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Department of Electrical & Electronics Engineering, Amrita Vishwa Vidyapeetham, Coimbatore.
Transformers
Types
Operation
EMF equation
Three phase transformers
Department of Electrical & Electronics Engineering, Amrita Vishwa Vidyapeetham, Coimbatore.
Department of Electrical & Electronics Engineering, Amrita Vishwa Vidyapeetham, Coimbatore.
Why transformers?
The electrical energy is generated and transmitted at
extremely high voltages.
The voltage is to be then reduced to a lower value for its
domestic and industrial use.
This is done by using a transformer.
Thus it is possible to reduce/increase the voltage level
using a transformer
Department of Electrical & Electronics Engineering, Amrita Vishwa Vidyapeetham, Coimbatore.
Transformers
static device which is used to transfer electrical energy
from one ac circuit to another ac circuit by
electromagnetic induction.
Increase or decrease in voltage/current but without
any change in frequency.
Department of Electrical & Electronics Engineering, Amrita Vishwa Vidyapeetham, Coimbatore.
Types
Based on application
Step-up transformer
Step-down transformer
Department of Electrical & Electronics Engineering, Amrita Vishwa Vidyapeetham, Coimbatore.
Types
Based on construction
Core type transformer
Shell type transformer
Department of Electrical & Electronics Engineering, Amrita Vishwa Vidyapeetham, Coimbatore.
Department of Electrical & Electronics Engineering, Amrita Vishwa Vidyapeetham, Coimbatore.
Types
Based on number of phases
Single phase transformer
Three phase transformer
Department of Electrical & Electronics Engineering, Amrita Vishwa Vidyapeetham, Coimbatore.
Principle of operation
The winding connected to ac supply is called as primary
winding whereas the other one is called as the
secondary winding
The ac primary current produces an alternating flux ø in
the core.
Most of this changing flux gets linked with the secondary
winding through the core and will induce voltage into the
secondary winding according to the Faraday’s law of
electromagnetic induction
Thus due to primary current, voltage is induced in the
secondary winding due to mutual induction.
Hence the induced emf in secondary is called as the
mutually induced emf.
Department of Electrical & Electronics Engineering, Amrita Vishwa Vidyapeetham, Coimbatore.
Can the transformer operate on DC?
NO.
With a DC primary current, the flux produced in the
core will not alternate, it is of constant value.
Department of Electrical & Electronics Engineering, Amrita Vishwa Vidyapeetham, Coimbatore.
Department of Electrical & Electronics Engineering, Amrita Vishwa Vidyapeetham, Coimbatore.
Transformer turns ratio
Ratio between the number of turns of the primary coil
divided by the number of turns of the secondary coil
Turns ratio value dictates the operation of the transformer
and the corresponding voltage available on the secondary
winding
For ideal transformer,
Department of Electrical & Electronics Engineering, Amrita Vishwa Vidyapeetham, Coimbatore.
Example #1
A voltage transformer has 1500 turns of wire on its
primary coil and 500 turns of wire for its secondary
coil. What will be the turns ratio of the transformer?
If 240 volts is applied to the primary winding of the
transformer, what will be the resulting secondary no
load voltage.
3:1
80 V
Department of Electrical & Electronics Engineering, Amrita Vishwa Vidyapeetham, Coimbatore.
EMF equation
Let, N1 and N2 be the number of turns of primary and
secondary,
ɸm be the maximum value of flux and f be the frequency
emf / number of turns is same for both primary and secondary
winding
Department of Electrical & Electronics Engineering, Amrita Vishwa Vidyapeetham, Coimbatore.
Example #2
Department of Electrical & Electronics Engineering, Amrita Vishwa Vidyapeetham, Coimbatore.
Three Phase Transformers
Generated and distribution of the electric power is three
phase.
The voltage generation takes place at very high level
typically at 13.2kV or 22 kV or even higher. The
transmission of electricity takes place at voltage level such
as 110kV, 132kV.
So it is necessary to use the step up transformer to raise
the voltage levels from 13.2kV to 132 kV etc. The three
phase transformer are used for the same.
Such high voltages can not be used by the domestic or
industrial users. So the voltage should be stepped down.
Three phase step down transformer are used as
distribution transformer.
Department of Electrical & Electronics Engineering, Amrita Vishwa Vidyapeetham, Coimbatore.
Three phase transformer
Department of Electrical & Electronics Engineering, Amrita Vishwa Vidyapeetham, Coimbatore.
Transformer connections
Department of Electrical & Electronics Engineering, Amrita Vishwa Vidyapeetham, Coimbatore.
Transformer connections
Department of Electrical & Electronics Engineering, Amrita Vishwa Vidyapeetham, Coimbatore.
Induction Motors
3 ɸ Induction motor
Types
Principle of operation
Department of Electrical & Electronics Engineering, Amrita Vishwa Vidyapeetham, Coimbatore.
Department of Electrical & Electronics Engineering, Amrita Vishwa Vidyapeetham, Coimbatore.
Introduction
Three-phase induction motors are the most common and
frequently encountered machines in industry
simple design, rugged, low-price, easy maintenance
wide range of power ratings: fractional horsepower to 10 MW
run essentially as constant speed from no-load to full load
No starting device required.
Department of Electrical & Electronics Engineering, Amrita Vishwa Vidyapeetham, Coimbatore.
Construction
Department of Electrical & Electronics Engineering, Amrita Vishwa Vidyapeetham, Coimbatore.
squirrel-cage rotor
conducting bars laid into slots
wound-rotor
complete set of three-phase windings
Department of Electrical & Electronics Engineering, Amrita Vishwa Vidyapeetham, Coimbatore.
Rotating Magnetic Field
Balanced three phase windings, i.e.
mechanically displaced 120 degrees
form each other, fed by balanced three
phase source
A rotating magnetic field with
constant magnitude is produced,
rotating with a speed
Where f is the supply frequency and
P is the no. of poles and nsync is called
the synchronous speed in rpm
(revolutions per minute)
Department of Electrical & Electronics Engineering, Amrita Vishwa Vidyapeetham, Coimbatore.
Rotating Magnetic Field
Department of Electrical & Electronics Engineering, Amrita Vishwa Vidyapeetham, Coimbatore.
Rotating Magnetic Field
Department of Electrical & Electronics Engineering, Amrita Vishwa Vidyapeetham, Coimbatore.
Rotating Magnetic Field
Department of Electrical & Electronics Engineering, Amrita Vishwa Vidyapeetham, Coimbatore.
Principle of operation
This rotating magnetic field cuts the rotor windings and
produces an induced voltage in the rotor windings
The rotor current produces another magnetic field
A torque is produced as a result of the interaction of those two
magnetic fields
Where ind is the induced torque and BR and BS are the magnetic
flux densities of the rotor and the stator respectively
ind R skB B
Department of Electrical & Electronics Engineering, Amrita Vishwa Vidyapeetham, Coimbatore.
Induction motor speed
At what speed will the IM run?
Can the IM run at the synchronous speed, why?
If rotor runs at the synchronous speed, which is the same
speed of the rotating magnetic field, then the rotor will
appear stationary to the rotating magnetic field and the
rotating magnetic field will not cut the rotor.
So, no induced current will flow in the rotor and no rotor
magnetic flux will be produced so no torque is generated
and the rotor speed will fall below the synchronous speed
When the speed falls, the rotating magnetic field will cut
the rotor windings and a torque is produced
Department of Electrical & Electronics Engineering, Amrita Vishwa Vidyapeetham, Coimbatore.
Induction motor speed
So, the IM will always run at a speed lower than the
synchronous speed
The difference between the motor speed and the synchronous
speed is called the Slip
Where nslip= slip speed
nsync= speed of the magnetic field
nm = mechanical shaft speed of the motor
nsync- nm = Slip Speed
slip sync mn n n
Department of Electrical & Electronics Engineering, Amrita Vishwa Vidyapeetham, Coimbatore.
The Slip
sync m
sync
n ns
n
Where s is the slip
Notice that : if the rotor runs at synchronous speed
s = 0
if the rotor is stationary
s = 1
Slip may be expressed as a percentage by multiplying the above
eq. by 100, notice that the slip is a ratio and doesn’t have units
Department of Electrical & Electronics Engineering, Amrita Vishwa Vidyapeetham, Coimbatore.
Frequency
The frequency of the voltage induced in the rotor is given by
Where fr = the rotor frequency (Hz)
P = number of stator poles
n = slip speed (rpm)
120r
P nf
Department of Electrical & Electronics Engineering, Amrita Vishwa Vidyapeetham, Coimbatore.
Frequency
What would be the frequency of the rotor’s induced voltage at
any speed nm?
When the rotor is blocked (s=1) , the frequency of the
induced voltage is equal to the supply frequency
On the other hand, if the rotor runs at synchronous speed
(s = 0), the frequency will be zero
Department of Electrical & Electronics Engineering, Amrita Vishwa Vidyapeetham, Coimbatore.
Torque
While the input to the induction motor is electrical power, its
output is mechanical power
Any mechanical load applied to the motor shaft will introduce a
Torque on the motor shaft. This torque is related to the motor
output power and the rotor speed as
.outload
m
PN m
2/
60
mm
nrad s
Department of Electrical & Electronics Engineering, Amrita Vishwa Vidyapeetham, Coimbatore.
Torque-speed characteristics
Typical torque-speed characteristics of induction motor