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Re: Rotary Spark Gap, the rpm's, and horsepower
Original poster: "Jolyon Vater Cox by way of Terry Fritz <teslalist-at-qwest-dot-net>" <jolyon-at-vatercox.freeserve.co.uk>
The reason why the torque of an induction motor falls to zero at synchronous
speed is that the machine is essentially a transformer and at synchronism,
the rotor would have to be turning as fast as the stator field is rotating;
at that speed, the frequency of the AC currents induced in the bars of the
squirrel-cage rotor would logically have to be zero i.e. DC and transformers
don't work with DC.
True synchronous machines i.e.salient-pole synchronous or
synchronous-induction (which are also used as power station generators) get
round this by having DC supplied to the rotor via slip-rings from an
The grinding of flats on the rotor of a squirrel-cage induction machine
converts it into what is properly termed a "reluctance motor" so-called
because the reduction in the proportion of magnetic material (iron) results
in a path of higher magnetic resistance or "reluctance" in the vicinity of
the flats. This kind of motor is synchronous because the field axis through
the rotor always lines up the path of least reluctance i.e.perpendicular to
the flats providing the load is not too great, when the motor will simply
stop. Motors used in electric clocks are usually of this type.
Hysteresis motors rely ability of "hard" magnetic materials like steel to
retain their magnetism for some time after they are magnetised,
as the stator field sweeps by of the field becomes in the magnetic field of
the resulting N-S field lines up with S-N axis of the stator and is
accelerated to synchronism by the rotating magnetic field. However, due to
the construction of the rotor (a simple cylinder or disc of magnetic
material) the position of the rotor and stator magnetic axes not fixed as
it is in synchronous or reluctance motors upon starting.
----- Original Message -----
From: "Tesla list" <tesla-at-pupman-dot-com>
Sent: Friday, June 13, 2003 3:39 AM
Subject: Re: Rotary Spark Gap, the rpm's, and horsepower
> Original poster: "Mark Broker by way of Terry Fritz <teslalist-at-qwest-dot-net>"
> >The link above is my first spark gap ever. (Yes, i did skip the static
> >completely in my learning path)
> >I'm using a 1/25HP, 1800 RPM hysteresis synchronous motor and it works
> >beautifully with power to spare. Whats a
> >hysteresis synchronous motor you ask? Well, in layman's terms, a
> >motor may not start up in the same position
> >everytime you run it. Therefore, you need to tune it every time you use
> >For me its not a problem, but some might rather use a true salient pole
> >synchronous motor.
> >Hope this helps.
> >The Captain
> Dan, I shared a similar learning curve. I had the motor, the machine
> someone else was paying for the materials, and I'm a quick study
> ;) ).
> A hysteresis motor is an interesting beast. It is able to deliver the
> amount of torque irrespective of the speed of the motor, right up to its
> synchronous speed. The torque of squirrel-cage induction motor OTOH drops
> to zero at the synchronous speed.
> Mark Broker
> Chief Engineer, The Geek Group