Alt-BEAM Archive
Message #01834
To: beam beam@corp.sgi.com
From: "Ben Hitchcock" beh01@uow.edu.au
Date: Mon, 29 Mar 1999 17:27:44 +0000
Subject: [alt-beam] Re: Small parts
>>
> > Excellent description of the gear boxes (deleted). However this ratio
> > may need correction. For an AC motor, the synchronous speed can be
> > calculated by the equation:
> >
> > (120 x f)/P
> >
> > Where f is the frequency of supply and P is the number of poles. So for a
> > motor with 2 poles operating with a 60 Hz supply, 3600 RPM is correct.
> Does that formula change with voltage?
> Why can't we rewind a small motor to get what we want? I gather it is to do
> with the gauge of the wire and the number of turns. The radio controlled
> car fanatics rewind their motors to get more torque so.....? The Maxon
> motors have the coils moving around the magnets so
> Rob
Sorry about the last (non) message. I guess my uni UNIX box got a bit
confused and decided not to post the stuff I typed. Here it is again, typed
on a MAC this time!
The speed of a synchronous AC motor is entirely dependant on the frequency
and number of poles, not the voltage. I remember a demonstration we were
shown at work where a small 415 volt 3-phase motor was made to run at 32
volts (3-phase). The speed was exactly the same as when 415 volts was used.
For DC motors, the voltage makes a lot of difference, but in an AC
synchronous motor the only thing that changes the speed is the frequency,
and the number of poles. (Unless the motor is stalled, of course!)
The reason for this is that an AC motor has a rotating magnetic field that
rotates around the central core at (frequency / poles * 2) times a second -
and the rotor will follow this speed exactly or be stalled.
Hope this helps,
Ben
------------------------------------------------------------------------
eGroup home: http://www.eGroups.com/list/alt-beam
Free Web-based e-mail groups by eGroups.com
Home