the
blower fan reaches full speed,
or, more technically, the motor’s
synchronous speed.
Let’s take a look inside a few overload relays to see how they work. We’ll review the
following overload relay types:
The Eutectic
Overload Relay
•
Eutectic (melting alloy)
•
Bimetallic
•
Solid State
The melting alloy (or eutectic) overload relay consists of a heater coil, a eutectic
alloy, and a mechanical mechanism to activate a tripping device when an overload
occurs. The relay measures the temperature of the motor by monitoring the amount
of current being drawn. This is done indirectly through a heater coil.
Many different types of heater coils are available, but the operating principle is the
same: A heater coil converts excess current into heat which is used to
determine whether the motor is in danger. The magnitude of the current and the
length of time it is present determine the amount of heat registered in the heater
coil.
13
STARTERS
Usually, a eutectic alloy tube is used in combination with a ratchet wheel to activate
a tripping device when an overload occurs. A eutectic alloy is a metal that has a
fixed temperature at which it changes directly from a solid to a liquid. When an
overload occurs, the heater coil heats the eutectic alloy tube. The heat melts the
alloy, freeing the ratchet wheel and allowing it to turn. This action opens the
normally closed contacts in the overload relay.
RATCHET WHEEL IS
LOCKED IN PLACE BY
SOLID EUTECTIC ALLOY
AND PAWL
INNER SHAFT
CONNECTED TO
RATCHET WHEEL
EUTECTIC
ALLOY TUBE
PAWL
NORMAL CURRENT
APPLIED TO
HEATER COIL
SOLID
EUTECTIC ALLOY
NORMAL
OPERATION
RATCHET WHEEL IS
FREE TO TURN
EUTECTIC
ALLOY TUBE
PAWL
EXCESSIVE CURRENT
APPLIED TO HEATER COIL
MELTS EUTECTIC ALLOY
MELTED
EUTECTIC ALLOY
OVERLOAD
CONDITION
FIGURE 11. EUTECTIC OVERLOAD RELAY: RATCHET WHEEL AND EUTECTIC ALLOY COMBINATION
14
STARTERS
The Bimetallic
Overload Relay
A bimetallic device is made up of two strips of different metals. The dissimilar
metals are permanently joined. Heating the bimetallic strip causes it to bend,
because the dissimilar metals expand and contract at different rates.
The bimetallic strip applies tension to a spring on a contact. If heat begins to rise,
the strip bends, and the spring pulls the contacts apart, breaking the circuit, as
shown in Figure 12.
CLOSED
CIRCUIT
TO LOAD
BIMETALLIC
STRIP
FROM LINE
NORMAL OPERATION
OPEN
CIRCUIT
TO LOAD
BIMETALLIC
STRIP
FROM LINE
OVERLOAD CONDITION
FIGURE 12. BIMETALLIC OVERLOAD RELAY: THE WARPING EFFECT OF THE BIMETALLIC STRIP
Once the tripping action has taken place, the bimetallic strip cools and reshapes
itself, automatically resetting the circuit. The motor restarts even when the overload
has not been cleared, and will trip and reset itself again and again. (This assumes
an automatic reset. This type of relay can also be equipped with a manual reset.)
As we mentioned, an overload relay is designed to prevent the motor from
overheating. The heat comes from two sources: heat generated within the motor,
and heat present in the area where the motor operates (ambient heat). Although
ambient heat contributes a relatively small portion of the total heat, it has a
significant effect on the operation of the overload relay bimetals. A properly
designed ambient-compensating element reduces the effects of ambient
temperature change on the overload relay.
This type of overload relay is commonly found in applications such as walk-in meat
coolers, remote pumping stations, and some chemical process equipment, where
the unit is operated in environments with varying ambient temperatures.
15
STARTERS
The Solid State
Overload Relay
Unlike the other two relay types, the solid state overload relay does not actually
generate heat to facilitate a trip. Instead, it measures current or a change in
resistance. The advantage of this method is that the overload relay doesn't waste
energy generating heat, and doesn't add to the cooling requirements of the panel.
Current can be measured via current transformers, then converted into a voltage
which is stored in memory inside the overload relay. If the relay notices that the
current is higher than it should be for too long a period of time, it trips.
Another type of solid state overload relay uses sensors to sense the heat generated
in the motor. When the sensor senses heat in excess of the preset value for too
long a period of time, it trips.
The solid state overload relay also provides some advanced functions.
1. It is possible to provide proactive functionality and improved protection against
special conditions. For example, when high ambient temperature conditions
exist, devices that use sensors can sense the effect the ambient temperature is
having on the motor.
2. Some solid state overload relays offer programmable trip time. This can be
useful when a load takes longer to accelerate than traditional overload relays
will allow, or when a trip time in between traditional trip classes is desired.
3. Some overload relays have a built in emergency override, to allow the motor to
be started even when it could be damaging to the motor to do so. This can be
useful in a situation where the process is more important than saving the motor.
4. Some solid-state overload relays can detect the change in current when a motor
suddenly becomes unloaded. In such a situation, the relay will trip to notify the
user that there is an application problem. Normally, this indicates a system
problem rather than a motor problem.
16
STARTERS
Tripping
Many overload protection devices have a trip indicator built into the unit to
indicate to the operator that an overload has taken place.
Overload relays can have either a manual or an automatic reset. A manual reset
requires operator intervention, such as pressing a button, to restart the motor. An
automatic reset allows the motor to restart automatically, usually after a “cooling off”
period, as in the case of the bimetallic strip.
Overload relays also have an assigned trip class. The trip class is the maximum
time in seconds at which the overload relay will trip when the carrying current is at
600% of its current rating. Bimetallic overload relays can be rated as Class 10,
meaning that they can be counted on to break the circuit no more than 10 seconds
after a locked rotor condition begins. Melting alloy overload relays are generally
Class 20.
Overload Relays
in Action
You will get motor protection with either a manual or a magnetic starter. However,
the actual mechanics of the overload protection work differently for each type of
starter.
When a manual starter experiences an overload, an overload trips a mechanical
latch, causing the contacts to open and disconnect the motor from the electrical
line.
In a magnetic motor starter (which we will discuss in the next section), an overload
results in the opening of a set of contacts within the overload relay itself. This set of
contacts is wired in series with the starter coil in the control circuit of the magnetic
motor starter. Breaking the coil circuit causes the starter contacts to open,
disconnecting the motor from the line. The motor is stopped and saved from
burning out.
17
STARTERS
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. Describe the purpose of overload protection for an electric motor.
_______________________________________________________________
_______________________________________________________________
2. A motor goes through three stages during normal operation. Name them, and
the typical percentage of full load current they draw.
_____________ ______%
_____________ ______%
_____________ ______%
3. Define and explain the importance of a motor’s synchronous speed.
_______________________________________________________________
_______________________________________________________________
_______________________________________________________________
4. Explain why fuses or circuit breakers cannot properly protect an electric motor.
_______________________________________________________________
_______________________________________________________________
5. Compare and contrast the function of the three overload relay types:
_______________________________________________________________
_______________________________________________________________
_______________________________________________________________
_______________________________________________________________
6. Define trip class.
Trip class is _____________________________________________________.
18
STARTERS
THE STARTER By combining the two elements we have discussed so far, the contactor and
overload protection, we have a new device: a starter.
=
STARTER
+
CONTROLLER
OVERLOAD PROTECTION
FIGURE 13. A STARTER IS MADE UP OF A CONTROLLER
(MOST OFTEN A CONTACTOR) AND OVERLOAD PROTECTION
A starter lets you turn an electric motor (or motor-controlled electrical equipment)
on or off, while providing overload protection. It represents another evolution in
control. Now, we have a power control device that offers more than just a manual
on/off control, such as a knife blade switch. The manual starter also provides a
means to protect the motor from burnout: overload protection.
There are two main types of starters: the manual starter and the magnetic
motor starter.
19
STARTERS
Manual Starter
As the name implies, a manual starter is operated manually. Operating a manual
starter is fairly simple and straightforward: you press a button or toggle (mounted
directly on the starter) to start or stop the connected electrical equipment.
Mechanical linkages from the buttons or toggle force the contacts to open and
close, starting and stopping the motor.
FIGURE 14. TOGGLE OPERATED STARTER
FIGURE 15. PUSHBUTTON OPERATED STARTER
Often, a manual starter is the best choice for an application, because it offers:
•
compact physical size
•
choice of enclosures
•
low initial cost
•
motor overload protection
•
safe and economical operation
Low-voltage protection (LVP), which prevents automatic restarting of equipment
after a power failure, is usually not possible with a manual starter.
This means, if the power fails, the power contacts remain closed (toggle or button in
ON position). When the power is restored, the motor automatically restarts itself.
This could create a dangerous situation, depending on the application.
Because of this, manual starters are generally used on smaller loads where low
voltage protection is not needed. On applications like pumps and blowers, where
the motor should run continuously, and restart automatically, this is actually an
advantage.
20
STARTERS
IN THE WORKPLACE
Inside this saw mill, a highspeed saw quickly reduces logs
to construction beams.
The saw uses a starter on the
motor to allow it to get up to
speed without tripping the
internal circuit breaker.
If the saw were to hit a knot or a
nail, and the motor was not able
to turn at its synchronous speed,
it would attempt to draw more
current in order to do so.
The resulting overload would
cause the saw to stop until the
overload condition was
removed.
Magnetic Motor
Starter
The other main type of starter is the AC magnetic motor starter. These are so
commonly used, that when we use the term “motor starter,” we mean “AC magnetic
motor starter” unless specifically stated otherwise.
We will begin with a look into what they are and how they work, and finish up with
an overview of specific types of motor starters.
Motor starters offer some additional capabilities not available in a manual
starter, most importantly, remote and automatic operation. In other words, the
magnetic motor starter did for manual starters what the magnetic contactor did for
manual controllers: it removed the operator from the immediate area.
Like the magnetic contactor, the motor starter depends on magnets and magnetism
for its operation. These additional capabilities are due, for the most part, to the
motor starter’s electromagnetic operation and the control circuit.
21
STARTERS
Starter Circuitry
Before we go any further, we need to discuss starter circuitry.
The motor starter has two circuits: the power circuit and the control circuit.
Figure 16 shows a three-phase, full voltage, non-reversing magnetic starter. The
thick lines are the power circuit, and the thin lines are the control circuit.
L1
L2
L3
AUXILIARY
CONTACTS
T1
T2
T3
MOTOR
FIGURE 16. CONTROL AND POWER CIRCUITS
22
STARTERS
The power circuit runs from the line to the motor. Electricity passes through the
contacts of a starter, the overload relay and out to the motor. The power (main)
contacts carry the motor current.
The control circuit operates the contactor (on/off). As shown in Figure 16, the
contacts that interrupt or allow the main current to flow to the motor are controlled
by opening or closing the contacts in the control circuit. The control circuit energizes
the coil creating an electromagnetic field that pulls the power contacts closed,
thereby connecting the motor to the line. The control circuit makes remote
operation possible.
The control circuit can get its power in one of two ways. If the control circuit gets its
power from the same source as the motor, this is termed common control. Figure 16
depicts a common control circuit configuration.
The other type is separate control. This is the most common form of control. In this
arrangement, the control circuit gets its power from a separate source, usually lower
in voltage than the motor’s power source.
In addition, there are two ways to wire the control circuit. One common method of
wiring the control circuit is known as Two-Wire. It uses a maintained contact type of
pilot device -- such as a thermostat, float switch, or presence sensor. This circuit
provides for an automatic operation (start - stop) of the load.
FIGURE 17. TWO WIRE CONTROL
23
STARTERS
The other common method of wiring the control circuit is Three-Wire control. It uses
momentary contact pilot devices and a holding circuit contact. The holding circuit
contact is commonly an auxiliary contact on the starter or contactor. If circuit power
is interrupted, the circuit must be restarted by an operator.
FIGURE 18. THREE WIRE CONTROL
24
STARTERS
Features of
Motor Starters
Types of Motor
Starters
All motor starters share the following power control functions:
•
Rated by current (amperes) or power (horsepower)
•
Remote ON/OFF control
•
Motor overload protection
•
Starting and stopping (electrical life)
•
Plugging and jogging (rapid making and breaking current)
Four particular varieties of motor starters are across-the-line, the reversing starter,
the multispeed starter, and the reduced voltage starter.
Across-the-line or full voltage non-reversing (FVNR) is the most commonly
used general purpose starter. This starter connects the incoming power directly to
the motor. It can be used in any application where the motor runs in only one
direction, at only one speed, and starting the motor directly across the line does not
create and “dips” In the power supply.
The reversing starter (FVR, for full-voltage reversing) reverses a motor by
reversing any two leads to the motor. This is accomplished with two contactors
and one overload relay. One contactor is for the forward direction and the other is
for reverse. It has both mechanically and electrically interlocked sets of contactors.
The multispeed starter is designed to be operated at constant frequency and
voltage. There are two ways to change the speed of an AC motor:
•
vary the frequency of the current applied to the motor
•
use a motor with windings that may be reconnected to form different numbers of
poles
The multispeed starter uses the latter option to change speed.
Reduced voltage starters (RVS) are used in applications that typically involve
large horsepower motors. The two main reasons to use a reduced voltage starter
are:
•
reduce the inrush current
•
limit the torque output and mechanical stress on the load
Power companies often won’t allow this sudden rise in power demand. The reduced
voltage starter addresses this inrush problem by allowing the motor to get up to
speed in smaller steps, drawing smaller increments of current. This starter is not a
speed controller. It reduces the shock transmitted to the load only upon start-up.
We will look at reduced voltage starters in much more depth in Module 21, Reduced
Voltage Starters.
25
STARTERS
Standards And
Ratings
NEMA and IEC
You will undoubtedly run across two acronyms when dealing with contactors and
starters : NEMA and IEC. These are two organizations that recommend design and
testing standards for electrical devices such as contactors and motor starters. It is
important to note that neither organization
performs actual testing of equipment.
NEMA is the National Electrical Manufacturers
Association. It has its headquarters in
Washington D.C., and is associated with
equipment used in North America. NEMA devices are built to a high level of
perfection, for use in a variety of applications.
NEMA devices, because of their conservative ratings, can be used in almost any
application. Being less application-sensitive and more durable, NEMA devices
tend to be larger, and therefore more expensive than IEC devices.
IEC is the International Electro-technical Commission. With
headquarters in Geneva, Switzerland, it is associated with equipment
used internationally. IEC devices are commonly used in OEM
machines, where specifications are known and not likely to change.
Because of their greater application sensitivity, IEC devices sometimes require
more care in selection than NEMA devices.
As we expand into a global economy, an increasing number of control products are
manufactured to IEC standards and conventions. Consequently, there is also more
confusion in understanding the differences the IEC and NEMA products. One
standard is not necessarily superior to the other, they are just different.
UL and NEC
In addition to IEC and NEMA standards, there is UL (Underwriters
Laboratories, Inc.). UL provides product standardization and testing.
Their goal is to verify (through testing) that equipment will not pose a
hazard to personnel or property when properly installed. They are
chiefly concerned with safety issues.
NEC (National Electrical Code) is a standard for applying electrical equipment in the
U.S. In the case of motors and starters, one of NEC’s requirements is that a motor
must be protected from destroying itself under overload conditions. And that is
where overload relays come in. The code is adopted and enforced by local
electrical inspectors.
CE and CSA
26
Finally, there are the CE and the CSA. CE relates the European market. CSA
(Canadian Standard Association) is very similar to UL. The CSA mark is required on
products for sale in Canada. Products receiving their approval are marked as such,
serving as an “entrance visa.” These marks indicate compliance with harmonized
European Standards.
STARTERS
HELPING THE Armed with your knowledge of starters, contactors, and overload relays, you are
now prepared to help the customer. The two most fundamental questions you need
CUSTOMER
to ask are:
1. Are you controlling a motor?
2. Do you need to remotely control the motor?
Based on the answer to these two questions, you will be able to easily navigate
through the various power control devices and accessories to narrow your selection.
Interviewing the
Customer
When you are selecting a device to recommend to the customer, you will need to
get a considerable amount of information about the customer’s application.
Consult the chart below for the type of device, and obtain customer data for each
topic. Some sample responses are given to help the customer understand what you
are asking about.
Contactor
Type of load (motor, lighting)
AC Manual Starter
Size (HP, amps)
Motor Starter
Starter type (FVR, RVS)
Size (HP, amps)
Motor Voltage (230, 460)
Motor voltage (230, 460)
Enclosure type
(NEMA 1, 12 etc.)
Horsepower (starter size or
amperage)
Enclosure type
(NEMA 1, 12 etc.)
Control voltage
Number of poles
Motor voltage (230, 460)
Common or separate control
If separate,
control voltage (24, 120)
Enclosure type
Needed accessories
Full Load Amps of motor
(for heater selection)
27
STARTERS
NEMA or IEC?
Part of your recommendation to the customer will be choosing a NEMA or an
IEC device. You will need to obtain the following information from the customer.
NEMA
Motor voltage (230, 460)
IEC
Motor voltage (230, 460)
Horsepower (5, 10)
Horsepower (5, 10)
Phase (1, 3)
Phase (1, 3)
Common or separate control
Common or separate control
Control voltage
Control voltage
Full Load Amps of motor
Frequency of stopping/starting
Starter type
Full Load Amps of motor
Enclosure type
Starter type
Enclosure type
Life expectancy required
There may be other considerations, depending on the application. But, remember
the general rule of thumb when dealing with IEC and NEMA: NEMA is simpler to
apply, but more expensive. IEC satisfies specific applications, and is less
expensive.
Checking the
The motor nameplate is a source of useful information. You can find the full load
Motor Nameplate current (FLC), the motor service factor (SF), and the trip class (10 or 20) there.
In the case of overload heater coil selection for continuous duty motors, refer
to the manufacturer’s tables, based on the motor’s nameplate FLC rating. The
size of the heater coil is based on the FLC, the motor SF, and trip class.
Armed with the right information from the customer, you can make an informed
recommendation to meet the customer’s needs.
28
STARTERS
Modifications
Often, a customer will ask if a contactor or motor starter can be modified. Generally,
they are looking to change its functionality. The answer is yes, as long as the
device does not require replacement.
Contactors and motor starters are easily modified by adding devices to expand
their capabilities. The modifications that can be made are analogous to adding
accessories to your car, like a CD-player. They change the performance of the
control device to meet a specific need.
Some devices that are commonly added include:
•
electrical auxiliary contacts
•
power poles
•
pneumatic timers
•
transient suppressors
•
control circuit fuse holders
29
STARTERS
REVIEW 3
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. What is the function of a starter?
______________________________________________________________
______________________________________________________________
2. :List the four main starter types.
______________________________________________________________
______________________________________________________________
______________________________________________________________
______________________________________________________________
3. Write out what organization each abbreviation stands for, then match it to the
proper definition.
NEC _________________________
A. Recommends design and testing
standards for North American electrical
devices.
NEMA ________________________
B. Recommends design and testing
standards for European electrical
devices.
UL __________________________
C. Provides product standardization
and testing.
IEC __________________________
D. A standard for specific electrical
applications.
4. In terms of application-sensitivity, explain the differences between a NEMArated device and an IEC-rated device.
______________________________________________________________
______________________________________________________________
30
STARTERS
GLOSSARY
Across-The-Line
The most commonly used general purpose starter. This
starter connects the incoming power directly to the motor
Ambient
The environmental conditions around a given piece of
equipment, such as a motor. The air temperature around a
motor is called the ambient temperature.
Arcing
A condition where high voltage leaps across the open
space between the contacts of a switch.
Armature
A component of a magnetic contactor that holds the
movable contacts.
Bimetallic Strip
A strip of two dissimilar metals that does the work of
tripping the relay on an overload in a bimetallic overload
relay.
Common Control
A control circuit that gets its power from the same source as
the motor.
Contact
The parts of a contactor that actual make and break the
electrical connection.
Contact Bounce
A condition caused by the contacts of a contactor slamming
against each other and rebounding, causing a bouncing
action.
Contactor
An operating device which connects or disconnects the
motor from the power supply. The term is used when the
power poles are operated by an electromagnetic circuit,
through the use of a coil and magnetic armature frame.
Control Circuit
The circuit that controls a relay or contactor.
Double-Break
Contacts
A pair of contacts that can open a circuit in two places
simultaneously
Electromagnet
A magnet formed by a coil of wire with an electrical current
running through it.
Eutectic Alloy
A metal that has a fixed temperature at which it changes
from a solid to a liquid.
Full Load Current
The current required by the motor to produce full-load
torque at the motor’s rated speed.
31
STARTERS
32
Heater Coil
A sensing device that monitors the heat generated by
excessive current, and by ambient temperature changes.
Inrush
The amount of current drawn when a motor is first switched
on. This can be 6 to 8 times the normal running current
draw.
Knife Blade
Switch
The first device used to stop and start an electric motor.
These were levers that would drop a strip of metal onto a
contact to make the electric circuit.
Load
The device being powered, such as a motor or heater.
Locked Rotor
A condition that occurs when a motor is so overloaded that
the rotor cannot turn, no matter how much current it draws.
Locked Rotor
Amps
The maximum amount of current a motor can draw when it
is so overloaded that the rotor cannot turn. This is generally
enough current to cause the insulation to fail and the motor
to burn up.
Low Voltage
Protection
In a three wire control setup, when the voltage on L1 - L2
drops to a low value, and is then restored, the contactor will
remain open.
Motor Service
Factor
The amount of extra horsepower a motor can generate
without overheating. This is typically expressed as 1.15.
Multispeed Starter
A starter designed to be operated at constant frequency
and voltage. It uses a motor with windings that may be
reconnected to form different numbers of poles to change
speed.
Overload
The application of excessive load to a motor.
Overload
Protection
A device or system that prevents an electric motor from
drawing too much current, overheating, and literally
“burning out.”
Overload Relay
A relay that responds to electrical overloads and operates
at a preset value.
Oxide
A build-up that forms over time on contacts that are
repeatedly opened and closed.
STARTERS
REFERENCE
Power Circuit
The part of a relay that actually provides power to the
output component (such as a motor).
Reduced Voltage
Starter
A starter used in applications that typically involve large
horsepower motors. It is used to reduce the inrush current
and limit the torque output and mechanical stress on the
load.
Reversing Starter
A starter that reverses a motor by reversing any two leads
to the motor.
Separate Control
A control circuit that gets its power from a separate source,
usually lower in voltage than the motor’s power source
Solid State
No moving, mechanical parts.
Starter
A device that controls the use of electrical power to
equipment, usually a motor.
Synchronous
Speed
The speed at which a motor is designed to run.
Trip/Tripping
The action an overload relay takes to protect a motor.
Trip Class
The maximum time in seconds at which the overload relay
will trip when the carrying current is at 600% of its current
rating. A Class 20 relay will trip in 20 seconds or less.
In preparing this training module, some material was taken from the publication
listed below:
Gary Rockis and Glenn A. Mazur, Electrical Motor Controls (Homewood, IL:
American Technical Publishers, Inc., 1997).
33
STARTERS
REVIEW 1
ANSWERS
1. Answer should basically say: “1. Arcing corroded the soft copper switches with
pits. Dirt and moisture compounded the problem. 2. As motors became larger, It
was physically dangerous to handle the switch.”
2. A contactor connects or disconnects the motor from the power supply.
3. Answer should basically say: “A magnetic contactor is operated
electromechanically without manual intervention. This means that the contactor
can be operated remotely, without the need for putting a person in a potentially
dangerous location.”
4. Answer should basically say: “1. As contacts open and close, an electrical arc is
created between them. The arcs produce additional heat, which, if continued,
can damage the contact surfaces. 2. Modern contactors close so quickly and
with such energy that the contacts slam against each other and rebound,
causing a bouncing action. When the contact bounces away, a secondary arc is
created.”
REVIEW 2
ANSWERS
1. Answer should basically say: “Overload protection prevents an electric motor
from drawing too much current, overheating, and literally ‘burning out’.”
2. Resting:
Starting:
Operating under load:
0%
600-800%
100%
3. Answer should basically say: “A motor is designed to run at a certain speed,
called its synchronous speed. If the load on the motor increases, the motor
draws more current to continue running at its synchronous speed. It is quite
possible to put so much load on a motor that it will draw more and more current
without being able to reach synchronous speed. If this happens for a long
enough period of time, the motor can melt its insulation and burn out.”
4. Answer should basically say: “A fuse or circuit breaker sized to handle the
normal running load of the motor will open the circuit during startup. Sizing the
fuse or circuit breaker for the spike in current draw would not permit smaller
overloads to trip the breakers, and the motor would burn out.”
5. Answer should basically say: The eutectic overload relay measures the
temperature of the motor by monitoring the amount of current being drawn
through a heater coil. The bimetallic overload relay uses a bimetallic strip to
apply tension to a spring on a contact. If heat begins to rise, the strip bends,
and the spring pulls the contacts apart, breaking the circuit. The solid state
overload relay does not actually generate heat to facilitate a trip. Instead, it
measures current or a change in resistance. The advantage of this method is
that the overload relay doesn't waste energy generating heat.”
6. the maximum time in seconds at which the overload relay will trip when the
carrying current is at 600% of its current rating
34
STARTERS
REVIEW 3
ANSWERS
1. Answer should basically say: “A starter lets you turn an electric motor (or motorcontrolled electrical equipment) on or off, while providing overload protection.”
2. Across The Line, Reversing, Multi-Speed, Reduced Voltage
3. NEC is National Electrical Code, matches with letter D.
NEMA is National Electrical Manufacturers Association, matches with letter A.
UL is Underwriters Laboratories, Inc, matches with letter C.
IEC is International Electro-technical Commission, matches with letter B.
4. NEMA is simpler to apply, but more expensive. IEC satisfies specific
applications, and is less expensive.
35
Cutler-Hammer
Milwaukee, Wisconsin U.S.A.
Publication No. TR.08.02.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.
Was this manual useful for you? yes no
Thank you for your participation!

* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project

Download PDF

advertising