Single phase induction motor 's construction, Operation, working, characteristics, speed control.

1. Ahmedabad Institute Of Technology
Branch : Electrical Engineering
Sem. : 7th
Subject: Design of Ac Machines
Topic : Design of Single Phase Induction Motor
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• 130020109046 Utsav Trivedi

2. Single-phase Induction Motor
•This “single-phase” motors are truly two phase machines.
•The motor uses a squirrel cage rotor, which has a laminated iron
core with slots.
•Aluminum bars are molded on the slots and short-circuited at
both ends with a ring.

3. Construction
• The three basic parts of an AC motor are the rotor, stator, and
enclosure.
• The stator and the rotor are electrical circuits that perform as
electromagnets.
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4. Construction (Stator construction)
• The stator is the stationary electrical part of the motor.
• The stator core of a National Electrical Manufacturers Association
(NEMA) motor is made up of several hundred thin laminations.
• Stator laminations are stacked together forming a hollow cylinder. Coils
of insulated wire are inserted into slots of the stator core.
• Electromagnetism is the principle behind motor operation. Each
grouping of coils, together with the steel core it surrounds, form an
electromagnet. The stator windings are connected directly to the power
source.
MZS
FKEE, UMP
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5. Rotating Magnetic Field
• When a 3 phase stator winding is connected to a 3 phase voltage
supply, 3 phase current will flow in the windings, which also will
induced 3 phase flux in the stator.
• These flux will rotate at a speed called a Synchronous Speed, ns.
The flux is called as Rotating magnetic Field
• Synchronous speed: speed of rotating flux
• Where; p = is the number of poles, and
f = the frequency of supply
p
f
ns
120

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6. OPERATING PRINCIPLE
• The single-phase induction motor operation can be described
by two methods: –Double revolving field theory; and –Cross-
field theory.
•Double revolving theory is perhaps the easier of the two
explanations to understand

7. Construction (Rotor construction)
• Induction motor types:
 Squirrel cage type:
Rotor winding is composed of copper bars embedded in
the rotor slots and shorted at both end by end rings
Simple, low cost, robust, low maintenance
 Wound rotor type:
Rotor winding is wound by wires. The winding terminals
can be connected to external circuits through slip rings
and brushes.
Easy to control speed, more expensive.
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8. Double revolving field theory
•A single-phase ac current supplies the main winding that
produces a pulsating magnetic field.
•Mathematically, the pulsating field could be divided into two
fields, which are rotating in opposite directions.
•The interaction between the fields and the current induced in
the rotor bars generates opposing torque

9. •The interaction between the fields and the current induced in
the rotor bars generates opposing torque.
•Under these conditions, with only the main field energized the
motor will not start
•However, if an external torque moves the motor in any
direction, the motor will begin to rotate

10. STARTING TORQUE
•The single-phase motor starting torque is zero because of the
pulsating single-phase magnetic flux.
•The starting of the motor requires the generation of a rotating
magnetic flux similar to the rotating flux in a three-phase motor.
•Two perpendicular coils that have currents 90°outof-phase can
generate the necessary rotating magnetic fields which start the
motor.
•Therefore, single-phase motors are built with two perpendicular
windings

11. Principle of Operation
• Torque producing mechanism
When a 3 phase stator winding is connected to a 3
phase voltage supply, 3 phase current will flow in the
windings, hence the stator is energized.
A rotating flux Φ is produced in the air gap. The flux
Φ induces a voltage Ea in the rotor winding (like a
transformer).
The induced voltage produces rotor current, if rotor
circuit is closed.
The rotor current interacts with the flux Φ, producing
torque. The rotor rotates in the direction of the
rotating flux.
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12. • •A less effective but more economical method using shaded
pole motors
•The motor has two salient poles excited by ac current.
•Each pole includes a small portion that has a short circuited
winding. This part of the pole is called the shaded pole.
•The main winding produces a pulsating flux that links with the
squirrel cage rotor.
•This flux induces a voltage in the shorted winding

13. •The induced voltage produces a current in the shorted winding.
•This current generates a flux that opposes the main flux in the
shaded pole (the part of the pole that carries the shorted
winding).
•The result is that the flux in the unshaded and shaded parts of
the pole will be unequal.
•Both the amplitude and the phase angle will be different.

14. CHARACTERISTICS
(i) The sinning torque is 2 times the full-loud torque mid (lie starting
current is 6 to 8 times the full-load current.
(ii) Due to their low cost, split-phase induction motors are most
popular single phase motors in the market.
(iii) Since the starting winding is made of fine wire, the current
density is high and the winding heats up quickly. If the starting period
exceeds 5 seconds, the winding may burn out unless the motor is
protected by built-in-thermal relay. This motor is, therefore, suitable
where starting periods are not frequent.
(iv) An important characteristic of these motors is that they are
essentially constant-speed motors

15. Speed Control
• There are 3 types of speed control of 3 phase
induction machines
i. Varying rotor resistance
ii. Varying supply voltage
iii. Varying supply voltage and supply frequency
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16. Varying rotor resistance
• For wound rotor only
• Speed is decreasing
• Constant maximum torque
• The speed at which max
torque occurs changes
• Disadvantages:
– large speed regulation
– Power loss in Rext – reduce
the efficiency
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T
ns~nNL
T
nr1nr2nr3
n
nr1< nr2< nr3
R1
R2
R3
R1< R2< R3

17. Varying supply voltage
• Maximum torque changes
• The speed which at max
torque occurs is constant (at
max torque, XR=RR/s
• Relatively simple method –
uses power electronics circuit
for voltage controller
• Suitable for fan type load
• Disadvantages :
– Large speed regulation since ~
ns
17
T
ns~nNL
T
nr1nr2nr3
n
nr1> nr2 > nr3
V1
V2
V3
V1> V2 > V3
V
decreasing

18. Varying supply voltage and supply frequency
• The best method since supply
voltage and supply frequency
is varied to keep V/f constant
• Maintain speed regulation
• uses power electronics circuit
for frequency and voltage
controller
• Constant maximum torque
18
T
nNL1
T
nr1nr2nr3
n
f
decreasing
nNL2nNL3