[Stoves] Calculating the RPM

ajheggie at gmail.com ajheggie at gmail.com
Tue Feb 12 16:30:35 CST 2013


[Default] On Tue, 12 Feb 2013 13:48:56 -0800,"Frank Shields"
<frank at compostlab.com> wrote:

>I wonder what they consider ?full load? when rating the motor. Must be some torque percentage of the HP value or something

Frank The no load speed of an induction motor is near enough the
supply frequency for a 2 pole motor( double the number of poles and
the synchronous rpm halves), so 60Hz is 60 x 60 rpm or 3600. At this
speed the motor is only drawing enough power from the grid to overcome
frictional losses. The voltage you apply to the motor  is matched by a
back electromotive force, a sort of opposing voltage, that is induced
in the windings by the magnetic  force induced in the rotor. As soon
as you put a load on the motor it slows  and the back emf is reduced
but the applied voltage is the same, the difference between these
voltages  is what drives a current through the motor and produces
power.

The rated power of an induction motor is thus developed when there is
an optimum slip, say about 10% which would give a speed of 3240rpm.

If you reduce the forward voltage below the designed voltage for the
same load the slip would increase beyond 10%, the motor would slow but
the current would not reduce as much, so the heating effect would
increase and ultimately the motor would burn out.


AJH




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