Re: VFD/Potentiometer/Motor Questions

Don,

You've answered some of my questions in the past, and I want to thank you once more for taking the trouble again.

I learned a lot from you regarding the shielded wiring. I had already ordered twenty feet of 18/4 shielded wire from Ebay ($3.99 for ten feet!) for both the pot and for the drum switch (the VFD does in fact have 10 volt control voltage). On page 2-10 of the users manual (http://web2.automationdirect.com/static/manuals/gs1m/ch2.pdf) it shows the shielded wire connected to the CM (common terminal I assume). So with this, would I still ground the shield to the box the pot is in?

The motor was made by Westinghouse. I'll set the max speed to 150% of rated RPM (to about 2500 RPM) and keep an eye on it. My VFD will go to 400 Hz, and I'll heed your advice to not go there. I really don't think I need that kind of speed anyway.

Thank you again!

Dave Young

DoN. Nichols wrote:
According to Dave Young <daveyoung@xxxxxxxxxxxxxxx>:

I'm hooking up a VFD to my 1961 vintage 9" South Bend lathe (with .5 hp, 208 vac, 3 phase motor) and I have a few questions.

I can hook up an external pot to control the speed of the lathe. The specs says the pot has to be between 3K - 5K ohms. What would be the difference in operation if I were to use a 3K ohm pot vs a 5K (or vice versa)?


Assuming a 10V control voltage, the 5K pot would dissipate 0.020
Watts, the 3K pot would dissipate 0.033 Watts. Essentially negligible.
Operationally -- *no* difference. The range given is selected so:

1) The resistance is not so low that it is drawing too much
current out of the reference output on the VFD.

2) The resistance is not so high that the wires pick up too
much electrical noise -- which would result in constant
variation of the speed.

Anything within the range which they specify will work without
problems, and can be considered equivalent to each other.

If you remote the pot some distance from the VFD, I would
suggest that you run the wiring through a shielded twisted pair wire.
The shield connects to ground and the low side of the pot, one side of
the twisted pair connects to the pot's high side, and the other to the
pot's wiper. The VFD's manual will tell you where to connect each wire
at the VFD end. And put the pot in a metal (aluminum will do) box at
the lathe end of things, with the shield connected to the box metal as
well. This will minimize noise pickup.


What would be a reasonable maximum speed for the motor? The nomenclature plate says 1725 operating rpm, but I understand with a VFD you can go much higher. I don't want to hurt the motor, but the extra speed would be nice when using carbide. I've also got the ramp up and deceleration speed currently set at two seconds; is this reasonable?


This depends on the motor itself. Often the same rotor is used
in several speeds of motors by the same manufacturer, so if you can find
data from the manufacturer of the motor indicating that the same part
number rotor is used in the 3450 RPM version of the motor, you should
have no problems running the existing motor up to twice its speed.
Anything beyond that would be a serious problem.

If you can't find such information, pick up a 850 RPM motor with
a bit more horsepower, change the pulleys to get the same spindle speed
with that motor, and you should be able to run that one to twice its
speed with no problems -- since you are unlikely to have a particularly
large motor on a 9" South Bend anyway. The larger the motor, the more
likely that overspeed will cause it to self-disassemble.

If your VFD can go up to 400 Hz (some can), *don't*. Stick to
no more than 120 Hz unless you have a motor *designed* for 400 Hz (such
as aircraft motors.)


I knew nothing about VFD's when I started the installation (I'm still not quite finished), but I have to say that it sure seems slick. Not sure of the advantages/disadvantages of using a VFD compared to a static phase converter (which is what I was using before). I understand with a static phase converter you lose 1/3 of your horsepower, not sure if the same thing happens with a VFD. I'm hoping not...


It should not lose any of the horsepower, as it is producing a
full three phase full time. The static phase converter only makes up a
pretend third phase for long enough to get the motor spinning, and then
turns it off -- pretty much akin to the centrifugal switch and start
winding on a single phase motor.

Good luck,
DoN.
.

Relevant Pages

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  • Re: VFD/Potentiometer/Motor Questions
    ... On Fri, 11 May 2007 01:50:01 GMT, Dave Young ... I can hook up an external pot to control the speed of the lathe. ... What would be a reasonable maximum speed for the motor? ... nomenclature plate says 1725 operating rpm, but I understand with a VFD ...
    (rec.crafts.metalworking)
  • Re: Electric motor on KBC mill
    ... speeds. ... shift at that end of the pole to nudge the motor to a start. ... VFD -- and you get some range of variable speed, ... In the case of this mill it has an instant reverse switch on the left ...
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  • Re: Electric motor on KBC mill
    ... A VFD is a big magic solid state box that delivers AC of from 10Hz ... There are smaller ones designed to *run* only from single phase, ... speeds. ... shift at that end of the pole to nudge the motor to a start. ...
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