Transcript How Electric Motors are made Three phase AC induction motor

AC Induction Motors
How does an Induction Motor work ?
https://www.youtube.com/watch?v=LtJoJBUSe28
How Electric Motors are made
https://www.youtube.com/watch?v=bCwu5KPVv54
Three phase AC induction motor demonstration
https://www.youtube.com/watch?v=P-eTLmJC2cQ
BALDOR - Athens, Georgia Plant
https://www.youtube.com/watch?v=CBFE-Bt7RjY
There are two types of rotor windings:
Squirrel cage as shown below
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.
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.
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 shortcircuited 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 woundrotor winding is similar to that of the stator. Sometimes the cagetype machine is called a brushless machine and the wound-rotor
machine is termed a slip-ring machine.
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.
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.
Equivalent
Circuit
Rotor Stationary:
Equivalent
Circuit
Rotor Running:
Torque and Power Relationships
Maximum or Breakdown Torque
For maximum power transfer from stator to rotor-load resistance, the
following relationship must be satisfied:
Industrial Considerations
Classificaiton of Induction Motors
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.
Mechanical Load Changes and their Effects on a Motor’s Parameters
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.
Soft-start
Plugging