Re: Help needed on understanding synchronous motors

Tesla List wrote:
> Original Poster: RWB355-at-aol-dot-com
> Hello All,
> I need some help to really understand how big sync. motors are built.
> The normal (capacitor start) induction motor (the ones with a short circuit
> armature .. sorry, don´t recall the english name for this)

Induction motors are what they are called... I assume you are referring
to the usual scheme with the rotor made from steel with aluminum bars
cast into it.  By changing the size and configuration of the aluminum
bars, you can change the torque/slip characteristic.  Not until you get
to 100 kW+ do you start to see wound rotor induction motors, where there
are actually copper windings in the rotor, often with a variable

 is not synchronous,
> if I understand it correctly.
> These motors run based on the fact, that there is always a difference
> the stator "speed" (giving by the mains frequency) and the rotor (that
> actually turns). The more difference between these two "speeds", the higher
> the output torque, which is why these motors have a high load starting
> capability.

The ratio of the speed difference to synchronous speed is called the
"slip" and since the torque output is roughly proportional to s/(k-s),
you can see that at 0 slip (at synchronous speed) they put out no
torque.... Interestingly, with negative slip (i.e. rotor running faster
than synchronous) the motor has negative torque, and puts power back
into the power line...

> Long, long ago, I took apart a sync motor that drove an analog wall
clock. If
> I remember it correctly, this motor had a set of stator windings and the
> was composed of a magnetic core. As the magnet will always follow the
> magnetic field of the stator coils exactly (i.e: no "speed" difference),
I can
> see how this motor is synchronous to the mains frequency.
> But how are higher powered motors (e.g. 1/2 hp.) built?
> A while ago I read a post on turning non sync motors into sync´d ones, by
> milling flats on the armature. How (by what principal) does this work ??

Similar to the variable reluctance stepper motors (which have an iron
rotor, not a magnetized one). The idea is that you build a motor where
the magnetic reluctance varies as the motor rotates. Put a DC current
through the stator.. The rotor will tend to move to a position with the
lowest stored energy. You can turn it by applying a torque and the force
gradually increases (exactly like tensioning a spring) as the stored
energy increases, then, as you go "past the hump", the force will
actually decrease and push the rotor along to the next stable point.

Now, if you make the magnetic field rotate, the rotor will tend to
follow the magnetic field. It will lag slightly behind in angle (but not
speed) by an amount related to the friction or load. If you put too much
load, it will exceed the "breakdown" torque, and just vibrate. 

Synchronous motors of this type have zero torque at zero speed, so
starting them with some external force is necessary. A secondary winding
to create an asymmetry in the field is one way. A helper motor is

They also don't have very much torque, compared to an induction motor,
or a regular synchronous motor with a excited rotor.

Big synchronous motors (as in large hydroelectric generating plants) use
DC excitation of the rotor winding, supplied by slip rings or by a
rectifier and a rotary transformer.

You can turn an automobile alternator into a 3phase synchronous motor
quite easily.

> Coiler greets from germany,
> Reinhard

Jim Lux                               Jet Propulsion Laboratory
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