Motor swap

[jwmay]

I've been thinking. I know that can be dangerous, but sometimes I can't help it.
Every since I got this Rockwell 21-100 milling machine I have been thinking on how to get variable speed. I can't ever see me needing any but one of the currently offered pulley configurations. I run high speed tooling mostly, and slow. So I got hung up on the fact that this motor it has, has a 5" long shaft on it. That's an expensive replacement. But really, if I just put a standard 3/4 hp 1725 rpm 3 phase C flange motor on it, with a single pulley sized for mid range speed, I'm all set. Right? The VFD will deal with the speed changes...no fuss. Correct?
I'm only asking because usually when something strikes me as simple, it just means I haven't thought it through well enough. Thanks for any replies.

 

[matthewsx]

Yes

 

[jwmay]

Well...that was easy.

 

[machPete99]

Longer answer...
Look on ebay for larger C-mount motors with enough shaft to do what you want. Ideally you want 4 or 6 pole, 220V, 3 phase, 1HP.
I picked up a special "washdown" motor that had enough shaft length, disassembled it, and cut down the shaft on the lathe to match the original.
Having 6 poles and extra HP overhead means you can run larger cutters and drills at slower RPMs and still have enough torque to get the job done.

See:

Rockwell 21-100 Spindle Rebuild

VFD is setup, using the Hitachi WJ200, 1HP model. Wiring is fairly simple, with 2 pole disconnect, 3 way forward/off/reverse switch, and speed control, all in one box. Also have new 4" CMT Vise mounted and rubber way covers installed. Just made fist chips cutting some flats on some heavy DOM...

www.hobby-machinist.com

 

[rabler]

Of course you need to wire the VFD up to suitable controls. I agree with @machPete99, go up to 1 HP or even a bit bigger, Think about your pulley size (what RPM your spindle will run at with the VFD at 60Hz). I'd suggest making sure you get an inverter rated motor that you can run down to 10Hz or slower. 10Hz - 120Hz gives you a 1:12 ratio between lowest RPM and highest. That's a ballpark but you get the idea. Remember that with a 1HP motor, at 20Hz you get 2/6's (1/3) HP, and 10Hz you get 1/6HP, so you loose power quickly at the bottom end

 

[strantor]

I can't ever see me needing any but one of the currently offered pulley configurations. I run high speed tooling mostly, and slow.

Are you saying you run high speed tools, slowly?
Or that you run HSS tooling, slowly?
Or that you run both high speed tools and low speed ones also?

Keep in mind that with belt/gearing changes as your mode of variable speed, you get a proportional increase in torque for a decrease in speed, and vice versa. With a fixed pulley/gear and VFD as your mode of variable speed, you don't; your torque is more-or-less fixed (fixed upper limit) at any given speed. Since HP is a function of RPM and torque, as speed goes down, useable HP goes down, as already mentioned. That is the reason for recommendations to go with a slightly larger motor.

I echo that recommendation and also suggest that you choose your fixed drive ratio somewhere in the lower ratios of what you have right now. That way you maintain plenty of available torque. A VFD enables you to exceed 60Hz, and most inverter-duty rated motors are given a "max safe speed" rating, which is often more than twice their nameplate base speed. So you could set your VFD for example to a max of 130Hz and still be able to hit the high speeds, without sacrificing much (or any) torque at low speeds.

 

[ConValSam]

If you plan to run low frequency for long periods you should make your motor can handle the heat generated and perhaps plan a cooling strategy.

 

[mksj]

Yes and No. You loose the mechanical advantage when you go below 60 Hz, so at 10 Hz you have 1/6th the applied torque that you would have by belting/gearing it down. Hp also falls off in a somewhat linear fashion below the base speed of 60Hz. Cooling is an issue below around 20 Hz for a TEFC motor, a TENV can dissipate the heat. At best with a vector type motor you can achieve maybe an 8-10 fold speed range and still low speed will be pretty anemic so you need to upsize the motor, probably 1.5 Hp as a single speed. You also want to run the motor to much higher speeds, the vector motors will run to 5-6K RPM. So belt it so you hit the maximum speed at ~200 Hz and a speed range of 20-200 Hz.
BALDOR IDNM3584T MOTOR 1.5HP 230/460V 1755RPM 7/8" DIA

Baldor IDNM3584T General Purpose Motor for sale online | eBay

Find many great new & used options and get the best deals for Baldor IDNM3584T General Purpose Motor at the best online prices at eBay! Free shipping for many products!

www.ebay.com

 

[Bi11Hudson]

The only comment I can make here is that a 4 pole motor is easy to find. For a 6 or 8 pole, have your bankroll ready.

.

 

[JimDawson]

It has taken me a long time to unlearn what I thought I knew about motors and running machine tools.

Horse power rating doesn't matter, it's actually a pretty useless number. HP describes the capacity to get work done over time. It is a useful number to compare motors, sort of, but not really descriptive. It only describes how much power the motor is rated to draw at full load. By definition 1 HP = 745.7 Watts

What is more important is the torque that the motor is capable of producing at a given RPM.
Here is the rated torque of three 1 HP, 3 phase motors. The motors below are all rated at 5400 RPM maximum, other motors max RPM may vary.
2 pole, 3600 nominal RPM @ 60Hz, torque = 1.5 ft/lb (2.033 Nm)
4 pole, 1800 nominal RPM @ 60Hz, torque = 3.0 ft/lb (4.067 Nm)
6 pole, 1200 nominal RPM @ 60Hz, torque = 4.5 ft/lb (6.101 Nm)
*Actual RPM will be less due to slip.

Using a modern sensorless vector VFD you can flatten the torque curve to produce nearly 100% rated torque from about 10 RPM up to the rated speed of the motor (constant torque mode), then above the rated speed the torque starts falling off (constant HP mode) until you reach the maximum rated RPM of the motor, at which point the motor is capable of producing very little torque.

The down side of operating a standard 3 phase motor at near maximum torque and below the rated RPM is the potential of overheating. Having said that, I have found that in most cases my mill motor runs at about 50% power input, unloaded, from about 10 RPM to 1800 RPM, and the power input drops off a bit as the RPM approaches 1800, and doesn't change that much when loaded in normal cutting. I have hit 100% input power when taking heavy cuts with large diameter cutters like a face mill. Above 1800 RPM the input power is pretty constant at around 50%. I have never had a problem of overheating. The motor will only produce enough torque to maintain the set RPM at a given load. High RPMs imply small cutters and thus require very little torque to run them.

I have a direct timing belt drive and it is geared up to 1.3:1 as I recall. I originally had it set up at 1:1, but found this not to be the best ratio, so I changed it.

So for your machine, I would choose a motor that has about double the torque of the original motor, put a sensorless vector VFD on it with a fixed belt drive and make chips.