AC Induction Motors

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AC Induction Motors




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Presentations text content in AC Induction Motors

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AC Induction Motors

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https://www.youtube.com/watch?v=P-eTLmJC2cQ

https://www.youtube.com/watch?v=LtJoJBUSe28

https://www.youtube.com/watch?v=bCwu5KPVv54

https://www.youtube.com/watch?v=CBFE-Bt7RjY

How does an Induction Motor work ?

How Electric Motors are made

Three phase AC induction motor demonstration

BALDOR - Athens, Georgia Plant

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There are two types of rotor windings:

Squirrel cage as shown below

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And wound-rotor as shown below here:

A very small percentage of induction machines have a wound rotor. The brushes can then be connected to an external variable resistor which controls torque/speed characteristics.

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When balanced three-phase currents are injected into the stator windings, they produce a rotating magnetic field that, unless the rotor is revolving at the same speed as the magnetic field, will induce voltages in the rotor windings. This results in rotor current and therefore rotor flux. The magnetic fields of the stator and rotor try to align their magnetic axes and in so doing develop torque.

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The induction motor is probably the most common of all motors. Like the de machine, an

induction motor

consists

of

a stator and a rotor, the latter mounted on bearings and separated from the stator by

an airgap

. The stator

core

, made up of

punchings

(

or laminations

), carries slot-embedded conductors.

These conductors

are interconnected in a predetermined fashion and constitute the armature

windings. Alternating

current is,

supplied

to the stator, windings, and the currents in the rotor windings

are induced

by the magnetic field of the stator currents. The rotor of the induction machine is cylindrical

and carries either

(1) conducting bars

short-circuited

at both ends by conducting rings, as in a cage-type

machine

or (2) a

poly-phase

winding with terminals brought out to slip rings

for external

connections,

as in

a wound-rotor

machine.

A wound-rotor winding is

similar

to that of the stator

.

Sometimes

the cage-type

machine is called a

brushless machine

and the wound-rotor machine is termed a slip-ring machine.

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An induction motor operates on the basis of interaction of induced rotor Currents and the airgap field.

If the rotor is allowed to run under the torque developed by this interaction, the machine

will

operate as

a motor

. On the other hand, the

motor

may be driven by

an

external agency beyond a speed such that

the machine

begins to deliver electric power; it then operates as an induction generator. Almost

invariably, induction

machines are used as motors.

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Slip

To achieve higher efficiency, the majority of three-phase induction motors are designed to operate at a very small slip (usually less than 5%).

When three-phase induction machines operate as induction generators, their actual rotor speed is higher than their synchronous speed and their velocity is in the same direction as the synchronously rotating stator field. Slip is negative.

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Equivalent Circuit

Rotor Stationary:

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Equivalent Circuit

Rotor Running:

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Torque and Power Relationships

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Maximum or Breakdown TorqueFor maximum power transfer from stator to rotor-load resistance, the following relationship must be satisfied:

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Industrial Considerations

Classificaiton

of Induction Motors

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The minimum value of the torque in the region between starting and maximum torque is called the pull-in torque. As can be seen from the general torque-speed characteristics, only class A and C motors have a meaningful pull-in torque of the motor, which must be lower than that of the motor itself.

The maximum torque developed by a motor indicates the capability of the machine to overcome high transient-load torques. A motor with a maximum torque that is relatively low may stall when a sudden load torque exceeds the motor’s breakdown torque.

A motor’s accelerating time is inversely proportional to its accelerating torque. During acceleration, a motor draws locked-rotor current. The longer a motor’s accelerating time, the greater the possibility of thermal damage. For this reason, the maximum number of starts should be determined.

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Mechanical Load Changes and their Effects on a Motor’s Parameters

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Torque at a given speed is proportional to the square of the applied voltage.

For small variations in the load requirements and for a constant ac voltage supply, the motor’s speed variation is limited.

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Soft-start

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Plugging


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