Do I need Constant Torque for my VFD use case?

[hustlebird]

I have an older (1992) Acer 3vk, which is a 3hp variable speed mill. I'd been using a cheap-o static phase converter on it, and today I went from drive to reverse and pop! blew a capacitor... Annoying, but it was the cheapest thing I could find on Amazon, I never expected much.

Anyway, I've got an extra Hitachi VFD, a WJ200-15..SF? Same one I have on my lathe per @mksj's great instructions there. I see that it can run 2.2kw in VT mode, and the mill is a variable speed lathe anyway, so am I missing great value if I don't run in CT mode? Basically I just want it to start/stop/reverse.

I thought maybe I could get that extra hp back with a relatively simple VFD conversion, plus use parts I have and don't otherwise need. If I need to or will want to have CT later, I guess I'll have a VFD for sale

Appreciate any feedback.

 

[mksj]

You should run it in constant torque, variable torque is used for fans and pumps where the power requirement is lower at low speeds/Hz and for start-up. Constant torque is needed on mills and lathe for low speed work where you need more grunt/power. This also effects the available amperage at different Hz and the overload points. You can compensate somewhat with a multi-speed mechanical drive and gear it lower, but the WJ200-015SF is really a 2 Hp drive and would not be a good fit for a 3 Hp mill in my opinion.

"Variable torque machines (pump and fans) are applications that allow for energy savings due the Affinity Law. This states, as I reduce the speed of my motor the torque demand decreases by the square of the decrease in speed AND the horsepower is reduced by the cube of the decrease in speed. In layman’s terms, this means for every decrease in speed, less energy (current) due to a decreased demand to perform the work. For a pump or centrifugal fan, this equates to real energy savings. In mathematical terms, a 20% decrease in speed equates to a 50% savings in energy costs; a 50% decrease in speed equates to an 80% savings in energy costs. When you use one of those payback calculators for VFDs…it is always based on a pump or centrifugal fan application. Why? Simply put: Physics. You can’t compress water and you can’t compress air with a standard centrifugal fan which is the key to energy savings…slower speed (Hz) equals less work (hp), less force required to do the work (torque) and less energy (kWh)."

 

[hustlebird]

Thanks Mark - I understand its not a great fit, but I thought it might be a 'working fit' that I could compensate for by keeping it at a fixed 60hz with light starting loads. I understand that negates the major benefit of VFD, but this is more for just the convenience of keeping the mill powered up than anything.

Guess I'll start looking for a replacement.

 

[mksj]

You could try it in constant torque mode and set the overload at say 110 or 120%, or 9.5A. Not sure if it will allow that with CT, but the VFD's tend to be protected from overload so may be worth a try. I would also set the motor voltage paramter to 230VAC.

 

[pacifica]

I have an older (1992) Acer 3vk, which is a 3hp variable speed mill. I'd been using a cheap-o static phase converter on it, and today I went from drive to reverse and pop! blew a capacitor... Annoying, but it was the cheapest thing I could find on Amazon, I never expected much.

Anyway, I've got an extra Hitachi VFD, a WJ200-15..SF? Same one I have on my lathe per @mksj's great instructions there. I see that it can run 2.2kw in VT mode, and the mill is a variable speed lathe anyway, so am I missing great value if I don't run in CT mode? Basically I just want it to start/stop/reverse.

I thought maybe I could get that extra hp back with a relatively simple VFD conversion, plus use parts I have and don't otherwise need. If I need to or will want to have CT later, I guess I'll have a VFD for sale

Appreciate any feedback.

There are other advantages to using the full range of a vfd:
Reducing speed to 20 to 100 rpm for tapping. I try not to use the variable speed built into the mill since a rebuild is not easy. Can do a slow ramp up to speed and a very quick slow down to a stop. Easy to put in a stop button.Various useful parameter adjustments for the motor.Of course changing the phase.

 

[hustlebird]

There are other advantages to using the full range of a vfd:

Absolutely! I'm unfortunately just not in the position to do it right, so I'm getting by with what I can. I would very much like to convert this to a properly sized VFD in the future. This particular machine is bigger in size (3hp) then what is affordable for me right now, so I was mostly hoping for a short-term solution that might keep it running until I can install the things I want to see on it.

Between the VFD, way covers, and a DRO I got a bill to save for (especially with Christmas around the corner!), and I would like to keep using it in the interim.