Industrial Power Protection & Controls
June 2021
Pushbutton
A pushbutton is a switch activated by finger pressure.
Two or more contacts open or close when the button is depressed. Push-buttons are usually
spring loaded so as to return to their normal position when pressure is re moved.
Control relays
A control relay is an electromagnetic switch that opens and closes a set of contacts when the
relay coil is energized. The relay coil produces a strong magnetic field which attracts a movable
armature bearing the contacts.
Control relays are mainly used in low-power circuits.
Power relays
Power relays come as the ideal solution for all applications requiring small, low-noise relays or
contactors at low costs. The power relays are suitable for basic controls and particularly for use
in large-scale series devices and controls. They are ideal for applications which require only one
auxiliary contact and no overload relay – and place increased requirements upon switching
capacity, switching voltage and service life. They include time-delay relays whose contacts open
or close after a definite time interval. Thus, a time-delay closing relay actuates its contacts after
the relay coil has been energized. On the other hand, a time-delay opening relay actuates its
contacts some time after the relay coil has been de-energized
Thermal relays
A thermal relay (or overload is a temperature sensitive device whose contacts open or close
when the motor current exceeds a preset limit. The current flows through a small, calibrated
heating element which raises the temperature of the relay. Thermal relays are inherent time-delay
devices because the temperature cannot follow the instantaneous changes in current.
Normally open and normally closed contacts
Control circuit diagrams always show components in a state of rest, that is, when they are not
energized (electrically) or activated (mechanically). In this state, some electrical contacts are
open while others are closed. They are respectively called normally open contacts (NO) and
normally closed contacts (NC) and are designated by the following symbols:
Normally Open contact (NO) Normally Closed contact (NC)
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Contactor
A contactor is an electrically controlled switch used for switching a power circuit, similar to a
relay except with higher current ratings.
They are used to connect and break power supply lines running through power lines or
repeatedly establish and interrupt electrical power circuits. These are used in light loads as well
as in complex machine controls. It can be considered as an intersection point between the control
circuit and power circuit because it is controlled by the control circuit, it also controls the circuit
between power and loads. Contactors are used to control electric motors, lighting, heating,
capacitor banks, thermal evaporators, and other electrical loads.
Components of a Contactor
The following three are crucial components of the contactor:
1. Coil or Electromagnet: This is the most crucial component of a contactor. The driving force
that is required to close the contacts is provided by the coil or electromagnet of the contactor.
The coil or electromagnet and contacts are protected by an enclosure.
2. Enclosure: Just like the enclosures used in any other application, contactors also feature an
enclosure, which provides insulation and protection from personnel touching the contacts. The
protective enclosure is made from different materials, such as polycarbonate, polyester, Nylon 6,
Bakelite, thermosetting plastics, and others. Generally, the open-frame contactor features an
additional enclosure, which protects the device from bad weather, hazards of explosion, dust, and
oil.
3. Contacts: The current carrying task of the contactor is done by the contacts. There are different
types of contacts in a contactor namely, contact springs, auxiliary contacts, and power contacts.
Each type of contact has an individual role to play.
The construction of a contactor.
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Operating Principle of a Contactor
An electromagnetic field is generated whenever current flows where the moving coils attract
each other. A large amount of current is drawn initially by an electromagnetic coil; this current
excites the electromagnet. The moving contact is pushed forward by moving core, as a result, the
force created by the electromagnet holds the moving and fixed contacts together there by
completing the circuit between the fixed contacts and the moving contacts. This permits the
current to pass through these contacts to the load.
When current is removed, the coil is de-energized and opens the circuit where the contactor coil
gravity or spring moves back the electro-magnetic coil to its initial position and there is no flow
of current in the circuit. The contacts of the contactors are known for their rapid open and close
action.
If contactors are energized with AC current, a small portion of the coil is the shaded coil, where
the magnetic flux in the core is slightly delayed. This effect is too average as it prevents the core
from buzzing at twice line frequency. There are internal tipping point processes to ensure rapid
action so that contactors can open and closed very rapidly.
From the block diagram below, the supply is given using a switch, that is when the switch is
closed current flows through the contactor coil and attaches the moving core. The contactor
attached to the moving core closes and the motor starts running. When the switch is released the
electromagnetic energizes spring arrangement pause the moving coil back to its initial position
and power supply to the motor is cut off.
The block diagram of a contactor.
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Types of Contactors
These are classified based on three factors;
 The load being used.
 The current capacity.
 The power rating.
1. Knife Blade Switch
It is the first contactor used to control an electric motor in the late 1800s. It consists of a metal
strip, which acts as a switch in connecting and disconnecting the connection. This switch had a
lever for pulling the switch down or pushing it up.
But the disadvantage of this method is that it switching process since it caused the contacts to
wear out quickly, since it was difficult to manually open and close the switch fast enough to
avoid arcing. As a result of this, the soft copper switches underwent corrosion, which further
made them vulnerable to moisture and dirt. Over the years, the size of the motors increased
which further created the need for larger currents to operate them, which leads to high physical
damage.
2. Manual Contactor
Since the knife blade switch became potentially dangerous to use, another contactor device,
which offered a number of features that were missing in the knife blade switch was developed.
This device was referred to as a manual controller.
These features included:
 Safe to operate
 Non-exposed unit, which is properly encased
 Physically smaller size
 Single break contacts replaced with double break contacts
As their name implies, double break contacts can open the circuit in two places at the same time.
Thus, even in smaller space, it allows you to work with more current. Double break contacts
divide the connection in such a way that it forms two sets of contacts.
The switch or button of the manual controller is not operated remotely and is attached to the
controller physically.
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The power circuit is engaged once the manual controller is activated by an operator. Once
activated, it carries the electricity to the load. Soon, manual contactors replaced knife blade
switches completely, and even today different variations of these types of contactors are being
used.
3. Magnetic Contactor
The magnetic contactor does not require human intervention and operates electromechanically.
This is one of the most advanced designs of a contactor, which can be operated remotely. Thus, it
helps eliminate the risks involved in operating it manually and putting operating personnel in
potential danger. Only a small amount of control current is required by the magnetic contactor to
open or close the circuit. This is the most common type of contactor used in industrial control
applications.
Difference between AC Contactors and DC Contactors
AC Contactors
DC Contactors
They are designed for the contactors with selfextinguishing arc is drawn whenever the contact
opens
They don’t use freewheel diode
Separation time is less
They are specially designed to suppress electrical
arching when there is switching in the DC circuit.
They use freewheel diode
Separation time is higher if the load is heavy a
shunt load is attached to the main contact.
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Advantages of a Contactor
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Disadvantages of a Contactor
Fast switching operation
Suitable for both AC and DC devices
Simple in construction.
High load capacity
Low power consumption
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In the absence of magnetic-filed, the coil
may burn
Aging of components causes corrosion
of materials when exposed to moisture.
Absence of overload protection
Application of Contactors
1. Lighting Control
Contactors are often used to provide central control of large lighting installations, such as an office
building or retail building. To reduce power consumption in the contactor coils, latching contactors are
used, which have two operating coils. One coil, momentarily energized, closes the power circuit contacts,
which are then mechanically held closed; the second coil opens the contacts
2. Electric Motor Starter
Contactors can be used as a magnetic starter. A magnetic starter is a device designed to provide power to
electric motors. It includes a contactor as an essential component, while also providing power-cutoff,
under-voltage, and overload protection.
3. Vacuum contactor
Vacuum contactors utilize vacuum bottle encapsulated contacts to suppress the arc.
This arc suppression allows the contacts to be much smaller and use less space than air break
contacts at higher currents. As the contacts are encapsulated, vacuum contactors are used fairly
extensively in dirty applications, such as mining. Vacuum contactors are only applicable for use
in AC systems.
4. Mercury relay
5. Mercury-wetted relay
How to Choose a Correct Replacement for a Contactor?
 Firstly, one should check the coil voltage, which is a voltage used to energize the contactor.
 Checking for auxiliary contacts available, that is how many open and closed nodes are used in the
contactor.
 Checking the rating which is mentioned in a table format on it.
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Control of Electric Motor with Contactor
When a relay is used to switch a large amount of electrical power through its contacts, it is designated by
a special name: contactor. Contactors typically have multiple contacts, and those contacts are usually (but
not always) normally-open, so that power to the load is shut off when the coil is de-energized. Perhaps the
most common industrial use for contactors is the control of electric motors.
The top three contacts switch the respective phases of the incoming 3-phase AC power, typically at least
480 Volts for motors 1 horsepower or greater. The lowest contact is an “auxiliary” contact which has a
current rating much lower than that of the large motor power contacts, but is actuated by the same
armature as the power contacts. The auxiliary contact is often used in a relay logic circuit, or for some
other part of the motor control scheme, typically switching 120 Volt AC power instead of the motor
voltage. One contactor may have several auxiliary contacts, either normally-open or normally-closed if
required.
The three “opposed-question-mark” shaped devices in series with each phase going to the motor are
called overload heaters. Each “heater” element is a low-resistance strip of metal intended to heat up as
the motor draws current. If the temperature of any of these heater elements reaches a critical point
(equivalent to a moderate overloading of the motor), a normally-closed switch contact (not shown in the
diagram) will spring open. This normally-closed contact is usually connected in series with the relay coil,
so that when it opens the relay will automatically de-energize, thereby shutting off power to the motor.
Overload heaters are intended to provide overcurrent protection for large electric motors, unlike circuit
breakers and fuses which serve the primary purpose of providing overcurrent protection for power
conductors.
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Jogging Control Circuits
Sometimes called “inching,” jogging is the term given to the momentary energization of a motor
only so long as an operator is pressing a button.
A jog circuit is a circuit that allows an operator to either start the motor or “jog” the motor and
are commonly used for motors controlling conveyor belts to allow for precise positioning of
materials.
Any motor starter that is used to jog a motor will be subjected to repetitive inrush currents,
which can cause overheating of the power contacts. If a motor is expected to be jogged more
than five times in a minute, the motor starter should be increased in size and horsepower rating
for this more severe operating condition.
To achieve a jog function, there are several common circuit designs, each with their own
advantages and disadvantages. A common feature that all jog circuits have is that they have some
method of disabling the holding contact used in the three-wire circuit. This is usually
accomplished by putting some component in series with the holding contact, such as a switch or
momentary pushbutton.
i)
Jog Circuit with Selector Switch
The most basic of the jog circuits, this is essentially a three-wire circuit with an SPST (singlepole, single-throw) switch connected in series with the holding contact.
In the closed position, the SPST switch offers no opposition to the flow of current and the circuit
behaves the same as a standard three-wire circuit. The normally open push button is acting as a
“start” or “run” button.
If the SPST switch is opened, then it has introduced an open in series with the 2-3 holding
contacts, effectively removing them from the circuit. Without the holding contact, the motor
starter will only be energized as long as an operator is pressing the normally open pushbutton,
which acts as a “jog” button in this position.
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The main advantage of this circuit is the ease of installation and the cheapness of equipment.
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The main disadvantage is that you must change the position of your selector switch to change
the function of your button.
ii)
Dangerous Jog Circuit
This uses a four-contact momentary pushbutton as the “Jog” button. This button has one set
of normally closed contacts which are wired in series with the 2-3 holding contact, and one set
of normally open contacts which are in series with only the stop button and the motor starter.
In normal operation, the stop and start buttons provide their standard functions in a three-wire
circuit, as the 2-3 holding contact is in series with the normally closed contacts of the jog button.
If the jog button is pressed, the normally closed contacts will open and the normally open
contacts will close, providing a path for current to energize the motor starter.
When the motor starter is energized, all contacts associated with it will change their state,
including the 2-3 holding contact, but because the jog button is being depressed the holding
contact cannot maintain power to the starter. Once the jog button is released the motor comes to
a stop.
This circuit is sometimes referred to as the “dangerous jog circuit” for reasons which may appear
obvious now. If the jog button’s normally closed contacts are able to return to their normal
condition before the motor starter’s armature has had a chance to drop out, then the coil will
remain energized and the motor will continue to run. This is dangerous because if an operator
pushes a jog button, expecting the motor to stop when they release the button, and the motor
continues to run, it could introduce a hazard to a person caught off guard. In short, we never
want machines to surprise people.
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This circuit has the advantage of being simple to install and has separate buttons dedicated to
starting and jogging the motor.
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The main disadvantage is the hazard introduced by the quick return of the jog button to its
normal state.
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iii)
Jog Circuit with Control Relay
A more sophisticated jogging circuit uses a control relay as shown in the figure below. Control
relays behave just like motor starters but lack overload protection and power contacts. Control
relays are loads that must be connected in parallel with the motor starter to ensure they get their
rated voltage.
In any schematic diagram, the current must find its way from Line 1 to Line 2 and energize
only one load along the way. Switches offer either infinite resistance when they are open or zero
resistance when they are closed, so some device must limit the current to prevent short circuits.
Notice that the current that passes through the control relay does not pass through the motor
starter. This means that they will both get their rated value of voltage and pull in their armatures.
As a rule, we NEVER connect loads in series.
Under normal conditions, if the start button is pressed, current will be able to complete the circuit
and energize the control relay. Once the relay is energized, the two normally open contacts
associated with it will change their state and close. This will provide a path for current to
energize the motor starter, closing the 2-3 holding contacts and running the motor.
The circuit will continue to operate as a standard three-wire circuit providing low-voltage
protection (LVP) until either the stop button is pressed, or an overload occurs.
If the jog button is pressed while the motor is running, there will be no change to the circuit.
If, however, the jog button is pressed while the motor is not running, it will provide a path for
current to energize the motor starter. The current will not be able to energize the control relay,
and so once the jog button is released, the starter drops out and the motor comes to a stop.
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The advantage of this circuit lies in its safety and reliability. Having two separate buttons for
“run” and “jog” functions increases ease of control for the operator.
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The disadvantage is the additional cost and installation time associated with the control relay and
additional current drawn in the control circuit due to the second coil.
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Electrical Interlocks
Electrical interlocking is accomplished by installing the normally closed contact of one
direction’s coil in series with the opposite direction’s coil, and vice versa. This ensures that when
the forward coil is energized, pushing the reverse pushbutton will not energize the reverse coil.
The same situation is in effect when the reverse coil is energized. In both situations the stop
button will need to be pressed to de-energize the running coil and return all its auxiliary contacts
back to their original state. Then the opposite direction coil can be engaged.
Pushbutton Interlocks
Pushbutton interlocking requires the use of four-contact momentary push buttons with each
pushbutton having a set of normally open and normally closed contacts.
To achieve pushbutton interlocking, simply wire the normally closed contacts of one pushbutton
in series with the normally open contacts of the other pushbutton, and the holding contacts will
be connected in parallel with the appropriate button’s normally open contacts.
This circuit still requires the installation of electrical interlocks.
Pushbutton interlocking doesn’t require the motor coils to be disengaged before reversing
direction because the normally closed forward contacts are in series with the normally open
reverse contacts, and vice-versa. Pushing one button simultaneously disengages one coil while
starting the other. This sudden reversal (plugging) can be hard on the motor, but if quick reversal
of the motor is required, this circuit can be a solution.
Mechanical Interlocks
Forward / reverse starters must never close their power contacts simultaneously. The best way to
provide this is through electrical interlocks, which prevent the one coil from being energized if
the other is engaged. A failure in electrical interlocking can cause both coils to be energized at
the same time.
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If both become energized, some form of mechanical interlock is required to prevent
both armatures from pulling in. Represented on schematic diagrams as a dotted line between
the two coils, a mechanical interlock is a physical barrier that is pushed into the path of one
coil’s armature by the movement of the adjacent coil. This means that even if both coils are
energized, only one armature will be able to pull in fully. The coil that is prevented from pulling
in will make a terrible chattering sound as it tries to complete the magnetic circuit.
Mechanical interlocks should be relied on as a last resort for protection.
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