Re: [CAD_CAM_EDM_DRO] Re: Converter or Inverter for 2hp Bridgeport mill

Bob McKnight wrote:

>I have been told and again never checked it out that if you over
>speed an induction motor it will become a generator. I kinda bought
>this based on residual magnetism.
>
>

Connected to the mains, this is true. If the rotor turns slower than the
synchronous frequency, power will be drawn from the mains and it
is a motor. Increasing the speed above synchronous, and it will start
feeding
power back into the mains. (I'm speaking about standard induction motors,
here.)

In the case where it is not connected to the mains, it needs current
through the
stator windings to induce the current in the rotor bars. This can be
accomplished
by connecting capacitors across the windings. Apparently, this works
better on
some motors than others, depending on the magnetic remanence of the rotor
iron. This is the induction alternator that has been used by a bunch of
independant
hope power types.

>So your explaination makes sense because I would be operating in
>the middle of the area between motor and generator which would
>no current flowing or no energy applied. Except I figured I would
>be supplying a little energy, but I figured I was supplying energy to
>overcome mainly friction forces, but I can see that is not the case.
>
>

The problem with homemade rotary converters is that many are undersized,
as the motors used are not really designed for this purpose. The commercial
units use specially designed motors that have lower leakage inductance or
something like that to make the generated phase a lot "stiffer", ie. immune
to voltage variations due to changes in load current. A guy who used the
handle "Fitch" on rec.crafts.metalworking some years ago wrote a number
of very detailed articles on how to properly set up a home made rotary
phase converter to get stable voltages on the generated phase. Some of his
stuff should still be lurking around, try Google and other search engines
to find it. Basically, he had a technique based on motor HP to
calculate the
capacitors needed to "balance" the phase converter, and some simple
measurements
to optimize it. You read voltage of the generated phase with and
without load
and adjust the capacitor bank based on the change, and it quickly converges
to almost no change.

>Which brings up another question. Most convertors are used to
>run motors. I am using this to run large a large welding transformer.
>See any problems with the weak leg. The local power company
>can supply 3 phase for a price and it will have a weak leg.
>
>
>

Well, the machine motor helps do some of the balancing, too, so you don't
need as big a motor on the converter in that case. For a welder or other
load that doesn't generate some 3rd phase itself, you probably need a
substantially larger motor. Don't worry about the "weak leg" from the
power company. Their open delta arrangement will be a LOT stiffer
than any rotary phase converter. The "open" legs of the delta will droop
no more than a volt or so more than the "real" legs of the delta, because
the transformers the power company uses are QUITE "stiff". But, their
price will almost always be WAY higher than even a brand-new rotary
converter, even if the 3-phase low-tension power is already available right
on the pole outside your property. The problem with open delta is you
don't get a standard 3-phase neutral. So, you can't run any equipment
that has loads run from line to neutral, as will be found in a fair amount
of CNC gear, and even machine tools that have 3-phase motors and
contactors powered from a control transformer. So, most installations
with open-delta power need a separate source of power for lighting
and office machinery. (That can be done with your own step-down
transformer, but that's just another hassle.)

(I hope this isn't considered too far off-topic.)

Jon