ECEG-3153: Electrical Workshop Practice II
Magnetic switch circuits basic components
Contactors
A contactor is an electrically controlled switch used for
switching a power circuit, similar to a relay except with
higher current ratings. A contactor is controlled by a circuit
which has a much lower power level than the switched
circuit. Contactors range from those having a breaking current of
several amperes to thousands of amperes and 24 V DC to many
kilovolts. The physical size of contactors ranges from a device small
enough to pick up with one hand, to large devices approximately a
meter (yard) on a side.
Unlike general-purpose relays, contactors are designed to be directly
connected to high-current load devices. Relays tend to be of lower
capacity and are usually designed for both normally closed and
normally open applications. Devices switching more than 15 amperes
or in circuits rated more than a few kilowatts are usually called
contactors. Apart from optional auxiliary low current contacts,
contactors are almost exclusively fitted with normally open ("form A")
contacts. Unlike relays, contactors are designed with features to
control and suppress the arc produced when interrupting heavy motor
currents.
When current passes through the electromagnet, a magnetic field is produced, which attracts the moving
core of the contactor. The electromagnet coil draws more current initially, until its inductance increases
when the metal core enters the coil. The moving contact is propelled by the moving core; the force
developed by the electromagnet holds the moving and fixed contacts together. When the contactor coil
is de-energized, gravity or a spring returns the electromagnet core to its initial position and opens the
contacts.
Overload relays
Overload relays are designed to meet the special
protective needs of motor control circuits.
Overload relays:
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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
Trip Class Overload relays are rated by a trip class which defines the length of time it will take for the relay
to trip in an overload condition. The most common trip classes are Class 10, Class 20, and Class 30. A Class
10 overload relay, for example, has to trip the motor off line in 10 seconds or less at 600% of the full load
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ECEG-3153: Electrical Workshop Practice II
amps (which is usually sufficient time for the motor to reach full speed). Many industrial loads, particularly
high inertia loads, require Class 30. Siemens offers overload relays in all three trip classes.
Overload Relay in a Motor Circuit:
Current flows through the overload relay
while the motor is running. Excess
current will cause the overload relay to
trip at a predetermined level, opening
the circuit between the power source
and the motor. After a predetermined
amount of time, the overload relay can
be reset. When the cause of the overload has been identified and corrected, the motor can be restarted.
Bimetal Overload Relays: Overload protection can be accomplished with the use of a bimetal overload
relay. This component consists of a small heater element wired in series with the motor and a bimetal
strip that can be used as a trip lever. The bimetal strip is made of two dissimilar metals bonded together.
The two metals have different thermal expansion characteristics, so the bimetal strip bends at a given
rate when heated.
Under normal operating conditions, the heat generated by the heater element will be insufficient to cause
the bimetal strip to bend enough to trip the overload relay.
As current rises, heat also rises. The hotter the bimetal strip becomes, the more it bends. In an overload
condition, the heat generated from the heater will cause the bimetal strip to bend until the mechanism is
tripped, stopping the motor.
Timers (Time Delay Relays)
In a standard control relay, contacts close immediately when voltage is
applied to the coil, and open immediately when voltage is removed. In a
variety of applications, it’s desirable to have the operation of the contacts
delayed following application or removal of voltage. A TDR solves the
problem handily. However, some TDRs postpone closing of the contacts
after voltage is applied while others close the contacts — and then reopen them after a delay.
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ECEG-3153: Electrical Workshop Practice II
Traditionally, TDRs were available only as single-function, single-time-range
devices. These devices are still available and are typically used in
applications where the timing needs to be locked in. Today, many TDRs are
also available with multiple timing ranges and functions. Costing little more
than single-function devices, these TDRs also have wide control voltage
ranges. In addition, newer multifunction IEC-style timers allow for reduced
inventories. Let’s take a closer look at a few of the more common types of
TDRs.
On-delay timers
With an on-delay timer, timing begins when voltage is applied. When the time
has expired, the contacts close — and remain closed until voltage is removed
from the coil. If voltage is removed before time-out, the time delay resets
Off-delay timers
When using an off-delay timer, nothing happens when voltage is applied. Closing the control input causes
the contacts to transfer. Opening the control input causes timing to begin, and the contacts remain closed.
On time-out, the contacts transfer. Closing the control input prior to time-out causes timing to reset.
Removing voltage prior to time-out resets the timing and opens the contacts. In addition, true off-delay
timers provide this functionality (keeping contacts closed) after input voltage is lost. They have capacitors
to keep contacts closed even if the timer loses power.
On/off push buttons
A push button is a simple type of switch that controls an action
in a machine or some type of process. Most of the time, the
buttons are plastic or metal. The shape of the push button may
conform to fingers or hands for easy use, or they may simply be
flat. It all depends on the individual design. The push button can
be normally open or normally closed.
Push button switches have three parts. The actuator, stationary
contacts, and the grooves. The actuator will go all the way
through the switch and into a thin cylinder at the bottom. Inside
is a movable contact and spring. When someone presses the
button, it touches with the stationary contacts, causing the
action to take place. In some cases, the user needs to keep
holding the button, or to press it repeatedly, for an action to
take place. With other push buttons, a latch connects and keeps
the switch on until the user presses the button again.
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ECEG-3153: Electrical Workshop Practice II
Transformers
What are transformers?
A TRANSFORMER is a device that transfers electrical energy from one circuit to another by
electromagnetic induction (transformer action). The electrical energy is always transferred without a
change in frequency, but may involve changes in magnitudes of voltage and current. Because a
transformer works on the principle of electromagnetic induction, it must be used with an input source
voltage that varies in amplitude. Transformers work on only AC systems.
Working principle and main components
The main principle of operation of a
transformer is mutual inductance between
two circuits which are linked by a common
magnetic flux. A basic transformer consists of
two coils that are electrically separate and
inductive, but are magnetically linked through
a path of reluctance. The working principle of
the transformer can be understood from the
figure shown here.
The transformer has primary and secondary
windings. The core laminations are joined in
the form of strips to form the magnetic core. A mutual electro-motive force is induced in the transformer
from the alternating flux that is set up in the laminated core, due to the coil that is connected to a source
of alternating voltage. Most of the alternating flux developed by this coil is linked with the other coil and
thus produces the mutual induced electro-motive force. The so produced electro-motive force can be
explained with the help of Faraday’s laws of Electromagnetic Induction as;
e=M*dI/dt
In its most basic form a transformer consists of:
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The CORE, which provides a path for the magnetic lines of flux.
The PRIMARY WINDING, which receives energy from the ac source.
The SECONDARY WINDING, which receives energy from the primary winding and delivers it to the
load.
The ENCLOSURE, which protects the above components from dirt, moisture, and mechanical
damage.
Basic types of transformers?
1. Based on core shape
Closed-core transformers are constructed in 'core form' or 'shell form'. When windings surround the core,
the transformer is core form; when windings are surrounded by the core, the transformer is shell form.
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ECEG-3153: Electrical Workshop Practice II
Shell form design may be more prevalent than core form design for
distribution transformer applications due to the relative ease in
stacking the core around winding coils. Core form design tends to, as
a general rule, be more economical, and therefore more prevalent,
than shell form design for high voltage power transformer
applications.
In the transformer shown in the cutaway view in figure below, the
primary consists of many turns of relatively small wire. The wire is
coated with varnish so that each turn of the winding is insulated from
every other turn. In a transformer designed for high-voltage
applications, sheets of insulating material, such as paper, are placed
between the layers of windings to provide additional insulation.
When the primary winding is completely wound, it is wrapped
in insulating paper or cloth. The secondary winding is then
wound on top of the primary winding. After the secondary
winding is complete, it too is covered with insulating paper.
Next, the E and I sections of the iron core are inserted into
and around the windings as shown.
The leads from the windings are normally brought out
through a hole in the enclosure of the transformer.
Sometimes, terminals may be provided on the enclosure for
connections to the windings. The figure shows four leads, two
from the primary and two from the secondary. These leads
are to be connected to the source and load, respectively.
2. Based on core material
The composition of a transformer core depends on such factors as voltage, current, and frequency. Size
limitations and construction costs are also factors to be considered. Commonly used core materials are
air, soft iron, and steel. Each of these materials is suitable for particular applications and unsuitable for
others.
Soft iron
"Soft" (annealed) iron is used in magnetic assemblies, electromagnets and in some electric motors; and it
can create a concentrated field that is as much as 50,000 times more intense than an air core. Iron is
desirable to make magnetic cores, as it can withstand high levels of magnetic field without saturating (up
to 2.16 teslas at ambient temperature.) It is also used because, unlike "hard" iron, it does not remain
magnetized when the field is removed, which is often important in applications where the magnetic field
is required to be repeatedly switched.
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ECEG-3153: Electrical Workshop Practice II
Steel
Because iron is a relatively good conductor, it cannot be used in bulk form with a rapidly changing field,
such as in a transformer, as intense eddy currents would appear due to the magnetic field, resulting in
huge losses (this is used in induction heating). Two techniques are commonly used together to increase
the resistivity of iron: lamination and alloying of the iron with silicon.
Lamination: Laminated magnetic cores are made of thin, insulated iron sheets, lying, as much as possible,
parallel with the lines of flux. Using this technique, the magnetic core is equivalent to many individual
magnetic circuits, each one receiving only a small fraction of the magnetic flux (because their section is a
fraction of the whole core section). Because eddy currents flow around lines of flux, the laminations
prevent most of the eddy currents from flowing at all, restricting any flow to much smaller and thinner,
thus higher resistance regions. From this, it can be seen that the thinner the laminations, the lower the
eddy currents.
Silicon alloying: A small addition of silicon to iron (around 3%) results in a dramatic increase of the
resistivity, up to four times higher.
Solid cores
Powdered iron cores are used in circuits such as switch-mode power supplies that operate above mains
frequencies and up to a few tens of kilohertz. These materials combine high magnetic permeability with
high bulk electrical resistivity. For frequencies extending beyond the VHF band, cores made from nonconductive magnetic ceramic materials called ferrites are common. Some radio-frequency transformers
also have movable cores (sometimes called 'slugs') which allow adjustment of the coupling coefficient
(and bandwidth) of tuned radio-frequency circuits.
Air cores
A physical core is not an absolute requisite and a functioning transformer can be produced simply by
placing the windings near each other, an arrangement termed an 'air-core' transformer. The air which
comprises the magnetic circuit is essentially lossless, and so an air-core transformer eliminates loss due
to hysteresis in the core material. The leakage inductance is inevitably high, resulting in very poor
regulation, and so such designs are unsuitable for use in power distribution.
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ECEG-3153: Electrical Workshop Practice II
3. Based on functionality and construction
Various specific electrical application designs require a variety of transformer types. Although they all
share the basic characteristic transformer principles, they are customize in construction or electrical
properties for certain installation requirements or circuit conditions.
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Autotransformer: Transformer in which part of the winding is common to both primary and
secondary circuits.
Capacitor voltage transformer: Transformer in which capacitor divider is used to reduce high
voltage before application to the primary winding.
Distribution transformer, power transformer: International standards make a distinction in terms
of distribution transformers being used to distribute energy from transmission lines and networks
for local consumption and power transformers being used to transfer electric energy between the
generator and distribution primary circuits.
Phase angle regulating transformer: A specialized transformer used to control the flow of real
power on three-phase electricity transmission networks.
Scott-T transformer: Transformer used for phase transformation from three-phase to two-phase
and vice versa.
Poly-phase transformer: Any transformer with more than one phase.
Grounding transformer: Transformer used for grounding three-phase circuits to create a neutral
in a three wire system, using a wye-delta transformer, or more commonly, a zigzag grounding
winding.
Leakage transformer: Transformer that has loosely coupled windings.
Resonant transformer: Transformer that uses resonance to generate a high secondary voltage.
Audio transformer: Transformer used in audio equipment.
Output transformer: Transformer used to match the output of a valve amplifier to its load.
Instrument transformer: Potential or current transformer used to accurately and safely represent
voltage, current or phase position of high voltage or high power circuits.
Design procedure of iron core transformers
1. Input data of design
• Total apparent power required at the secondary side (VA)
The sizing power is calculated as follows
𝑆𝑑 =
𝑃
; (where cos 𝜙 is the power factor of the load and P is the active power rating)
cos 𝜙
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ECEG-3153: Electrical Workshop Practice II
2. Dimensioning
• Utilization (or machine) factor Ku
Transformer
Ku
Core type single-phase
Shell type single-phase
Core type three-phase
Shell type three-phase
(1.2 - 1.9)10-2
(2.5 - 4) 10-2
(1 - 1.6) 10-2
(2 - 3) 10-2
High Ku values are related to much iron and few copper, and vice versa.
Maximum flux per column;
𝑆𝑑 × 10−3
(𝑊𝑏) 𝑊ℎ𝑒𝑟𝑒 𝑓 𝑖𝑠 𝑡ℎ𝑒 𝑠𝑢𝑝𝑝𝑙𝑦 𝑓𝑟𝑒𝑞𝑢𝑒𝑛𝑐𝑦
𝜙 = 𝐾𝑢√
𝑓
3. Determine the value of the induction B and current density J in the iron
Operation
B (Wb/m2 )
J (A/mm2 )
Continuous
Intermittent
Pulsed
0.8 - 1
0.9 - 1.1
1 - 1.3
1.8 - 3
3-4
4–5
To calculate the area of the net iron section;
Air = 𝜙/B
4. Determine approximate Width of column
𝐶 = 4√𝑆𝑑
Hence choose the commercial sheet. The net thickness of the iron package is Lp = Air/C
5. Kind of lamination
Continuous operation: Thickness 0.35 mm (1.3 W/Kg)
Intermittent operation: Thickness 0.5 mm (2.3 W/Kg)
Insulation
Thickness
Packing coefficient Ks
Paper
0.5
0.35
0.88-0.91
0.85 - 0.88
Paint
0.5
0.35
0.90 - 0.93
0.88 - 0.9
Assuming a packing coefficient Ks the gross thickness is;
Lpo = Lp/Ks
The total number of laminations is
N sheet = Lpo/t (sheets)
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ECEG-3153: Electrical Workshop Practice II
6. Voltage drop and efficiency
Power Sd
ΔV%
η%
5 - 30
30 - 50
50 - 100
100 - 500
500 - 1000
1000 - 5000
25 - 15
15 - 9
9-7
7-4
4-3
3-2
65 -75
75 - 80
80 - 85
85 - 90
90 - 94
94 – 95
7. Filling coefficient
Kr
Diameter (mm)
1.1
1.05
0.05 - 0.30
0.30 – 3
8. The voltage per turn is
e = 4.44 f φ (volt/ turn)
The number of primary turns is therefore;
N1 = V1/e
Assuming a voltage drop from no-load to load condition is ΔV, the number of secondary turns is
N2 = (V2+ΔV)/e
9. Calculate the current in the windings,
primary winding, I1 =
𝑆𝑑
(𝐴)
𝑉1 𝜂
&
secondary winding, I2 =
𝑎𝑛𝑑
𝐴𝑐𝑢2 =
𝑆𝑑
(𝐴)
𝑉2
10. Determine the section of the conductor
𝐴𝑐𝑢1 =
I1
𝑗
I2
𝑗
Copper current density can be taken from step 3. Therefore we can select the commercial diameter.
11. Winding overall dimensions.
Being h the useful height of the reel, we can therefore calculate the number of turns per layer. We have
to keep the air layer between turns into account, through the filling coefficient.
𝑛1 =
ℎ
𝐾𝑟 ∙ 𝐷𝑐𝑢𝑝
&
𝑛2 =
ℎ
𝐾𝑟 ∙ 𝐷𝑐𝑢𝑠
The number of layers is therefore
𝑆1 =
𝑁1
𝑛1
&
𝑆2 =
𝑁2
𝑛2
Therefore the radial diameter of the winding is;
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