[Milsurplus] High frequency power.. more

[J. Forster]

Just a bit more on the maximum speed of AC motors:

AC is generally a sine wave. If you phase shift it with a capacitor, you can
also generate a cosine wave, which is a sine phase shifted by 90 degrees, If you
plot a sine on the X axis and a cosine on the Y axis of a scope (and the waves
are of equal amplitude) you will get a circle. At any instant of time, the spot
will be at some point on that circle and with changing time,  the spot  will
travel around the circle. It goes around once per cycle.

For simplicity, consider a two phase motor. Such things exist and were widely
used in WW II. They are called servo motors. If you apply the sine signal to one
coil and the cosine signal to the other coil, which is rotated 90 degrees in
phase with respect to the first coil, you will get a rotating magnetic field
that makes one full rotation per cycle of the applied voltages. A bar of
magnetic material placed in the middle and on bearings, will follow the magnetic
field and rotate once per cycle. It CANNOT rotate MORE than once per cycle,
unless it is driven faster by some external device. That said, the fastest a
motor can turn is once per cycle. This is called the 'synchronous speed' (It's
how electric clocks of the non digital type, and traffic lights stay
synchronized). Synchronous speed is 3600 RPM for 60 Hz and 24,000 for 400 Hz.

An AC motor can run slower than synchronous speed, but not faster.  The above is
not meant to be a rigorous treatise on AC motors.

-John



antqradio at juno.com wrote:

> My Black and Decker router is rated at 25,000 RPM with 120 VAC 60 Hz.
> Must be witchcraft!
> Jim