PM-833T Motor RPM?

[davidpbest]

There is at least one data point on YouTube with an 833T being converted to CNC, where the owner was driving the stock head at 8K RPM. Inspecting the innards of the geard head after what appears to be a lot of run time, the gearing held up just fine, but the spindle bearings were fried.

Who's to know. Maybe someone could post the FEA the factory surely did of the geared head on the 833T.

 

[markba633csi]

I thought the specs on that nameplate seemed rather optimistic too- Like Mark said the current draw should be something like 12 amps at least for 2 HP
-Mark

 

[ahazi]

I thought the specs on that nameplate seemed rather optimistic too- Like Mark said the current draw should be something like 12 amps at least for 2 HP
-Mark

Look carefully, maybe it is an upside down 7 for -
7 amp x 220 volts = 1,540 watts ~ 1.5KW which matches the plate and
1.5 KW = 2 HP by calculating
0.746 KW x 2 HP = 1.5 KW

All very simplistic without accounting for power factor, service factor losses and current under actual load. All I was after is the nominal RPM as I think that actual HP ratings are questionable.

Ariel

 

[JimDawson]

I think Asian horses are smaller than USA horses.

 

[ahazi]

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.

Mark, could it be that a contributing factor for heat at very high RPM are the low quality spindle bearings? They might be seizing and creating extra load on the gearbox? From what I managed to find everything is pointing to overstressed spindle bearings. Unlike the gearbox that is full of oil, the spindle bearings are greased which reduces the ability of the bearings to dissipate heat especially if they are low quality variety that generates excessive heat from friction.

Just a thought, comments are welcome.

Ariel

 

[mksj]

The limitation is often the design of the gears, material/hardening of surfaces, bearings used, tolerances, lubrication, etc. The YouTube fellow who was pushing his 833T to 8K was on the path of self destruction, he also was replacing the gear drive with a belt system. Most people that CNC their machines go that route. I would not want to be around if something lets go in the head at speed. Assuming the quality of the gears and the bearings are better on the Taiwanese gear heads, than you may be OK with 3K top end. This is similar to the RF45 which when ordered with a 2 speed motor with a 6 gear speeds has a speed range of 65-3100 RPM. Still just a guess, and he also burned up his spindle bearings. In the end he will have nothing left of the stock drive system. My gearhead at 3K was annoyingly loud, can't imagine at 8K.

After break in you may want to switch to a better gear oil, I used the AMSOIL Synthetic Marine Gear Lube 75W-90, this is often used in stern drives which undergo extreme drive conditions. I did contact Amsoil with regard to EP additives and they indicated it did not effect yellow metals "Marine Gear Lube is designed to prevent rust and is compatible with aluminum, copper and brass alloys". There was a very noticeable decrease in head temperature, a lot less oil foaming, and I picked up around 100 RPM on the top end.

AMSOIL Synthetic Marine Gear Lube 75W-90

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The stock single and 3 phase motors on many of these Asian mills (and lathes) are metric frame, the problem I have encountered in the past is that the replacement metric frame motors (usually Leeson) are lower Hp for the same size frame/shaft. As the YouTube gentleman indicated he had to turn down the shaft and most likely had to adapt the motor key. Seems like a lot of ifs and pretty much you are on your own if something goes. If there was a drop in 3 phase motor with similar ot better specs. that fit, than it would be a simpler VFD conversion.

There VFD conversion is pretty straight forward, there are several threads on VFD conversions for the PM-932/940, two on the PM-932:

PM932 Mill Motor Upgrade

Hello group. I haven't posted in quite a while; however, I read most of the posts here daily. I have a PM932PDF mill that I am in the formative stages of getting ready to think about upgrading the motor to 3 phase with a VFD. I figured the best place to start would be with the motor. Below is...

www.hobby-machinist.com

Pm 932pdf 3ph Vfd Conversion

After many evenings of tinkering and emailing MKSJ (who I'd like to think very much for his help with the conversion ) the mill is once again usable. Some fine-tuning still needs to be completed but that will happen in time. I'm extremely happy on how it performs. The soft start and...

www.hobby-machinist.com

 

[mksj]

If someone was interested in replacing the stock single phase motor with a 3 phase direct fit motor rated at 1800 RPM, 4P, D80 frame with a B5 flange there is only one I could find in a 1.5 Hp rating that matched the stock 833T mounting specifications. Most are 1 Hp in a D80 frame, going to a D90 metric frame size allows for 1.5 - 2.0 Hp motors with the same mounting flange specs. but would require turning down the motor shaft and cutting a new key slot.

TechTop GR3-AL-TF-80B35-4-B-D-1.5 Inverter rated 3 phase motor, should be good for 2X it's base speed in a mill application. Since it is a TEFC motor the low end of operation would be around 20 Hz to minimize cooling issues.
Aluminum, TEFC, 80 FRAME, 1800 RPM,BALL BEARING, 230/460V, 1.5 HP, Removable Feet, B5 flange face

GR3-AL-TF-80B35-4-B-D-1.5 – Electric Motor Superstore

There may be higher output BLDC options that would require a custom mounting plate and controller.