Ahmedabad Institute Of Technology

Branch : Electrical EngineeringSem. : 7th

Subject: Design of Ac MachinesTopic : Design of Single Phase Induction Motor

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• 130020109046 Utsav Trivedi

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.

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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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 4

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

pfns

120

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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

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 ringsSimple, 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.7

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

•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

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

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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• •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

•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.

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

Speed Control• There are 3 types of speed control of 3 phase

induction machinesi. Varying rotor resistanceii. Varying supply voltageiii. Varying supply voltage and supply frequency

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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< nr3R1R2R3

R1< R2< R3

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

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T

ns~nNL

T

nr1nr2nr3 n

nr1> nr2 > nr3

V1

V2

V3

V1> V2 > V3

V decreasing

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

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T

nNL1

T

nr1nr2nr3 n

f decreasing

nNL2nNL3

Thank you