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Mach Discussion => General Mach Discussion => Topic started by: M250cnc on April 18, 2008, 07:22:41 AM

Title: Lathe motor question "Variable Speed"
Post by: M250cnc on April 18, 2008, 07:22:41 AM
Ok  more advice needed.

I am starting converting my lathe shortly, it is single phase 240 volt. Harrison M250

I want some control over the spindle speed, what is the best way to get this regardless of cost. Cheaper options will be considered also.

Any links to circuits for spindle speed encoders also welcome.

Thanks
Phil_H
Title: Re: Lathe motor question "Variable Speed"
Post by: Hood on April 18, 2008, 08:55:39 AM
Best option is an AC Servo, torque from zero to full revs, certainly not cheap though unless you find suitable drive and motor on eBay.
Hood
Title: Re: Lathe motor question "Variable Speed"
Post by: M250cnc on April 19, 2008, 03:01:05 AM
Thanks Hood.

What am i looking for, can you be more specific ?

Phil_H
Title: Re: Lathe motor question "Variable Speed"
Post by: jimpinder on April 19, 2008, 06:35:55 AM
I will suggest the alternative  - which is  - replace your spindle motor with a three phase motor (about twice the power) then use a single phase to 3 phase inverter to control the speed. PWM and M3 and M4 signals are readily available from Mach 3.

I say "twice the power" becasue you do loose torque at lower speeds with an inverter - but I am assuming you have a gearbox on the Harrisson

I only say this becasue I've done it, and I know Hood has done the servo option - the choice is yours.
Title: Re: Lathe motor question "Variable Speed"
Post by: M250cnc on April 19, 2008, 08:15:10 AM
Thanks for the answer Jim.

What i want to know is how much power is lost, i know that is a really tough question to quantify.

Can you give any more clues or real world examples, in the main i will be setting the speed via the gearbox and using the speed control to fine tune the speed.

Phil_H
Title: Re: Lathe motor question "Variable Speed"
Post by: Hood on April 19, 2008, 08:51:05 AM
I have a VFD and induction motor on my mill. It is a good quality Mitsubishi drive but just the standard Bridgeport 2HP motor so I dont know if its an inverter duty motor. I have heard that the inverter duty motors dont drop off so much in torque but how good they are I dont know. What I do know is with my VFD/motor combination I lose too much torque if I go down below 50% of the motors rated frequency. I still use the vari speed pulleys on the mill to get to the rough speed range and then fine tune by the VFD (external potentiometer). I was considering hooking the VFD to Mach but after using the mill for a while I decided there was no point as the drop off in torque would be no use for the varying types of work I do.
 As Jim has said you could go up a few HP on the motor and that would help things a great deal.

An AC Servo for the spindle however is a different thing alltogether, the torque is almost constant from 0RPM to Full speed, it does drop off slightly but rather than it dropping off at low revs it is high revs it drops off at. This is perfect for both a lathe an mill as it is the lower revs you are wanting the real torque anyway. However when I say the torque drops off at higher revs I am not talking a lot, below is the torque curve of the motor I am about to fit to the new mill I am retrofitting. It is a 5.5KW motor and its continuous torque rating is 18Nm (160lbs in approx), as it goes above 3000RPM it starts to drop off a bit but as said the torque is not so important way up there. One other advantage is no brake is needed, the AC Servo will have full rated torque when at 0 RPM so when you tell it to stop it will go to zero speed at the acelleration you have tuned it and sit there. I have been testing the motor I have and although its not connected to the spindle it still is impressive. I have tested by running the motor at 4000RPM then gone to -4000RPM with the acelleration set to 40,000RPM per second. That means from full speed CW to full speed CCW is only 2/10ths of a second and the motors out of position error doing that is zero counts and that is with the 8000count encoder (2000line) I have not tried the acell higher as it takes a bit to hold the motor down on the floor when it changes direction :D

Hood
Title: Re: Lathe motor question "Variable Speed"
Post by: Hood on April 19, 2008, 09:24:23 AM
Just a quick update, I decided to get brave and have just about dislocated my knee LOL
 Anyway I have just done a full speed CW (4000RPM) to Full speed CCW (-400RPM) with an acell set to 160,000rpm/second. It did that no problem and it is impressive to see the motor go 4000rpm CW to 4000rpm CCW in 0.05 seconds :D
 Obviously with it hooked up to a spindle with a big face cutter it might not do so well but in reality this kind of acelleration is probably not needed.
 Away to rest my knee now (http://forums.pcper.com/images/smilies/extras/yllol.gif)

Hood
Title: Re: Lathe motor question "Variable Speed"
Post by: M250cnc on April 19, 2008, 09:43:09 AM
Thanks Hood

I did a search for that motor, one link that came up was on ebay a guy called HoodScotland bought it at good price, is that you ? ;D ;D ;D

That doesn't seem to be a lot of power for a spindle motor, or does it, what is the equivalent in HP

Would it be feasible to run it with timing belts to make use of the positioning feedback.

Phil_H
Title: Re: Lathe motor question "Variable Speed"
Post by: Hood on April 19, 2008, 10:17:59 AM
Yes thats me :)
5.5KW is approx 7.33HP this is going  on my mill which currently has a 4HP inducion motor, I think it will do fine :)
Yes I am planning on timing belt, main reason I want an AC Servo spindle, other than the speed control and continuous torque, is I am planning on making a toolchanger in the future and so will need positioning accuracy. Also I am hoping that rigid tapping should be possible with a controlled spindle but not thought too much into how that would be done yet.
Hood
Title: Re: Lathe motor question "Variable Speed"
Post by: jimpinder on April 20, 2008, 05:54:59 AM
Yes - Hood is about right - much under 50% frequency and torque drops off quite quickly - which is why I asked if you had a gearbox. I am just having to make one to turn wheels for my miniature railway.

6" wheels at 100 ft per min mean something in the order of 70 to 100 rpm. My motor runs up to 1300 rpm so I could do a 10 or 12 to 1 reduction to keep the torque.

The figures for Hoods motor are pretty impressive. He's a Scott - make him an offer for it  ;D ;D ;D :D :-\
Title: Re: Lathe motor question "Variable Speed"
Post by: Hood on April 20, 2008, 06:04:49 AM
No Jim I am not a Scott, I am a Scot (http://forums.pcper.com/images/smilies/extras/yllol.gif)
The problem I see with using a gearbox is not being able to change speeds on the fly, you will have to stop the motion then change the spindle then resume. Granted if there is no tool changer then that may not be a problem as you will have to stop for the toolchange anyway.
 My lathe has a gearbox but its controlled with electromagnetic clutches so speeds can be changed on the fly, however I would dearly love CSS so I am on the lookout for an AC Servo and drive, only problem is its a big lathe and the current motor is 11KW so there wont be many going about at the prices I like :(
 Hood

Hood
Title: Re: Lathe motor question "Variable Speed"
Post by: jimpinder on April 21, 2008, 04:42:38 AM
Sorry Hood - Finger twitch. I knew you would pick it up as soon as I saw it - at least I didn't say Scotch !! ;D
Title: Re: Lathe motor question "Variable Speed"
Post by: Hood on April 21, 2008, 06:21:56 AM
Sorry Hood - Finger twitch. I knew you would pick it up as soon as I saw it - at least I didn't say Scotch !! ;D

Yes that would have been war :D

Hood
Title: Re: Lathe motor question "Variable Speed"
Post by: Dan13 on February 13, 2010, 12:52:30 PM
I have a VFD and induction motor on my mill. It is a good quality Mitsubishi drive but just the standard Bridgeport 2HP motor so I dont know if its an inverter duty motor. I have heard that the inverter duty motors dont drop off so much in torque but how good they are I dont know. What I do know is with my VFD/motor combination I lose too much torque if I go down below 50% of the motors rated frequency. I still use the vari speed pulleys on the mill to get to the rough speed range and then fine tune by the VFD (external potentiometer). I was considering hooking the VFD to Mach but after using the mill for a while I decided there was no point as the drop off in torque would be no use for the varying types of work I do.
 

Hood,

Saw this post and it made me wonder. Could you not permanently set the pulleys transmission to say 4:1 reduction and thus have a low speed of 370RPM (assuming a 2 pole motor) - which is at 50% rated frequency like you said - 30Hz. Then for higher speeds, good inverters can output frequencies as high as 400Hz, which would mean a top speed of almost 5000RPM with the same pulleys. Or did you figure the motor wouldn't last long at 20kRPM. But if it were a 4 pole motor then I think it could handle 10kRPM no problem.

I am asking this because I'm considering doing this - using a 3 phase, 4 pole motor (or even better a 6 pole motor if I can find such) with an inverter to give speeds from 1500RPM (50% rated frequency for a 2 pole motor) to about 10,000RPM  (at about 160Hz). What do you think?

Daniel

Title: Re: Lathe motor question "Variable Speed"
Post by: Hood on February 13, 2010, 03:51:10 PM
I dont know enough about it Daniel, going to be redoing my Bridgeport with servos anyway , so will fit a servo to the spindle as I have a 5.5KW one sitting here doing nothing :)

Hood
Title: Re: Lathe motor question "Variable Speed"
Post by: jimpinder on February 15, 2010, 12:54:14 AM
I think you will have to look at the torque figures produced over the range of sppeds you are talking about. As I say, I use a 2HP three phase motor in place of my 1HP single phase. My Omron inverter worls up to 60 HZ. Below 30Hz the torque drops off quite quickly, so if I want to turn 6 inch diameter wheels, which need a low speed,  I must gear down the motor to the spindle shaft to maintain the torque, (and therefore run the motor faster) to keep the torque, otherwise if I take anything of a reasonable cut, the motor says "I'm not playing", and stalls.

If you can maintain the rated torque over such a wide speed rahge, then fair enough - but if you manage it, patent it.
Title: Re: Lathe motor question "Variable Speed"
Post by: Dan13 on February 15, 2010, 01:31:26 AM
Hi Jim,

I haven't been able to find such charts... it seems that the charts I need are torque against frequency, but those that I see published by manufacturers are torque against speed, where the speed seems to be the percent of the nominal speed - these show the startup process of the motor.

Daniel
Title: Re: Lathe motor question "Variable Speed"
Post by: jimpinder on February 15, 2010, 09:00:04 AM
The problems would appear to be with the motor then, which is normal. Torque is a function of the speed. DC motors tend to be higher torque at low speeds - e.g. railway traction motors, whereas I think most AC motors tend to produce their maximum torque at higher speeds.

I am not saying that your idea is necessarily bad, indeed for us it would be ideal - maximum torque over a whole range of speeds - and I am not in touch with the latest developments, it may have been that modern electronics had managed to produce more torque over a wider range of speeds, but as I say, I do not think it is necessarily achievable at the moment.

In the miniature railway scene, many years ago square wave was produced in a pulsed type of system, instead of merely increasing voltage, to produce more torque at lower speeds, and, indeed most traction motor controls use a similar system today, where full voltage in pulses is fed to the motor.

I don't think you can do that with AC yet, the speed controls using frequency control still have a sine wave type of output.

Title: Re: Lathe motor question "Variable Speed"
Post by: Dan13 on February 15, 2010, 09:53:54 AM
You may be right saying that at very low speeds an AC motor has very low torque, but then please note that I said that I was looking for a 6 pole motor, which means that its rated speed at the rated 50Hz is about 900RPM. Then I can decrease the frequency about 50% to 25Hz before the torque starts dropping significantly and get 450RPM, which is almost low enough for me. And then for the high speeds the frequency can be increased up to 400Hz which would give me around 8000RPM.

Daniel
Title: Re: Lathe motor question "Variable Speed"
Post by: jimpinder on February 16, 2010, 03:43:19 AM
Sounds pretty good to me - let us know how you go on.