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AC Induction MotorsSlide2
How does an Induction Motor work ?
How Electric Motors are made
Three phase AC induction motor demonstration
BALDOR - Athens, Georgia PlantSlide3Slide4
There are two types of rotor windings:
Squirrel cage as shown belowSlide5
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.Slide6Slide7Slide8
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.Slide9
The induction motor is probably the most common of all motors. Like the de machine, an
a stator and a rotor, the latter mounted on bearings and separated from the stator by
. The stator
, made up of
), carries slot-embedded conductors.
are interconnected in a predetermined fashion and constitute the armature
to the stator, windings, and the currents in the rotor windings
by the magnetic field of the stator currents. The rotor of the induction machine is cylindrical
and carries either
(1) conducting bars
at both ends by conducting rings, as in a cage-type
or (2) a
winding with terminals brought out to slip rings
A wound-rotor winding is
to that of the stator
machine is called a
and the wound-rotor machine is termed a slip-ring machine.Slide10
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
. On the other hand, the
may be driven by
external agency beyond a speed such that
begins to deliver electric power; it then operates as an induction generator. Almost
machines are used as motors.Slide11Slide12Slide13Slide14Slide15Slide16
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.Slide17Slide18Slide19Slide20Slide21
Torque and Power RelationshipsSlide24Slide25Slide26
Maximum or Breakdown TorqueFor maximum power transfer from stator to rotor-load resistance, the following relationship must be satisfied:Slide27Slide28Slide29Slide30Slide31Slide32Slide33Slide34Slide35
of Induction MotorsSlide36Slide37Slide38
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.Slide39Slide40
Mechanical Load Changes and their Effects on a Motor’s ParametersSlide41
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.Slide42Slide43Slide44Slide45Slide46Slide47Slide48Slide49Slide50Slide51Slide52Slide53Slide54Slide55Slide56Slide57Slide58Slide59Slide60Slide61Slide62