More on Squirrel Cage Motors and Slip

In my previous post I mentioned the impact of motor slip on the
performance of a centrifugal pump. When I bring this topic up
in classes, there are often questions.  So,to better
understand what is going on, a quick review of how a multi-phase
“squirrel cage” induction motor like the ones we typically use for
our HVAC equipment works may be in order.  A very cool
discussion of this including some really great animated
graphics can be found on the University of New South Wales
website, as illustrated below.

Their is a link from that page to a
related page that is also very cool and shows the guts of a lot of
different motor types and discusses their operating principle
as illustrated below.

What follows is my somewhat simplistic understanding of how a
squirrel cage motor works.  It seems to work for me so maybe
it will be helpful to you.  The discussion assumes we are
talking about a 3 phase motor.

A typical squirrel cage motor contains stationary windings which
we’ll call stators, that are connected to the three phase power
supply. Suspended inside the stators are rotating windings which
we’ll call the rotor, that drives the output shaft.

As you will recall from your physics class, when current flows
through a wire, it generates a magnetic field. As a result, the
current flowing through the stator windings causes them to act like
magnets with the strength and frequency of the magnetic field
varying as a function of the alternating current supplied by the
power company. In addition, since the power is three phase – we’ll
call the phases A, B and C – and since the current in the phases
peaks consecutively in time (versus concurrently), the magnetic
field in the windings connected to the A phase will be rising
toward its peak when the field associated with the windings
connected to the B phase is at its peak. Meanwhile, the field
associated with the windings connected to the C phase will be
declining.

Of course, since the current is continuously alternating, the
fields in the windings will be continuously varying, one after the
other. If you consider that for a minute, you can probably see how
if one arrange the windings properly, you could create a rotating
magnetic field whose frequency was a direct multiple of the utility
company frequency and the number of windings. For instance the
rotating magnetic field in a two pole motor operating on a 60 hz
utility system will spin at 3,600 rpm, which is termed its
synchronous speed. For a four pole motor on a 60 hz system, the
synchronous speed is 1,800 rpm; for a 6 pole motor its 1,200
rpm.

Now lets talk about the rotor for a minute. There is no physical
connection between the rotor and the power supply. But because the
rotors’ windings are located inside the field created by the
stators and because that field expands and collapses due to the
alternating current supplied by our utility companies, currents are
induced in the windings of the rotor. And since current flowing
through wire creates a magnetic field, the rotor itself becomes a
magnet.

Now recall from physics that if you place two magnets in close
proximity to each other, they try to align themselves opposite pole
to opposite pole. So, if you visualize what might happen when you
put a magnet inside the rotating field we just discussed, you can
probably see that the magnet would start to spin as it tried to
line itself up with the rotating poles created by the rotating
magnetic field. This is exactly what the rotor does and it is why
the output shaft of the motor spins.

But, there is a catch. Remember that a wire must be moving
relative to the lines of flux associated with a magnetic field to
generate a current. That means that if the rotor ever caught up
with the rotating magnetic field created by the stators; 

Its windings would no longer be moving
relative to the stator’s lines of flux, which would mean that

No current would be generated in the windings of the
rotor, which would mean that

The rotor would cease to be a magnet,
which would mean that

It would stop trying to rotate.

Of course, if it stopped rotating, then the stator’s lines of
flux would again be moving relative to the windings in the rotor
and it would start to become a magnet again. The bottom line on all
of this is that the output speed of a squirrel cage induction motor
will approach but never equal the synchronous speed of the rotating
field in the stators associated with it. The difference between the
two speeds is termed slip, and it varies with the load on the
output shaft of the motor.

The reason it varies with load is that as you apply a load to
the output shaft, it will tend to slow down relative to the
rotating field.  As a result, the rotor’s winding are cutting
more lines of flux in the stator’s field.  If more lines of
flux are cut, then the current in the field in the rotor increases,
increasing its magnetic strength and allowing more power to be
transfered. 

With nothing attached to the shaft of the motor, the only
resistance is the bearing resistance, windage (aerodynamic losses
associated with rotor spinning in air), the work being done by the
cooling fan to move air through the motor, and similar losses, all
of which are part of the losses associated with the motor’s rated
efficiency.  Under this condition, the rotor spins at nearly
the synchronous speed.

As load is applied, the slip increases until at the rated load,
the difference between the synchronous speed of the rotating stator
field and the slip equals the rated motor speed.  If load
continues to be applied, the rotor speed will continue to drop
relative to the synchronous speed and the motor will actually
produce more than its rated horsepower.

But, if you keep trying to take more power out of the shaft, at
some point, things will break down and the strength of the field
induced in the rotor will not be enough to overcome the resistance
to rotation associated with the load, no matter how many lines of
flux are cut, and the rotor stops rotating.  This point is
generally termed the break down torque for the motor.