On 2008-09-29, David Lesher wrote:
Vernon writes:


Joseph,

I'm still trying to wrap my electrically challenged brain around the
differences / advantages / disadvantages between rotary converters and
VFDs. As soon as I'm able to articulate an intelligent question I'll
ask one!

A VFD is a big magic solid state box that delivers AC of from {say} 10Hz
to 100Hz. It USUALLY will be one able to deliver 3 phase power. [But
there may well be smaller single phase ones, so cabbage emperor...]

There are smaller ones designed to *run* only from single phase,
and power a 1/8 HP motor (I have one -- about the volume of a brick and
a lot lighter).

But not for driving normal single phase motors at variable
speeds. The tricks which start single phase motors are rather frequency
sensitive. Most capacitor start ones will need a different value of
capacitor for each frequency range -- rather impractical.

Shaded-pole induction motors (such as old phonograph motors or
little fan motors (pre muffin fan style) use a trick consisting of a
heavy shorted winding around only part of each pole, causing a phase
shift at that end of the pole to nudge the motor to a start. This is
rather frequency sensitive as well. I've played with such a motor and a
VFD -- and you get some range of variable speed, but for below say 60%
speed, or above perhaps 120% speed, the motor will simply stall. So it
is not practical.

How it does that.... Well think of it as an audio oscillator and a
BIG amplifier for the speakers err motor.

Except that it is normally done by PWM (Pulse Width Modulation)
so the transistors driving it can simply switch fully on or off (both low
power conditions), and avoid the half-on state where the maximum power
is dissipated in the transistor -- generating heat and leading to
failure.

Its major plus is the variable frequency means whatever motor it feeds
will be variable speed.

Whatever *three* *phase* motor will be. Single phase will not
be happy. :-)

It's almost a waste to buy one just for a single
speed task, but a friend did to drive an air compressor.

Air compressors can draw a *lot* of power, and running one from
a VFD (if you run it a lot) will probably cover the cost of the VFD in
power bill savings over two or three years. They are close to 100%
efficient because of the switching mode output, while a RPC is rather
inefficient.

A rotary converter is a semi-kludgey way of getting 3 phase from a single
speed source, at the line freq. [60 Hz]. Ditto the capacitor-based
schemes. They may work well enough for your case. But neither one will
deliver the large starting torque a true 3ph supply will offer.

Actually -- a *larger* idler motor than the load motor (1.5x or
2x) will probably provide all the torque you need -- especially if you
take time to balance the converter by adding capacitors between the
generated phase and the other two legs to produce approximately equal
current in each leg.

The capacitor start only certainly throws away motor horsepower,
at least for full-time operation. Someone here has measured produced
horsepower from the cap start trick and found that the motor will
produce that full horsepower -- but the current through the only powered
winding is much higher than that when it is driven from true three
phase, so long term operation can wind up burning out the motor's
windings.

Enjoy,
DoN.

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