AC Motor - Basics of AC Motor Design Engineering
A synchronous and synchronous electric motors are the two main
categories of ac motors. The induction ac motor is a common form
of asynchronous motor and is basically an ac transformer with a
rotating secondary. The primary winding (stator) is connected to
the power source and the shorted secondary (rotor) carries the induced
secondary current. Torque is produced by the action of the rotor
(secondary) currents on the air-gap flux. The synchronous motor
differs greatly in design and operational characteristics, and is
considered a separate class of ac motor.
Induction AC Motors: Induction
ac motors are the simplest and most rugged electric motor and consists
of two basic electrical assemblies: the wound stator and the rotor
assembly. The induction ac motor derives its name from currents
flowing in the secondary member (rotor) that are induced by alternating
currents flowing in the primary member (stator). The combined electromagnetic
effects of the stator and rotor currents produce the force to create
rotation.
AC motors typically feature rotors, which consist of a laminated,
cylindrical iron core with slots for receiving the conductors. The
most common type of rotor has cast-aluminum conductors and short-circuiting
end rings. This ac motor "squirrel cage" rotates when
the moving magnetic field induces a current in the shorted conductors.
The speed at which the ac motor magnetic field rotates is the synchronous
speed of the ac motor and is determined by the number of poles in
the stator and the frequency of the power supply: ns
= 120f/p, where ns = synchronous
speed, f = frequency, and p
= the number of poles.
Synchronous speed is the absolute upper limit of ac motor speed.
If the ac motor's rotor turns exactly as fast as the rotating magnetic
field, then no lines of force are cut by the rotor conductors, and
torque is zero. When ac motors are running, the rotor always rotates
slower than the magnetic field. The ac motor's rotor speed is just
slow enough to cause the proper amount of rotor current to flow,
so that the resulting torque is sufficient to overcome windage and
friction losses, and drive the load. The speed difference between
the ac motor's rotor and magnetic field, called slip, is normally
referred to as a percentage of synchronous speed: s = 100
(ns - na)/ns,
where s = slip, ns = synchronous speed,
and na = actual speed.
AC Motor: Basics of AC Motor Design Engineering
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