This simplest form of ac induction
motor or asynchronous motor is the basic, universal workhorse of
industry. Its general construction is shown in Fig. 10.7. It is
usually designed for fixed-speed operation, larger ratings having
such features as deep rotor bars to limit
Direct on Line (DOL) starting currents.
Electronic variable speed drive technology is able to provide the
necessary variable voltage, current and frequency that the induction
motor requires for efficient, dynamic and stable variable speed
control.
Modern electronic control technology is
able not only to render the ac induction motor satisfactory for many
modern drive applications but also to extend greatly its application
and enable users to take advantage of its low capital and maintenance
costs. More striking still, microelectronic developments have made
possible the highly dynamic operation of induction motors by the
application of flux vector control.
The practical effect is that it is now
possible to drive an ac induction motor in such a way as to obtain a
dynamic performance in all respects better than could be obtained
with a phase-controlled dc drive combination.
The stator winding of the standard
industrial induction motor in the integral kilowatt range is three
phase and is sinusoidally distributed. With a symmetrical three-phase
supply connected to these windings, the resulting currents set up, in
the air-gap between the stator and the rotor, a travelling wave
magnetic field of constant magnitude and moving at synchronous speed.
The rotational speed of this field is
f/p revolutions per second, where f is the supply frequency (hertz)
and p is the number of pole pairs (a four-pole motor, for instance,
having two pole pairs). It is more usual to express speed in
revolutions per minute, as 60 f/p (rpm).
The emf generated in a rotor conductor
is at a maximum in the region of maximum flux density and the emf
generated in each single rotor conductor produces a current, the
consequence being a force exerted on the rotor which tends to turn it
in the direction of the flux rotation.
The higher the speed of the rotor, the
lower the speed of the rotating stator flux field relative to the
rotor winding, and therefore the smaller is the emf and the current
generated in the rotor cage or winding.
The speed when the rotor turns at the
same rate as that of the rotating field is known as synchronous speed
and the rotor conductors are then stationary in relation to the
rotating flux. This produces no emf and no rotor current and
therefore no torque on the rotor.
Because of friction and windage the
rotor cannot continue to rotate at synchronous speed; the speed must
therefore fall and as it does so, rotor emf and current, and
therefore torque, will increase until it matches that required by the
losses and by any load on the motor shaft.
The difference in rotor speed relative
to that of the rotating stator flux is known as the slip. It is usual
to express slip as a percentage of the synchronous speed. Slip is
closely proportional to torque from zero to full load.
The most popular squirrel cage
induction motor is of a 4-pole design. Its synchronous speed with a
50 Hz supply is therefore 60 f/p, or 1500 rpm. For a full-load
operating slip of 3 per cent, the speed will then be (1 – s)60 f/p,
or 1455 rpm.
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