Re: DC versus 3 Phase

On 20 Feb, CAD_CAM_EDM_DRO@yahoogroups.com wrote:

> I'm getting really tired of changing belt pulleys and want to change
> to a variable speed system on my shoptask machine.
>
> Can someone out there help me with the pros and cons of DC and 3
> phase systems. I would probably buy the controller and motor on E-
> bay, 220 v is no problem, single phase supply, want about 1 HP.

I've converted my Shoptask to a 3phase VFD, also being tired of
changing belts. Here's a few comments ...

1. As I have CNC'd the machine, there is never a need to have both the
mill motor and the lathe motor running at the same time. Therefore, I
replaced both the original motors and run one or the other off the same
controller. ( I'm cheap and VFD controllers are expensive.) A 3 pole
double throw relay was the trick here.

I'm also kinda pleased with myself - I used the original Shoptask
switches. The upper Fwd-Off-Rev is now Lathe-Off-Mill, the lower is now
Fwd-STOP-Rev.

That is, AC power is switched on and off ( through another relay ) by
the upper. And as the VFD has dynamic braking capability, the lower
switch truly has STOP capability

2. My VFD ( obtained NEW, largely for free - sometimes a guy gets
lucky ) will control 1/2 HP motors. I then got some Baldor 1/2 HP
motors. I was quite surprised to find that at full speed, these motors
are not nearly so powerful as the original Shoptask motors, rated as
3/4 HP.

We're often quick to condemn "crappy Chinese", but in my case, the
Shoptask motors are all they're rated for.

Therefore, buy at LEAST a 3/4HP VFD and motors. This motor will be
oversized, IMHO, for the mill but about the right size for the lathe.

3. I LOVE the VFD on the mill. It is so nice to set an approximate
speed ( using the belts ), then dial it right down for use with edge
finder, then wind it back up again to mill.

4. I'm less pleased with the VFD on the lathe, although that is
largely due to the now lower power. I CAN stall the motor with an
agressive cut and that almost always results in a damaged insert.

Also, I still change belts on occasion, although I no longer use the
intermediate pulley. ( The bearings in that pulley were trashed
anyway ) With the belts configured for about 2400 RPM max speed, there
is really no power left if you "dial" it down to 100 RPM. All the more
reason to buy bigger motors than you think you need.

However, with that intermediate pulley out of the way, belt changes are
not so onerous.

5. My VFD has overcurrent protection, as I suspect most do. This is
kinda nice, as you can stall a motor, repeatedly, without overheating
it.

OTOH, you do run a risk of overheating the motor if run well loaded at
low speeds. The motor's fan may not be turning fast enough to
adequately cool it. I haven't noticed this with the lathe (strangely)
but have noted it with the mill.

I fabbed up a couple of brackets that bolt to the mill motor mount and
attached some 5" computer-like fans to either side to blow air at the
sides of the motor. Works well.

6. You'll need a tach. Easily added from many sources. DanM sells a
good one for use with 3in1 machines. I built my own and added what I
think is a clever feature.

My tach has a "set" button.

I turn the power on, wait for the motor to come up to speed. Once at
speed, I press "set" and proceed to CNC. Should the motor speed ever
drop to 1/2 the "set" speed, an ESTOP signal is sent to CNC controller.

Ultimately, I'll wire this up so that the CNC software, through an
output switch, will "press" the set button for me.

7. DC motors typically make power through RPM. This makes them
somewhat unsuitable for those occasions when low speed work is required.
On the Shoptask, you'd be back to using the idler pulley again AND the
low speed kit - more belts, not less.

Hope this helps.

Alan

--

Alan Rothenbush | The Spartans do not ask the number of the
Academic Computing Services | enemy, only where they are.
Simon Fraser University |
Burnaby, B.C., Canada | Agix of Sparta