Speed control of a universal motor

I need a cheap beefy adjustable speed motor setup for a DIY lathe I'm building. I was considering using a universal motor perhaps from a table saw or other source of large universal motor. I've been told that universal motors only operate at high speed and I know that they are almost exclusively used for high speed applications (table saws, angle grinders, dremel tools, blenders, etc) but I don't see why it couldn't be made to run at lower speed by controlling the power to it.

So my plan is to use an encoder or DC analog tach to read the speed of the motor and build a cricuit that would read the speed, compare it to setpoint, and use proportional control to adjust a phase angle firing triac/SSR to maintain a constant speed as the load changes.

In my mind, this seems like it would work. But there's the potential issue of my motor likely being designed to spin 20,000RPM and I'm trying to keep the speed down around 500RPM (or maybe more, if need be I can use mechanical gear reduction, but that only goes so far).

From what I understand about series wound motors, they are more or less "constant mechanical HP" - that is to say (possibly not 100% accurately) that as torque demand increases, RPM drops. So if I were to apply the maximum load that the motor could sustain without burning up and read the speed @ that load, I could call this my "maximum speed" (4000rpm for example)and design my circuit and my machine accordingly. At 75% of this load, I would need to use my phase angle firing circuit to deliver 75% of full load amps and therefore maintain 4000rpm. The purpose of this, would be that if I did not do it this way, I could find myself in a situation where as I'm increasing the load (say I'm running 6000RPM) and I hit an invisible wall where my motor can no longer maintain 6000RPM and the speed starts to fall off - that's not good.

So, do you think it can be done or what?

I've done a similar thing Strantor, and 500 RPM is totally unsuitable for a 20k RPM universal motor. The nature of series would motors makes them have very low power and/or unstable when under about 1/4 of their normal RPM.

You really need a DC motor, pref a shunt wound or perm magnet type. There are a lot available including treadmill motors which are specifically designed for fairly low RPMs.

And even with a DC motor 500RPM is far from ideal, so you are better with a DC motor at (say) 1500 RPM and 3:1 reduction gearing. What are you driving?

Mr RB said:

I've done a similar thing Strantor, and 500 RPM is totally unsuitable for a 20k RPM universal motor. The nature of series would motors makes them have very low power and/or unstable when under about 1/4 of their normal RPM.

You really need a DC motor, pref a shunt wound or perm magnet type. There are a lot available including treadmill motors which are specifically designed for fairly low RPMs.

And even with a DC motor 500RPM is far from ideal, so you are better with a DC motor at (say) 1500 RPM and 3:1 reduction gearing. What are you driving?

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Well in the immediate future it's going to be driving just a lathe chuck & 2.5" steel shaft - I estimate 75lbs of rotational weight all inside a radius of <4". That is, until I start gouging into it with tooling, then I expect the load to increase quite a bit. Then, later, I will be adding a lead screw to it, so it will br driving that as well (possibly, may use a stepper for that, not sure)

I've been told elsewhere that treadmill motors are also fairly high RPM (in the many thousands).

good to hear from someone who's tried it, your info will be invaluable.

I've been told elsewhere that treadmill motors are also fairly high RPM (in the many thousands).

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All motors run in the thousands.
You need a DC Gear motor.

Of course, the universal motor can also be run on DC. We're experimenting here, so why not build a beefy rectifier/filter circuit and follow it up with a PWM speed control just to check things out. If it works well enough, you can add the speed feedback/control later.

I am with Dean on this. If the motor runs on DC, then I would use PWM as he suggests.

I have used both 3-phase with VFD and DC with PWM for lathe power. They both give good speed control and low-speed torque, but I prefer the DC power for my very small Prazi. You will be in good company. The Monarch 10EE used both DC and VFD 3-phase depending on the model (http://www.lathes.co.uk/monarch/page2.html).

The major difference between the two types of supply are in the stopping. The little Prazi does not have split nuts for threading, so one needs to stop the lathe -- at least that is how I do it when RH treading to a shoulder. DC provides almost instant stop and start.

John

I would use a 90V 1/2 HP motor with PWM control. I have used them successfully in 3000PSI pumps and cable winches. They are quite inexpensive and can be found used. Gearboxes are readily available. These motors are also used in Treadmills and high-powered sub-base woofers. E
ps: PM me if you would like a copy of my design

Hi,

I have controlled motors with brushes using triacs. Triacs allow adjusting the firing angle to change the power getting to the motor and thus slow it down. You will find a problem at low speed however because the motors dont have as much torque as we'd like. To help this situation you can build a control system with speed measurement and feed that back to the control circuit. That will keep even a slower speed working pretty well, as long as the armature current is not raised too high (you'll have to limit that so you dont burn up the motor windings).

I always meant to do this with my Dremel, but it would require taking the whole thing apart to remove the speed control in there already and i didnt feel like doing that. I also thought about making a foot pedal for the speed control, so i could raise or lower the speed as needed and still have hands free. Funny, sewing machines had this for years

Yeah a nice PWM system would work pretty well. Just have to watch the motor current level so it doesnt go too high for too long.

I have used triac control on a small Unimat lathe with a universal motor. In my opinion, triac control is almost totally unsuitable for speed control on a lathe because of the torque issue. Just try threading or a heavy cut with triac control -- that is, unless you a have a geared motor for very low speed.

John

Mr RB said:

500 RPM is totally unsuitable for a 20k RPM universal motor. The nature of series would motors makes them have very low power and/or unstable when under about 1/4 of their normal RPM.

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jpanhalt said:

I have used triac control on a small Unimat lathe with a universal motor. In my opinion, triac control is almost totally unsuitable for speed control on a lathe because of the torque issue.

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Dean Huster said:

Of course, the universal motor can also be run on DC. We're experimenting here, so why not build a beefy rectifier/filter circuit and follow it up with a PWM speed control just to check things out. If it works well enough, you can add the speed feedback/control later.

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Ok, So for the time being I'm 100% sold on the idea of using DC. I trust you guys and your first hand experience, but I've been digesting your comments for the past half day and I'm wondering (theoretically) why the universal motor with triac control at low speed is a no-go. Is it because of the nasty half sawtooth/half sine wave ? because if so, I don't see why that would be any different than 60Hz PWM (or possibly PWM that low freq would also be problematic). Is it possibly because the little "blips" of on time might not fall cleanly inside a commutation period, instead they may fall between periods and be "wasted" in the form of an arc; sort of "hit & miss" of the pulses?

Given your rough dimensions and weights you plan to run I doubt that a 1/2 Hp Dc motor is going to be anywhere near capable of running your lathe unless you don't mind only being able to take a few thousands of an inch cut with each pass which gets old really fast.

I have a Smithy 1340I Lathe/Mill combo unit that came with a 2 HP DC motor from the factory. It was crap and I killed it and its overly complicated speed control unit after only a few dozen running hours. After that I switched it over to a full VFD unit and 2 HP three phase motor which made a world of difference in the low end power and speed control aspects plus a good VFD unit has loads of programmable functions including ramp up time, ramp down time, auto reverse, upper speed limit, lower speed limit, timed torque boost, and external potentiometer based speed control interface.

I for one vote for three phase motor and VFD unit if you are looking for sustained high torque and low RPM operating capacity. A good used 2 Hp four pole (1740 RPM) three phase motor will set you back about $20 or less and a good used VFD to run it can be anywhere from free to around $100 at most.

tcmtech said:

Given your rough dimensions and weights you plan to run I doubt that a 1/2 Hp Dc motor is going to be anywhere near capable of running your lathe unless you don't mind only being able to take a few thousands of an inch cut with each pass which gets old really fast.

I have a Smithy 1340I Lathe/Mill combo unit that came with a 2 HP DC motor from the factory. It was crap and I killed it and its overly complicated speed control unit after only a few dozen running hours. After that I switched it over to a full VFD unit and 2 HP three phase motor which made a world of difference in the low end power and speed control aspects plus a good VFD unit has loads of programmable functions including ramp up time, ramp down time, auto reverse, upper speed limit, lower speed limit, timed torque boost, and external potentiometer based speed control interface.

I for one vote for three phase motor and VFD unit if you are looking for sustained high torque and low RPM operating capacity.

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I actually already have several 3ph motors and I have 2 VFDs; one Yaskawa 5hp and one Emerson Unidrive SP 5hp. Problem with both is, they won't accept <380V. I could turn off phase loss detection and I'm pretty confident I could run them on single phase but I would need a big transformer to step up to 480V to get the benefits of them. I don't want to pay for a transformer, unless I can get one cheap (which I doubt).

tcmtech said:

A good used 2 Hp four pole (1740 RPM) three phase motor will set you back about $20 or less and a good used VFD to run it can be anywhere from free to around $100 at most.

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Whaaa? Where do you shop for motors & drives? Do you have any you'd like to sell at that price?

Actually going from 380 to 480 is cheap and easy. Just use a simple buck boost transformer to make up the difference that way it only has to carry the amp load of the 100 volt boost instead of the full load.
My math says something around 1 - 1.5 KVA would easily carry the 100 volt difference to power the 480 volt 5 hp motors.

I think a fairly common 120/240 : 120/240 isolation transformer configured right would get you close enough.

As far as getting the motors and VFD units apparently either you don't know the right people or the right places to look!

tcmtech said:

Actually going from 380 to 480 is cheap and easy.

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What I meant was, I need to step up 220V (1ph) to (minimum) 380V or (optimal) 480v.

tcmtech said:

As far as getting the motors and VFD units apparently either you don't know the right people or the right places to look!

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I agree, I don't. Ebay is my source, and I haven't seen anything near those prices on ebay. Any pointers where I should be looking?

**broken link removed**

Search on VFD Drive and list by price. There are quite a few new ones for less than $100 that take single-phase input.

Edit; BTW, is this a wood lathe or metal working lathe you are building? If metal, will you have threading capability? Will you use half/split nuts on the lead screw? If you need instant stop, that may affect your choice of control type.

John

jpanhalt said:

**broken link removed**

Search on VFD Drive and list by price. There are quite a few new ones for less than $100 that take single-phase input.

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Yes, I see that. That one and others like it all seem to come from china so I see that as a gamble. I looked at a couple different models and searched around for customer reviews and couldn't find much. I found **broken link removed**and this corresponding review:

let me warn you that the Huanyang drives are very low quality and I've personally had trouble with them and know of others the same.
I would strongly recommend you buy the VFD in UK,

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The reason I didn't know that these inexpensive drives exist is probably because when I searched for them previously, I was probably searching by brand name for drives that I was familiar with (Allen Bradley, Yaskawa, Emerson, Eurotherm)

I'll give it my due dilligence though and see if I can find a cheap-o that's being praised by it's customers.

jpanhalt said:

Edit; BTW, is this a wood lathe or metal working lathe you are building? If metal, will you have threading capability?

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It's a metal lathe and I plant to be able to thread on it. That area (driving of spindle VS lead screw) is still up in the air right now. I'm thinking I might do it seperately, with a servo on the leadscrew. Not quite sure, have to look into my options for gearing.

jpanhalt said:

Will you use half/split nuts on the lead screw? If you need instant stop, that may affect your choice of control type.

John

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I don't know. maybe 2 acme nuts with a spring between them? Can you tell me how does it effect, so that I can use this information to decide?

strantor said:

I don't know. maybe 2 acme nuts with a spring between them? Can you tell me how does it effect, so that I can use this information to decide?

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The most common thread you will make will be an external, right-hand thread. Internal threads, particularly to a dead end, are considerably more difficult to cut on a lathe, and you may just choose to use a tap.

With a normal configuration, cutting a RH thread will involve cutting toward the chuck. Given some pitch, say 28 tpi, your cutter will be moving pretty quickly across the work, and everything stops quite quickly when it hits the chuck. So, you try to avoid doing that. Slow spindle rpm's help. Half-nuts (split-nuts) grab the lead screw and power the carriage. When they are closed, the carriage moves. When they are opened, the carriage stops instantly. Thus, with a little coordination, you can thread toward the chuck or to a shoulder on the piece a not jam the cutter into the chuck/shoulder.

The little Prazi I mentioned above does to have half-nuts. You need to stop the lathe from turning and withdraw the cutter simultaneously to give a nice tapered end to the thread. A DC motor will typically stop of faster than a 3-phase motor. That is why I used a DC motor in that application. After making this thing:
View attachment 64913
I decided that relying on my old eyes and coordination was too much. So, I then made a little gadget working off an electronic dial gauge to detect "zero" (i.e., the last thread) and stop the motor so I could concentrate on withdrawing the cross-slide to give a nice taper to the end of the thread.

An alternative to threading toward the chuck for RH threads, is to reverse the spindle and thread off the backside away from the chuck. In theory, you might consider just turning the cutter upside down on the front side with the lathe reversed, but that will tend to lift the cutter and carriage from the bed, and most lathes are not built rigidly for that. In designing your lathe, you might consider sizing it to allow cutting off the back side. My Prazi won't allow me to do much more that 1/4" diameter that way.

One other thing. In threading, if you disconnect the lead screw, as with half-nuts, you will need a way to re-establish where to start the thread relative to the leadscrew and spindle. That is what the "thread dial" on a lathe is for. With care, you can figure out where the thread starts without a dial.

John

As far as getting 380 - 480 out of the 220 line the same transformer set up in an autotransformer configuration will work just fine.

Either stack the 120 secondary on the 240 primary and get 360 VAC or stack the 240 secondary on the 240 primary and get 480. Either way the transformer only has to carry 1/3 to 1/2 the total KVA load.

Relating to finding used VFD's and motors around here getting to know the guys in the savage yards will save you loads of money! In many places they will give you the electronics for the scrap price of the aluminum heatsinks just to get rid of all the non recyclable materials attached to them.

strantor said:

Ok, So for the time being I'm 100% sold on the idea of using DC. I trust you guys and your first hand experience, but I've been digesting your comments for the past half day and I'm wondering (theoretically) why the universal motor with triac control at low speed is a no-go.
...

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It's worse than that, the universal motor even on DC is still terrible for low RPM torque and stability. Because it is series wound the torque at low RPM is terrible, so your closed loop system pumps in tons of power trying to make the torque at low RPM and it will be nasty and unstable.

You need a DC motor, like a perm magnet motor and treadmill motors are some of the best for lathes as they are lower RPM than other applications.

Also for a lathe you need huge torques, I have a small lathe and it's way too easy to stall the chuck when working steel, especially drilling or thread cutting.

Look for a good sized treadmill motor Strantor, or pay the $$ for a good VFD motor and drive.