Now I went and did it. Let the fun begin!

[jwmelvin]

Generally speaking with the mechanical variable drive you will be getting full horsepower at all speeds whereas in VFD variable you lose power at reduced speeds. The step pulleys offset much of this though.

Mechanical variable has some additional potential for wear/maintenance. The step pulley takes more time to change speeds when a pulley change is required (less than a minute extra).

My old step pulley mill has a VFD and covers all speeds 40-4200 rpm using VFD, pulleys, and back gears.

Oh also: the mechanical-variable approach will be a little louder in my experience (none of which is with new, more modern systems). But the VFD makes some higher-pitch whine that many find objectionable. That whine can be minimized by running a higher carrier frequency in the VFD. Mine was bad at default and just fine after one parameter adjustment.

 

[davidpbest]

I have a PM-935TS driven by a Hitachi WJ200-015SF VFD and have never had to change belt positions to get 50-3700 RPM. Personally, I would not want mechanical variable speed because of the longer term maintenance issues. To get a sense for that aspect watch this video.

 

[jwmelvin]

I have a PM-935TS driven by a Hitachi WJ200-015SF VFD and have never had to change belt positions to get 50-3700 RPM.

That’s a good point - that modern motors can handle a wider frequency range, reducing or apparently eliminating the need to change pulleys. David, what frequency range do you use for the VFD?

 

[davidpbest]

That’s a good point - that modern motors can handle a wider frequency range, reducing or apparently eliminating the need to change pulleys. David, what frequency range do you use for the VFD?

30-100 Hz if I remember correctly - it's been a while since I set up the VFD. On my 1340 I have a vector-rated Baldor motor and can run that with a much broader range (20-130) and not loose torque or HP.

 

[Cletus]

I've seen that video and glad I chose the TS. So far, never had to change belt position. I love my PM-935TS

 

[Just for fun]

Thanks for the video Dave. That doesn't look like a fun job.

 

[Just for fun]

I made the deal this afternoon and ordered the 935TS 3 phase, I also ordered a Hitachi VFD at the same time.

 

[jwmelvin]

30-100 Hz if I remember correctly - it's been a while since I set up the VFD. On my 1340 I have a vector-rated Baldor motor and can run that with a much broader range (20-130) and not loose torque or HP.

I apologize in advance for being pedantic, and should have done some estimates before asking; I'm not intending to argue. It seems likely that the range you describe (50-3700 rpm) would require a pulley change according to the PM-935 manual (page 10). My mill runs at very similar speeds on the four pulleys.

Let's say the VFD runs 22-120 Hz. On pulley 2 in back gear (135 rpm at 60 Hz), that would get you 50 to 270 rpm. High gear would go from 400 to 2230 rpm. That's a wide range, but I still could see wanting to change pulleys, and that frequency range would be pushing some motors.

Now in reality, I agree that precise speed isn't so important for most things. I seem to keep my belt on pulley 2 or 3 most of the time (I run the VFD from 30-90 Hz, as I'm trying to be kind to the 40-year-old motor). I made a little chart to let me know the min and max for each pulley, as I don't have a tachometer built in, just a handheld one:

David, I will say that your approach to your machines and tooling is an inspiration. I appreciate how much you put into both what you do and how you share it. Thank you.

I made the deal this afternoon and ordered the 935TS 3 phase, I also ordered a Hitachi VFD at the same time.

Congrats! It will be awesome.

 

[Larry$]

I was wondering what some of the other advantages were other then the RPM range.

I've got a Jet 949 knee mill that has two ranges and variable speed. I don't know what a VSD would add but I sure wouldn't like changing belts. The gear & variable speed drive keeps all the power/torque regardless of what speed. I'll never have to figure out why the electronics isn't working.

Yes changing the VS belt is a PIA but I bought a well used mill. The belt was original from 2004 when it was made. Bought the belt for $60 and 2 hours for 2 of us to change. It will out live me.

 

[davidpbest]

I apologize in advance for being pedantic, and should have done some estimates before asking; I'm not intending to argue. It seems likely that the range you describe (50-3700 rpm) would require a pulley change according to the PM-935 manual (page 10). My mill runs at very similar speeds on the four pulleys.

Let's say the VFD runs 22-120 Hz. On pulley 2 in back gear (135 rpm at 60 Hz), that would get you 50 to 270 rpm. High gear would go from 400 to 2230 rpm. That's a wide range, but I still could see wanting to change pulleys, and that frequency range would be pushing some motors.

Now in reality, I agree that precise speed isn't so important for most things. I seem to keep my belt on pulley 2 or 3 most of the time (I run the VFD from 30-90 Hz, as I'm trying to be kind to the 40-year-old motor). I made a little chart to let me know the min and max for each pulley, as I don't have a tachometer built in, just a handheld one:
View attachment 385905

David, I will say that your approach to your machines and tooling is an inspiration. I appreciate how much you put into both what you do and how you share it. Thank you.


Congrats! It will be awesome.

OK, I couldn't stand the controversy any longer, so I went to the shop and made a video of my PM-935TS with factory 3-phase motor, Mark Jacobs control system which includes the Hitachi VFD. Here's the link to the video - starts out with the backgear disengaged:

https://flic.kr/p/2mKKwQu

As you can see in the video, without moving the belt, I get 54-436 RPM with the backgear engaged, and 474-3780 RPM with the backgear disengaged. Those speeds are with the belt positions configured as shown in the photo below. I honestly do not remember the parameters I put into the VFD, but they can't be far off from what I posted above. Clearly there is a broader speed range possible in both directions with the belt repositioned, but I haven't needed to touch it in the 3+ years it's been in service.

FWIW, I have found the speed charts in the PM manuals to be imprecise, and off by a substantial amount on some of the benchtop mills sold by PM. Hope this helps.

David