Re: [CAD_CAM_EDM_DRO] Re: VFD question

On Mon, 2004-04-19 at 12:01, washcomp wrote:

> Unless I'm mistaken, you're mixing metifores.
>
> The VFD itself generally is able to reverse the rotation and the
> reversing starter is no longer needed. (The initial direction of
> rotation can be sellected on three phase by reversing two of the
> phase legs, but after that the VFD should be used).

Just a bit more info. The incoming power is rectified and filtered
into DC. Thus it doesn't matter what the phasing is for the input.
This is also why you can hook up single phase and it'll still work.
However, as mentioned below you may have to derate the current output.
The DC is chopped, making a square wave signal. This is then filtered
(or not on the really cheap units), making a somewhat sine wave.
For 3 phase output you have 3 choppers and the timing is changed between
the 3 to control the direction of rotation. The VFD itself couldn't care
less which leg goes first, 2nd, and then 3rd. It's just a matter of
timing.
I'm most familiar with the Mitsubishi line, I've got two of them.
There are a whole lot of controls and program registers. If you wire
up the 3 legs wrong and forward is really reverse you can usually
program the VFD to swap what it thinks forward with reverse. Or you
can rewire, ;-). You can also easily hook them up to a computer. It
isn't hard to set up a run signal to start and stop the motor on
command.
One thing that I find fascinating is watching the current draw
of the motor depending on what I'm doing.

>
> If using a three phase VFD on single phase, with a three phase motor,
> check with the manufacturer on sizing. Some can be used for their
> rated hoursepower, some have to be de-rated by as much as 1/2. I am
> told that it has to do with the size of capacitors the manufacturer
> elected to use. At a given HP, a single phase hookup will draw more
> current per phase than a three phase feed.

Definitely check the VFD's specs for single phase and use the motor's
amps from the plate. Don't go by horsepower. Amps won't lie! Read the
VFD's specs closely. Sometimes within the same product line they will
support single and 3 phase inputs for the lower capacity units, but
not for the higher capacity units. Read the fine print closely.

I have one for my Bridgeport mill and another for my Cincinatti lathe.
VFDs *MUST* be permanently wired to a motor. If you turn one on without
a motor you will probably damage something. One of the really nice
features is that you can rewire the machine's push buttons/direction
switch to run the VFD. Thus the machine operates just like it was hooked
to real 3 phase power. On my Bridgeport I use the original switch up on
the head for forward, reverse, and stop. On the Cincinatti lathe the
forward and reverse start pushbutton switches start the VFD in the
correct direction and the stop PB stops it. If I'm running one direction
and press the other PB, the VFD just drops the frequency to 0 and then
restarts in the other direction. If I want to change speed then I go to
the VFD, but usually I don't. And since it's true 3 phase power you get
100% of the motor's power.

Hope this helps,
Brian Botton