PM-833T Motor RPM?

[ahazi]

I am considering getting a PM-833T and replacing the original single phase motor with a 3 phase motor and a VFD so I can get to a higher RPM (around 3,000.) It seems that the PM-833TV (electronic speed control version) will lose substantial power at low RPM due to the fact that it is the same gear ratio for the entire range. So my conclusion after checking and scouring the web for information that I am better off with a modified PM-833T vs getting the TV version.

I am looking for information about the original single phase motor rated RPM. I could not find this information anywhere. I will greatly appreciate if one of the PM-833T owners can take a look and report back.

Any other ideas, shared experience or warnings are welcome.

Thank you!
Ariel

 

[Long Roof]

1720 rpm. Your VFD idea sounds like an interestig idea but a lot of work. If you go ahead be sure to keep us posted on the progress.

 

[7milesup]

Interesting...I have an 833 but they did not have the TV version when I purchased mine.

Not sure about it losing power though at low RPM. I would think that it depends on the electonics Matt is using in that. I'm thinking that that is already a VFD drive on that mill?

 

[davidpbest]

Interesting...I have an 833 but they did not have the TV version when I purchased mine.

Not sure about it losing power though at low RPM. I would think that it depends on the electonics Matt is using in that. I'm thinking that that is already a VFD drive on that mill?

Maybe this will help in understanding why running a 3-phase motor at 3Hz ( three Hz is the required frequency to get the 833TV to 50 RPM ) is unlikely to yield much power. More info here: https://www.hobby-machinist.com/threads/pm935tv.82461/page-2#post-720799

 

[ahazi]

1720 rpm. Your VFD idea sounds like an interestig idea but a lot of work. If you go ahead be sure to keep us posted on the progress.

Thank you so much "Long Roof". Does not seem like a lot of work to me... I found a drop-in metric size, VFD rated 3 phase replacement motor that I will order and electronics is my thing... So I think the a VFD and a gearbox will give me plenty of options in terms of power, spindle speed and probably noise control. I am not planning to go crazy high in RPM but at least I will have a good range and options. I will probably will replace the spindle bearings at some point to higher quality, high RPM rated set.

I will keep you all posted.

 

[JimDawson]

@davidpbest I've not see a chart like that before, thank you for posting. The best illustration of the torque / power curve that I have seen of a 3 phase motor when running on a sensorless vector VFD. Constant torque (100%) up to the base speed, and constant HP above that.

 

[ahazi]

Maybe this will help in understanding why running a 3-phase motor at 3Hz ( three Hz is the required frequency to get the 833TV to 50 RPM ) is unlikely to yield much power. More info here: https://www.hobby-machinist.com/threads/pm935tv.82461/page-2#post-720799
View attachment 314401

Thank you David!
The longer explanation is pretty simple:
Power(HP) = Speed (RPM) x Torque
Unless you can generate MORE torque as you go down in RPM you lose power, so almost 0 RPM = almost 0 HP

It is actually worse because there is only so much that you can reduce the speed of an AC motor and still get rotation. Also the magnetic losses of the motor start to go up dramatically so the motor heats up and the built in fan is barely rotating. This is why BLDC motors are used in applications that require wider speed range down to zero and high torque (think electric cars) but they are very proprietary with proprietary controller.

As an electrical engineer I like BLDC motor solutions for their efficiency and elegance but I am also thinking of how unreliable is the power electronics in a low cost devices. So my preference is to go as much as possible with non-proprietary solution that if it fails I can fix myself or just replace.

I hope this helps.

Ariel

 

[7milesup]

Very cool. Always something to learn.
Electronics are not my strong suit...Yet.

 

[davidpbest]

Totally agree Ariel. There are a number of horror stories about equipment with BLDC motors and proprietary electronic packages to drive them, keeping you hostage to the original supplier for any repair parts. Some distributors (Grizzly comes to mind) make a killing on jacking up prices on out of warranty proprietary parts like BLDC motor drives. I will follow your 833T upgrade with interest.

 

[mksj]

Single phase motor name plate, I would question the operating amps which seems to be 4, I assume it is missing a 1 in front of the 4. Single phase 2 Hp motor would draw around 12-14A @220V. It is hard to do a comparison to a BLDC motor to a 3 phase AC motor, the low end torque would be higher, but I cannot find any data on the Hp vs. RPM. I did have a 3 Hp (2.2kW) BLDC on my last mill, it had a 3 speed gearhead and covered 50-3000 RPM. I could stall it when in high gear pushed hard, low gear you couldn't. Concern about proprietary control systems is a real issue, replacement control boards are very expensive. In a lot of cases people just switched to a belt drive and VFD with 3 phase motor.

The main concern with converting the gearhead to VFD drive and expecting to double the operating speed is that the gears are not designed to operate at that speed. My gearhead mill which was factory designed for a 3000 RPM top end got very hot and the original oil would have extreme foaming. Switching to a high end synthetic gear oil helped. I have worked with a few people on PM-932 and 940 VFD conversions and I usually recommend not exceed 30-40% of the stock speed range.