electrical-engineering-portal.com
http://electrical-engineering-portal.com/the-cage-induction-motor-explainedin-details
The Cage Induction Motor Explained In Details
This simplest form of ac induction motor or
asynchronous motor is the basic, universal
workhorse of industry. Its general construction is
shown in Fig. 1. 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.
Three phase squire cage induction motor, fully enclosed
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
most
Fig. 1 - Sectional view of a totally enclosed induction motor
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.
Torque characteristics
A disadvantage of the squirrel cage machine is its fixed rotor characteristic. The starting torque is directly
related to the rotor circuit impedance, as is the percentage slip when running at load and speed. Ideally, a
relatively high rotor impedance is required for good starting performance (torque against current) and a low
rotor impedance provides low full-load speed slip and high efficiency.
This problem can be overcome
to a useful extent for DOL
to a useful extent for DOL
application by designing the
rotor bars with special cross
sections as shown in Fig. 2 so
that rotor eddy currents
increase the impedance at
starting when the rotor flux (slip)
frequency is high.
Alternatively, for special high
starting torque motors, two or
even three concentric sets of
rotor bars are used. Relatively
costly in construction but
Fig. 2 - Typical rotor bar profiles
capable of a substantial
improvement in starting
performance, this form of design produces an increase in full load slip. Since machine losses are closely
proportional to working speed slip, increased losses may require such a high starting torque machine to be
derated.
The curves in Fig. 3 indicate
squirrel cage motor
characteristics. In the general
case, the higher the starting
torque the greater the full load
slip. This is one of the
important parameters of
squirrel cage design as it
influences the operating
efficiency.
SOURCE: Newnes Electrical
Power Engineers Handbook –
Warne
Fig. 3 - Typical torque-speed and current-speed curves (a - standard motor, b - high
torque motor (6 per cent slip))