On 2008-09-29, Vernon wrote:
On Sep 27, 12:38*am, Joseph Gwinn wrote:

[ ... ]

Buy a VFD to run the mill. *Three phase works very well. *Many VFDs can
be driven from 220 volt single phase, with a derating.

[ ... ]

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!

Quick and dirty:

VFD
Pros: Gives you the ability to vary speed while running over a wide
range.

Very efficient compared to a rotary converter.

Can be set up to accelerate the motor over a programmable time,
so you don't draw as heavy a current from the wall to get the
motor up to speed.

Quiet.

Cons: Does not like having switching between the motor and the VFD,
this generates high voltage spikes during the switching, and can
blow the output transistors.

So -- rewire the switch on the machine to command
start/stop/reverse to the VFD's control terminals and you are
fine for driving a *single* machine at a time. Best to have a
separate VFD per machine -- among other things so you can adjust
the speed of one without affecting the others.

ROTARY Converter
Pros: Fairly simple to make.

Can be tuned to minimize current draw and maximize three phase
quality.

Can easily run multiple machines (if large enough) because it
does not care whether you have switches between it and the load
motors. The other machines running actually add to the capacity
of the RPC so you can handle a larger motor on the next machine
turned on.

If the idler motor in the RPC is enough larger than the motor in
the machine, it can handle "plug" reversing (switching to
reverse while at full speed forward). If the idler motor is not
large enough (say the same HP as the machine's motor), attempts
to plug reverse sometimes result in the RPC reversing instead of
the machine. This can get exciting if your machine is about to
hit something (more likely on a lathe than a mill).

Cons: Wastes power idling when there is no load turned on, so you
probably want to remember to turn it off.

Some of that waste power is turned to noise which can be
annoying (but which will remind you to shut it off when you are
done -- if you don't banish it to a box outside the house to
keep the shop quieter. :-)

Draws a heavy current spike when first starting before you ever
turn on your machine.

Depending on the nature of your particular motor, you may need
to provide additional fan cooling.

There are probably other things which I have not thought of, but
others may add to the list.

Enjoy,
DoN.

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