Re: [CAD_CAM_EDM_DRO] Single phase mains

Bill Darby wrote:

> Single phase mains.
>
> I have been running a five horse Top Well mill (5 VK I think they call
> it) with an Anilam 1100 controller and I have always had a problem
> with the bearings in the variable speed unit. (replaced
> twice) So I am searching for a way around the problem and hopefully I
> can convert to an electronic variable speed system. I would appreciate
> very much hearing form someone who has taken this route
> and knows the pitfalls.

I used an el-cheapo unit first, and while it worked, it caused the motor
to run
hot. It had excellent torque, though. I then upgraded from the 1/2 Hp
M head
to the 1 Hp J head on the mill, and connected the 1/2 Hp rated VSD. It
worked
fine, until one day I tried to do powered tapping, and wanted to run the
motor at
very low speed direct drive so it would stop without coasting. The VSD
blew
up! Well, that's what I get for trying to run a 1 Hp motor with a 1/2
Hp VSD.

Then, I got a 1 Hp rated unit, made by Magnetek. This is an
industrial-grade
PWM inverter, with a vast array of settings available in the
microprocessor.
I think it is actually made by Yaskawa, as the plug-in programming
keypad
had their label on it. It works great, the motor runs cool for hours,
even at
low speed. I was running it at 9 Hz (15% of nominal 60 Hz) for over an
hour to get slow rotation of an EDM electrode, and the motor was
completely
cool at the end. No load, but that operation with older square wave
inverters
will make the motor heat up even without load.

I can strongly recommend one of the newer, programmable VSDs for this
application. My unit is rated for 3-Phase input only, but I figure I
rarely
run that power level for more than a few seconds, if ever, so I can
easily
get by with this.

> These single to three phase variable frequency control units do not
> seem to be available in the five horse variety so I was wondering if I
> could down size to a three horse arrangement (I never do any
> heavy duty hogging and believe that I would be much better off with a
> less powerful motor that could run about twice the speed of this unit
> that tops out at 3K rpm. Additionally I would dearly love
> to do away with having to run a rotophase.

Well, I would suggest using a 5 Hp VSD, even though it specifies 3-phase
only input.
You will have much better results with this than an undersized unit.
One of the problems
I ran into when talking to Magnetek about using it with a high-speed
spindle motor at
400 Hz is that these units are very sensitive to motor inductance.
Larger HP motors and
high frequency motors have lower inductance, and you may need to add
series inductors
to run a 3 Hp VSD with a 5 Hp motor, even if you only want 3 Hp output!
The worst
case duty cycle of the PWM technique is tuned for the lowest inductance
expected on
motors of the rated capacity of the VSD. If you try to run a larger Hp
motor, it may
saturate the iron in the stator, cause a high current pulse, and
possibly pop transistors
in the VSD. Not a good thing to happen. If you intend to change to a 3
Hp motor,
then the 3 Hp VSD should work OK. A 5 Hp unit that is fully
programmable will work
fine with a 3 Hp motor, as you can program the current limit lower.

> The last query that I have is about how the hookup between the VFD and
> my controller is done. Can it only be done by an expert, or could I do
> it myself??

If you want to have spindle RPM control from the CNC, that is possible,
if your
CNC provides 4-20 mA current loop or 0-10V DC output. But, you need to
find out what your CNC provides, and then select the appropriate VFD and
options.
The Magnetek GPD-333 I got was a dealer sample, and so it had EVERY darn

bell and whistle they made already on it. Most won't, as the options
add to
the cost. There is an option on the Magnetek that will take BOTH 4-20
mA
AND 0-10V. It also can select from several start/direction command
schemes.
One is one contact for run/stop, one for direction, another is one
contact for
start forward, and one for start reverse, and I think there is one more.

The start stop controls just require a relay contact closure from the
CNC.

Jon