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Electrical Workshop practice II –EEng -3203
2011
Push Buttons - momentary contact switches
A push-button is a simple switch mechanism for controlling some aspect of a machine or
a process. Buttons are typically made out of hard material, usually plastic or metal. The
surface is usually flat or shaped to accommodate the human finger or hand, so as to be
easily depressed or pushed. Buttons are most often biased switches, though even many
un-biased buttons (due to their physical nature) require a spring to return to their unpushed state. Different people use different terms for the "pushing" of the button, such as
press, depress, mash, and punch. Different types of push buttons are shown below in
figure 1.1.
Figure 1.1. Push –buttons
Push buttons can be either normally open (NO) or normally (NC). Some types can be
stacked to have multiple sets of NO and NC contacts.
Open State: The pins on either side are electrically the same point. With the button
released, there is no path for electrons between pins 1, 4 and 2, 3.
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Closed State: With the button pressed, a conductive material bridges the gap allowing
electrons, and thus current, to flow.
Indicator Light Symbols
An indicator light (sometimes referred to as a pilot light) is a small electric light
used to indicate a specific condition of a circuit. For example, a red light might be
used to indicate a motor is running. The letter in the center of the indicator light
symbol indicates the color of the light.
Different types of push buttons are shown below in figure 1.2.
Figure 1.2.Signaling lamps
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Electromagnetic Relays
- An electromagnetic relay is a switch, which is driven electromagnetically and is
used in cases of low power control. Those relays are mainly applied in control circuits,
communication systems and data transmission. The structure of a relay is shown in
fig.1.3.
Figure .1.3.Structure of a Relay.
Function:
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An octal base relay (like the one shown fig 1.4 below) is one of the most common
electrical devices in use today. Also referred to as general purpose relays, they're widely
available in both 8 and 11 pin models, with 8 being the most common. The base of the
relay is designed to plug into a socket, which makes installation and replacement (if
required) very easy.
Figure 1.4 An octal base relay
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This basic relay is constructed of 5 main parts the Coil, Armature, Contacts, the Base
(which consists of the socket pins), and the Molded Plastic Frame.
The relay works on the principle of electromagnetic force. When the coil is energized, it
becomes magnetized. The armature (made of a ferromagnetic material and in close
proximity to the coil) - is attracted to the coil by this magnetic force and moves towards it
until it comes to rest against the coil's iron core.
Attached to the pivoting end of the armature is a spring. The purpose of the spring is to
return the armature to its original position (away from the coil) when the coil is deenergized. Also attached to the armature are the arms of the movable set of contacts (the
common contacts). See the illustration in fig.1.5 below:
Figure 1.5 An 8 pin Relay
In an 8 pin relay (as shown in fig.1.5) there are 2 common contacts, 2 normally open
contacts, and 2 normally closed contacts. In an 11 pin relay, there are 3 of each of the
aforementioned. These contacts are made of an electrically conductive material such as
copper. The common contacts have this material embedded on both sides of the movable
arms. The relationship of all of the contacts is explained as follows:
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The common contacts carry the supply voltage that is to be connected to another
electrical device(s). In the de-energized state of the relay, these common contacts are in
contact with (touching) the normally closed contacts.
Before we go too far, it's important to think of a relay as an electrical switch. That is, a
remote controlled switch, designed to direct the current path from one part of the circuit
to another. You must also understand that although there are (in this case) 2 of each type
of contact (common, normally open and normally closed), each is designed to complete a
path independent and separate from the other contact of similar type.
Also, when we use the term "normally closed" it means that in the normal state of the
relay (de-energized) the common contacts are providing conductive paths to their
normally closed contact partners and only to these contacts. At the same time, there is no
completed path to the "normally open" contacts.
So then, when the relay is de-energized, the common contact #1 is making contact with
the normally closed contact #1 and the common contact #2 is completing a circuit path
with the normally closed contact #2. When the relay is energized, the situation is
reversed. Now the common contact #1 is completing a conductive path to the normally
open contact #1 and at the same time, the common contact #2 is making contact with the
normally open contact #2. The electrical conductive paths that did exist with the
"normally closed" contacts have now been "opened". The paths no longer exist. The Pin
Out connections on the base of the relay are as follows:
Base of Octal Pin Relay
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Time Delay Relay
-A time delay relay is a relay that stays ON for a certain amount of time once activated.
This time delay relay is made up of a simple adjustable timer circuit which controls the
actual relay. The time is adjustable from 0 about certain seconds or hours with the parts
specified. Fig.1.6 and Fig.1.7 shows analog and digital display relays.
Figure 1.6. Analog display relays
Figure 1.7. Digital display relays
Motor –Protective Relays
Thermal Overload Relays:
Overload protection (as shown in fig.1.8) is installed in the motor circuit and/or motor to
protect the motor from damage from mechanical overload conditions when it is
operating/running. The effect of an overload is an excessive rise in temperature in the
motor windings due to current higher than full load current. Properly sized overload
protection disconnects the motor from the power supply when the heat generated in the
motor circuit or windings approaches a damaging level for any reason. Since the switch
contacts remain closed if power is removed from the circuit without operating the switch,
the motor restarts when power is reapplied which can be a safety concern.
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Thermal Overload Relays:
 Allow harmless temporary overloads (such as motor starting) without disrupting
the circuit.
 Will trip and open a circuit if current is high enough to cause motor damage over a
period of time.
 Can be reset once the overload is removed.
Figure 1.8 Thermal Overload Relay
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2. Contactors
-A contactor is an electromagnetically driven switch applied for electrical power
engineering.
Figure 1.9 Structure of a Contactor
Function:
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Contactors may have different arrangements of contacts. One has to differentiate
between primary contacts (main contacts) and secondary contacts (auxiliary / control
contacts).as shown in the fig.2.1.
Figure 2.1. Arrangements of contacts
Figure 2.2 parts of a contactor
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Figure 2.3. A contactor with two one NO and one NC auxiliary contacts.
You can control contactor circuits by timers with on –or off delay. This is important for
motor control circuits like star-delta start circuit.
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Difference between Contactor and Relay

Since a contactor is required for a higher load, a relay is always cheaper than a
contactor.

A relay is normally used in appliances below 5KW, while a contactor is preferred
when the appliance is heavier.

A relay is used only in control circuit while a contactor can be used in both
control and power circuits.

In general contactors are little slower than relays

Contactor is so designed that it can be repaired while it is not normally done in
the case of relays.
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Motor starting Methods
a) Direct-on-line starting (D.O.L).
This is by far the most common starting method. The starting equipment consists of only
a main Contactor and thermal or electronic overload relay as shown in fig.2.4 below.
Figure 2.4 Direct-on-line starting (D.O.L).
This stating method is not used for motors with power rating of greater than 5KW. Thus
is due to high starting current and starting torque.
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D.O.L-Power and control circuit
Figure 2.5 D.O.L starting power and control circuits
As shown in Fig.2.5 power circuit, when the coil of the contactor energized the
contactor closes the contacts and the motor receives a three phase supply. In the
control circuit of Fig.2.3 separate push-buttons are provided for energising and deenergising a contactor. For starting, a push-button with an NO contact (S3) and for
stopping another push-button with an NC contact (S2) is used. A normally open (NO)
auxiliary contact (-K1-13-14) of the contactor is connected in parallel with the start
push-button S3 so that the contactor should remain energised even when the pressure
on the start push-button is with drown. Thus the contactor would continue to remain
energized by receiving supply through its own contact –K1. Contact –K1 here has been
used as a self-hold -on contact as it holds on the supply even after the pressure on the
push-button has been released. Thus the motor continues to rotate once the start push
button has been pressed. For stopping the motor S2 is pressed. Here –S1 is the
emergency switch and –F5 is overload relay for overload protection.
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b) Star -Delta starting principle
Figure 2.6 Star- delta starting contactors, push-buttons, and overload relays
What is Star Delta starting?
 Star Delta starting is when the motor is connected (normally externally from the
motor) in STAR during the starting sequence. When the motor has accelerated to
close to the normal running speed, the motor is connected in DELTA. Pictures 1
and 2 show the two connections for a series connected three phase motor and
fig.2.5 show the overall power circuit of star-delta.
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Why is Star Delta Starting used?
 Let’s consider an example motor: 120kW, 4 Pole, 380 Volt, Delta connected, 3
Phase, 50 Hz. To truly grasp the differences between these two starting methods,
we will list the values next to each other in table.
Immediately we notice the primary reasons for using star delta starters on electric motors:
 The starting power is reduced from 98 kW to 33 kW (by approximately 67%).
 The starting current is reduced from 1495 A to 500 A (by approximately 67%).
 Because the motor is not intended to actually run in this connection, the reduction
in full load speed, power factor and efficiency is not significant for this discussion.
The reason for these 67% changes becomes clear when we examine the phase
voltage on the motor, we see that the phase voltage when the motor is connected in
Delta is 380 Volt.
 When the motor is however connected in Star, the Phase Voltage will be 219.3
Volt. Thus, when the motor is started in the star connection, the phase voltage of
the motor is reduced by a factor of √3.
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The relations for star and delta connections are as listed in a Table as follow.
The reductions in starting current, starting power and starting torques for a reduced
Voltage can each be calculated as follow:
If we apply this equation for the star delta starting,
What are the advantages of using Star Delta starting?
- The most significant advantage of using Star-Delta starting is the huge reduction in the
starting current of the motor, which will result in a significant cost saving on cables,
transformers and switch gear.
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Power and Control circuit of Star-Delta starting.
Figure 2.7 Power and Control circuit of Star-Delta starting
As shown in the control circuit of fig.2.7 when push-button –S2 is pressed contactor –K3
energised thus, intern makes NO(13,14) of contactor –K3 to close and NC(21,22) to
open, as a result contactor –K1 energised and contactor –K2 de- energised . This helps
the motor to run in star connection. When push-button –S3 is pressed contactor –K3
de-energised, both –K1 and –K2 energised, and the motor operated in delta.
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Figure 2.8 Star- delta control circuit using timer
As shown in the control circuit of fig.2.8 when push-button –S2 is pressed both
contactors –K3 and –K1 energised and the motor runs in star after a pre defined time NC
contact of the off delay timer (-K4T) opens, thus makes contactor –K3 to de-energise and
contactors -K2 and -K1 to energise, and the motor operates in delta.
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Reversing a Three-Phase Motor
The rotation of a three phase motor can be reversed by interchanging the connection of
any motor terminals to the supply lines. As shown in the fig.2.7, two separate contactor
are used for this purpose, one for forward and another for reverse operation. To
prevent both the contactors from being energized simultaneously and causing a short
circuit, some preventive methods called interlocking methods are employed. Two
different interlocking methods generally used are described below.
Figure 2.9 Power and Control circuit of motor reversing
Push button interlocking
Push-button interlocking is one of the methods of preventing two contactors from being
energising simultaneously. As shown in fig.2.9 power circuit, through contactor –K1, the
motor is fed with a three phase supply for forward rotation whereas through contactor
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–K2, the motor is fed for reverse direction rotation. Both the NO and NC contacts of the
forward and reverse Push-button have been used. When the forward Push-button (-S3)
is pressed, contactor –K1 get energized by getting supply through the NC contact of the
reverse Push-button (-S2). The contactor remains energized even when the pressure on
the Push-button is withdrawn as the NO contact (-K1:13-14) of the contactor –K1 is now
closed (this is called self-hold-on contact). When the reverse Push-button (-S2) is
pressed contactor –K1 gets de-energised first and then contactor –K2 B gets energized.
This ensures that it is not possible to energise both the contactors simultaneously.
Auxiliary contact interlocking
In this method, interlocking is done by connecting a normally closed auxiliary contact
of the forward contactor in series with the coil of the reverse contactor and vice versa,
as shown in fig2.9, thus preventing simultaneous energisation of both the contactors.
When the Push-button -S3 is pressed, contactor –K1 get energized and provides a threephase supply to the motor for forward operation. When pressure on the Push-button
-S3 is released the contactor remains energized because of the closing of the self hold
on contact -K1. It should be noted that when a contactor –K1 is energized it is not
possible to energise contactor -K2 because NC of contactor –K1 is used in contactor -K2.
Similarly, when contactor –K2 is energized it is not possible to energized contactor –K1
because NC of contactor –K2 is used in contactor –K1. In this circuit changing the
direction of the motor is achieved either by using the stop push button in between or by
using push-buttons –S2 and –S2. It is necessary to stop the motor before receiving its
direction of rotation. The motor stops running when stop push-button (–S1) is pressed.
Sequence start of motors
Sequence control of motors is required in situations where it is necessary to start a
particular motor first before the second motor can be started. This type of control is
necessary where one machine, before starting must have all its auxiliary equipments
operating. For example in a grinding machine, the auxiliary equipments such as the
coolant pump and the lubricant pump should start operating before the rotation of the
grinding wheel.
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Figure 2.1.1 power and control circuit of sequence start of two motors
Let us consider the operation of two motors one (-K1) and two (-K2) in sequence. The
condition is such that it should not be possible to start motor two unless motor one is
first started. The control circuit is shown in fig.2.1.1 It is to be noted that contactor –K2
has been connected to the supply through –K1:23, 24 NO contact of contactor –K1.
Hence, unless contactor –K1 is energized, which closes –K1:23, 24 NO contact, it is not
possible to energise contactor –K2 by pressing Push-button S2. In the same manner it
should not be possible to stop motor one unless motor two is first stopped. Hence
contactor –K1 has been connected to the supply through –K2:13, 14 NO contact of
contactor –K2 it is not possible to de-energise contactor –K1 by pressing Push-button S4.
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