ac motors.ppt

7/27/2019 AC Motors.ppt

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AC Motors


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Classification of AC motors


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AC motors include 3-phase and single phase types.

3-phase AC induction motors are the most widely used motors in industrialand commercial applications. They are divided into two sub-categories:

Squirrel cage motors

Wound rotor motors

3-phase Synchronous motors are most commonly used in very largeindustrial applications or where exact speed is required.

Single phase induction motors are used where three phase power is notavailable; typically in residential, commercial and agricultural applications.They are also used in applications with power requirements below 1horsepower (HP). The mainmsub-categories include:

Split phase Capacitor run

Capacitor start

Capacitor start capacitor run

Shaded pole

Universal motors

Universal motors are mostly operated on AC power, but they can operateon either AC or DC. Tools and appliances are among the most frequentapplications.


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Principles of operation

All motors have two basic parts:

The STATOR (stationary part)

The ROTOR (rotating part)


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F = BIL , where

F Force (newtons)

I Current (Amperes)

L Length (metres)

B Magnetic Flux (webers/m2)


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Torque-Speed Characteristics of

Motors

Torque produced by a motor typically varies with speed.

Each motor type has its own torque speed relationship which

when plotted as torque vs. speed helps in the selection process


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Some important points found on a torque-speed graph include:

(a) Starting torque the torque produced at zerospeed.

If the motor is to turn a load that is difficult to start (a highinertia load) one would choose a motor with high starting torque.

(b) Pull-up torque the minimum torque producedduring acceleration from standstill to operating speed.

This may be critical for an application that needs power to go

through some temporary barriers before achieving the working leveloutput.

(c) Breakdown torque the maximum torque that themotor can produce before stalling.

(d) Full load torque (also braking torque) the torqueproduced at full load speed that gives the rated output ofthe motor. At this point the torque times the speedequals the nameplate horsepower rating.


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AC motors Working

A common feature of all AC motors is a rotating magnetic field

produced by the stator windings.


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Direction of rotation of motor

The rotor sees the net rotating magnetic field

created by the three coils and rotates, creating

the torque on the motor drive shaft. This field

rotates either clockwise or counter clockwise,depending on the order of the phases connected

to the motor.

Reversing a 3-phase motors direction is simply

achieved by changing the connection order oftwo of the three conductors.


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Synchronous speed

The rotating field speed depends on the

number of magnetic poles in the stator and

is referred to as the synchronous speed.

(120*frequency)/No. of poles

The number of magnetic poles (or simply

poles) is the principal design factor

affecting speed in AC motors.


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3-Phase Induction MotorsSynchronous speed

The rotor of an induction motor does not rotate at synchronousspeed or the speed of the magnetic field of the stator, but lagsslightly. This lag is usually expressed as a percentage of thesynchronous speed called slip.

Motor slip is the result of the interaction between the magnetic fieldof the stator and the magnetic field resultant from the inducedcurrents flowing in the rotor. The rotor bars cut through the magnetic

lines of force resulting in useful torque being produced. As the motorslows down (i.e. slip increases) when load is added, more torque iscreated.

Slip=Synchronous speed-running speed

%slip=(synchronous speed-running speed)*100----------------------------------------------------

synchronous speed


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3-phase induction motors are very robust

and reliable and are the most common

type of motor in use.

Unfortunately, power factor tends to be

poor for reduced loads. This is due to the

current that is supplied to just maintain

magnetic field.


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Types

Sqirrel cage rotor

Wound rotor or slip ring type


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Squirrel Cage Motors

The rotor of a squirrel cage motor is made of conductive

bars that are parallel to the shaft and short circuited bythe end rings in which they are physically supported


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The squirrel cage is skewed so that the force applied tothe rotor it continuous. If they were straight then theforce would be jerky, as whole of the bar is cutting themagnetic field lines at the same time. With the bars

skewed the amount of the bar cutting the field line growscontinuously and the next bar starts cutting the field linesas the first finishes.

Also, slots are skewed to get uniform torque ,reduce the

magnetic locking (also called COGGING EFFECT)between stator and rotor,and thirdly to reduce themagnetic humming noise during running condition


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There are 4 types

Design A

Design B

Design C

Design D

These classifications are according to

speed torque characteristics


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Design type B is the most common and

suits the majority of motor applications.

Design A motors are not generally

specified today due to the high starting

current.


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Motors are also referred to as being general, definite orspecial purpose.

A general purpose motoris any motor which isdesigned in standard ratings, such as those specified in

the National Electrical Manufacturers Association(NEMA) Standards Publication MG1-1993, paragraph14.02.4 AC Motors25

A definite purpose motoris any motor designed instandard ratings with standard operating characteristics

or standard mechanical construction for use underservice conditions other than usual, such as thosespecified in NEMA Standards Publication MG1-1993.

A special purpose motoris any motor (other than ageneral purpose motor or definite purpose motor) which

has special operating characteristics or specialmechanical construction (or both) designed for aparticular application. Motors over 500HP are usuallyconsidered special purpose rather than general purpose,and are designed for the specific application.


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Wound Rotor Induction Motor

The wound rotor induction motor operates on the sameprinciples as the squirrel cage motor, but differs in theconstruction of the rotor.

Instead of shorted bars, the rotor is made up ofwindings which terminate at slip rings on the shaft.

This type of motor is used in special applications wherea high starting torque is required.

Connection of external resistance to the rotor circuit viathe slip rings permits variation of motor torque-speedcharacteristics

After starting, the slip rings are shorted together.

Shorting the external connection results in operationsimilar to squirrel cage motors.

S d i ti f b t 5 1 b


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Speed range variation of about 5:1 can be

achieved by adding external resistance to the

rotor circuit; however, this is at the expense of

electrical efficiency unless a slip energyrecovery circuit is used.


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Torque speed characteristics The maximum torque that a wound rotor motor can produce is

determined by the design of its rotor, but the speed at which this

torque is developed depends on external rotor resistance.

Each wound rotor design has a family of torque-speed curves that

correspond to various values of external rotor resistance.


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Starting circuits

What is starting?

Starting is a process in which motor rotor isbrought from zero to rated speed

Speed of rotor is given by

speed=120f/p

Starting torque proportional to applied

voltageStarting circuits used to reduce the starting

current taken by the motor.


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Types

DOL starter

Primary resistance starter

Autotransformer starterStar delta starter

Rotor resistance starter


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Speed control

Stator side control

Stator voltage control

Using autotransformer

Primary resistors in series with stator winding

Stator frequency control

V/f control

Pole changing method

Rotor side controlCascade control

Adding external resistance


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Differences between squirrel cage and slip ring inductionmotor

Squirrel cage Slip ring

Low starting torque Much higher

Higher efficiency Lower efficiency

No speed control Speed control

High starting current Low starting current

Needs a starter Can be started directly

Minimum maintenance High maintenance

Rugged Not as rugged

Cheaper Higher cost


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Single Phase Induction Motors

When a single phase induction motor is running itdevelops a rotating magnetic field, but before the rotorbegins to turn, the stator produces only a pulsatingstationary field.

To produce a rotating field, and thus a starting torque, an

auxiliary starting winding is placed at right angles to themain stator winding so that the currents through themare out of phase by 90degree (1/4 of a period in time).

As a result, the rotor wants to align the magnetic poles,which creates a starting torque.

A failed start winding circuit will result in a motor thatmakes a low humming sound and will start in eitherdirection by carefully providing a slight spin by hand


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Types

Shaded pole

Split phase

Capacitor start

Capacitor start/run

Permanent split capacitor


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Split phaseSplit phase motors use a starting winding

with a different resistance/reactanceratio than that of the main stator

winding to produce the phase difference

required for starting.The phase difference is not the desired

90, and the magnetic fields are not equal.

This results in a lower starting torquethan other motor designs


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Applications

Split phase motor starting torque,

however, is sufficient for many

applications such as refrigerated display

case circulation fans and some powertools (e.g. drill press).

This type of motor is cheap to produce

and is therefore a favourite in OEMproducts.1/2 HP


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Capacitor Motors Many single phase motors use a capacitor in series with

one of the stator windings to optimize the field phasedifference for starting.

Capacitive current leads voltage by 90. Addingcapacitance causes a phase shift in one winding relativeto the other.

The result is a higher starting torque than a split phasemotor can produce.

Applications Capacitor motors are used in high startingtorque applications such as compressors and airconditioners. Typical sizes range up to about 10 HP.

Types Capacitor run

Capacitor start


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Capacitor run motors

Capacitor run motors use a capacitorpermanently connected in series with one ofthe start windings, to achieve a compromisebetween good starting torque and good running

characteristics . This design is lower in cost than other capacitor

motors that incorporate capacitor switchingsystems.

These are sometimes called permanent splitcapacitor (PSC) motors.

Application New furnace fan motors sometimesuse capacitor run motors.


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Capacitor start motor

In capacitor start motors, a capacitor connected

in series with the starting winding is sized to

maximize starting torque.

The starting winding is removed from the circuitby a centrifugal switch or electronic relay

when the motor reaches running speed.

Starting torque is higher than for capacitor run

motors, with running performance similar to a

split phase motor.


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Capacitor Start Capacitor Run Motors

This design uses a capacitor optimized forrunning characteristics in series with themain stator winding

A second capacitor in series with thestarting winding optimizes starting torque.The starting capacitor is switched out ofthe circuit at running speed.

Both starting torque and runningcharacteristics are optimized


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Shaded Pole Motors

A shaded pole motor is the simplest form of a single phase motorand isvery low in cost .

It develops a rotating field by delaying the build up of magnetic flux throughpart of the pole structure.

The shaded portion of the pole is isolated from the rest of the pole by acopper conductorthat forms a single turn around it.

The magnetic flux in the unshaded portion increases with the currentthrough its winding. Magnetic flux increases in the shaded portion; however,it is delayed by the current induced in the copper field.

The magnetic field sweeps across the pole face from the unshaded portionto the shaded portion, developing a torque in the squirrel cage.

To maximize torque, the rotor is made with relatively high resistance.

Applications Shaded pole motors are used where low torque is acceptable

(such as fans) and are usually less than 1/4 HP. Due to their very lowefficiency, shaded pole motors should only be used in applications wherethe motor is either very small or operates for very short periods of time (e.g.shower fan motor).


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Universal motors

Universal motors are series wound, with rotor circuitry similar to DC motors

The term universal results from their ability to operate on either DC or AC power.

The operation and construction of these motors closely resemble DC motors, with components designed for

efficiency on AC up to the line frequency

Operating speeds typically range from 3,000 to 15,000 RPM.

The speed will drop with increasing load.

A high horsepower to size ratio is characteristic of this design.

Maintenance requirements per hour of operation are higher than other designs due to the brush/commutatorsetup.

A universal motor has both a wound field (on the stator) and a wound armature (on the rotor). It cannot use a

permanent magnet to create the stator field because it needs to reverse its magnetic polarity every half cycle of

the mains. The voltage being fed to the commutator brushes is also changing polarity every half cycle, and in fact

is simply in series with the field winding.

This motor only has two poles, and so does not have a lot of starting torque. Thats why you sometimes need to

turn the big wheel on the end of the sewing machine to get it started.

Common uses of these motors include low duty cycle applications such as power saws, drills, vacuum cleaners

and lawn mowers. Sizes up to about 2 HP are common.

They'll be where you want something compact that spins fast (food processor, coffee grinder, electric nose-hair

trimmer) or needs variable speed (sewing machine, hand-held electric drill).


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Compensated type

The compensated type motor consists of distributed field winding and thestator core is similar to that of split-phase motor. We already know that splitphase motors consist of an auxiliary winding in addition to main winding.Similar to the split phase motors, the compensated type also consists of anadditional winding. The compensating winding helps in reducing thereactance voltage which is caused due to alternating flux, when the motor

runs with the aid of an AC supply. Both the types of motors develop unidirectional torque regardless of the

supply with which they run. The supply may be AC or DC but the directionof torque is same.


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Non compensated


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ac motors.ppt

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