Author Topic: DC motor for my lathe has low torque  (Read 875 times)

Offline vtsteam

  • Global Moderator
  • Hero Member
  • *****
  • Posts: 6349
  • Country: us
  • Republic of Vermont
DC motor for my lathe has low torque
« on: February 16, 2022, 10:36:58 PM »
I've run into a problem at low speeds on my new homemade lathe. Just not enough torque for cleaning up my iron sand castings.

I'm thinking about possibly going to a vfd and and three phase motor of 1 hp. One issue though (besides the cost) is that the small shop only has single phase 120 V, though I guess that there are a few inverter VFD's that will work for that situation.

What do you guys think -- is it worth making the change?

I guess the alternative would be making a jackshaft for the present motor and belting it down 2:1 in hopes that that will give enough torque (toothed belt).
I love it when a Plan B comes together!
Steve
https://www.youtube.com/watch?v=4sDubB0-REg

Offline SwarfnStuff

  • Hero Member
  • *****
  • Posts: 575
  • Country: au
Re: DC motor for my lathe has low torque
« Reply #1 on: February 17, 2022, 12:58:42 AM »
Hi Steve,
      For what it's worth I just changed from a 360W DC Brushed motor to a 600W Industrial sewing machine motor that came with all controls needed. Sure, it is designed for a foot pedal to operate the speed etc but a bit of messing around and I now have my little 7x12 Seig clone almost unstoppable. Still a bit to do to get the speed control fitted to get easily set speeds from 100 - 3500 RPM.
  What I have suits the 240V here in OZ but am sure it works 110-240V.
Hopefully insert Pic from eBay works.
Regards,
John B

Converting good metal into swarf sometimes ending up with something useful. ;-)

Offline chipenter

  • Hero Member
  • *****
  • Posts: 909
  • Country: gb
Re: DC motor for my lathe has low torque
« Reply #2 on: February 17, 2022, 01:31:23 AM »
My solar powered lathe runs off of two 12 volt battery's , I have found that power is very voltage dependant at low rpm , I use a e bike controller for speed control and if I use the battery charger the voltage goes up to 28 volts giving more power .
Jeff

Offline vtsteam

  • Global Moderator
  • Hero Member
  • *****
  • Posts: 6349
  • Country: us
  • Republic of Vermont
Re: DC motor for my lathe has low torque
« Reply #3 on: February 17, 2022, 08:17:29 AM »
Thanks John and Jeff, I should have explained that I already have a 90VDC 10 amp treadmill motor running the lathe with a 5 to 1 belt reduction and a variable DC controller. If I remember correctly that motor was label listed for 10,000 RPM. I made the 5 to 1 reduction decision to bring it to 2000 RPM max (which I'm guessing is open speed anyway).

At 100 RPM, however it just doesn't have enough torque when doing iron work of about 3" diameter.

Could that be the fault of the controller? Maybe I need a better one?

Not quite sure how these controllers work. Do they limit the current always and just vary the voltage? In that case If a controller is rated for say 2 amps, does that mean that at a low voltage (to keep the speed low) it will keep the amperage at 2 amps?

In other words, would a 10 amp controller supply more torque at low speed than a 5 amp controller? Or would the 5 amp controller simply cut out, no matter what the speed if you work started to draw  5 amps?

I guess I may be thinking wrongly about the amperage rating of a controller --- I imagined it was just a cutoff type of rating, not a normal continuous current supply.
I love it when a Plan B comes together!
Steve
https://www.youtube.com/watch?v=4sDubB0-REg

Offline awemawson

  • Global Moderator
  • Hero Member
  • *****
  • Posts: 8651
  • Country: gb
  • East Sussex, UK
Re: DC motor for my lathe has low torque
« Reply #4 on: February 17, 2022, 08:31:48 AM »
Steve,

If the main issue is getting 'under' the hard crust and sandy inclusions have you considered a 'tool post grinder'. The cutting forces are much reduced, as is the damage to your tooling, but cover your ways well against the dust.

I don't usually like the idea of grinding in the lathe but I had once to thread an HSS tool for a screw in collet chuck and was amazed how well a 'jury rigged' tool post grinder did, and my grinding disk' was just an abrasive angle grinder disk dressed to 55 degrees!
Andrew Mawson
East Sussex

Offline vtsteam

  • Global Moderator
  • Hero Member
  • *****
  • Posts: 6349
  • Country: us
  • Republic of Vermont
Re: DC motor for my lathe has low torque
« Reply #5 on: February 17, 2022, 09:21:38 AM »
Thanks Andrew, no the problem is that there isn't enough torque to turn even 3" dia of iron reasonably quickly even if cleaned up already. The motor slows if I take more than a tiny cut.

I'd like to be able to shift more metal, and the new lathe definitely has the stiffness for it, the motor slowdown is greatly limiting depth of cut when running at slow speed. A conventional lathe has back gears for this. This direct drive variable speed method needs motor torque at low speeds to do the equivalent.

My question is -- is this DC motor DC/controller method inherently problematic at low speeds vs AC 3 phase motor VFD drive?

Or is my problem simply that my motor is adequate, but the DC controller is undersized/inadequate and I would get usable results by getting a higher rated one?

My question there is a result of not truly understanding how they work. Does the current rating of a DC controller mean that it will supply full rated current at any controlled voltage (speed)? Because if so, then it may well be that a higher rated controller would give me the torque I need.

This is the speed controller I have -- rated at "400 watts". My motor is actually rated at 900 watts.

https://www.amazon.com/24V-90V-Controller-Electric-Regulator-Permanent/dp/B076KPP92X
I love it when a Plan B comes together!
Steve
https://www.youtube.com/watch?v=4sDubB0-REg

Offline John Rudd

  • Hero Member
  • *****
  • Posts: 2480
  • Country: gb
Re: DC motor for my lathe has low torque
« Reply #6 on: February 17, 2022, 10:01:29 AM »
Torque is proportional to speed which in the case of a dc motor is also a function of applied voltage.
So the lower the voltage the lower the speed and torque.
So to effectively cut a diameter of 3” you would need to speed the motor up but reduce the spindle speed to maximise the torque of the motor….( jack shaft?)
I fear that the controller is part of the problem too,in that there is no feedback to the controller of the motor’s speed or torque.

The KB electronics controllers ( Florida based..) are great for dc motor application once set up correctly,which is why they are favoured by the Chinese in their lathes and mills
eccentric millionaire financed by 'er indoors
Location:  Backworth Newcastle

Skype: chippiejnr

Offline vtsteam

  • Global Moderator
  • Hero Member
  • *****
  • Posts: 6349
  • Country: us
  • Republic of Vermont
Re: DC motor for my lathe has low torque
« Reply #7 on: February 17, 2022, 11:09:45 AM »
So John, what about a 3 phase motor and VFD? Would that be an advantage, or is it the same problem?
I love it when a Plan B comes together!
Steve
https://www.youtube.com/watch?v=4sDubB0-REg

Offline chipenter

  • Hero Member
  • *****
  • Posts: 909
  • Country: gb
Re: DC motor for my lathe has low torque
« Reply #8 on: February 17, 2022, 11:53:38 AM »
My voltage stays the same at all speeds , a controller chops the square wave and supplies shorter amounts the slower the speed .
Jeff

Offline John Rudd

  • Hero Member
  • *****
  • Posts: 2480
  • Country: gb
Re: DC motor for my lathe has low torque
« Reply #9 on: February 17, 2022, 11:55:48 AM »
Similar kind of issue, but a 3 phase motor controlled by a vfd is frequency controlled rather than voltage....

Secondary to the lack of torque at low speeds is the lack of cooling too....which in the case of many of the Chinese dc motors, is the reason for their demise.. Folk run them at low speeds, reducing what air flow is provided by the useless fan and inevitably killing the motor by its overheating...
3ph motors run at low speeds via a vfd have a similar problem, but rather than the armature windings overheating,  its the stator of the 3 ph motor that suffers but to a lesser degree because they are afforded the cooling of the outer motor casing....
eccentric millionaire financed by 'er indoors
Location:  Backworth Newcastle

Skype: chippiejnr

Offline John Rudd

  • Hero Member
  • *****
  • Posts: 2480
  • Country: gb
Re: DC motor for my lathe has low torque
« Reply #10 on: February 17, 2022, 11:57:41 AM »
My voltage stays the same at all speeds , a controller chops the square wave and supplies shorter amounts the slower the speed .

That is PWM control....the DC supply is chopped into 'segments', whose width varies to provide the different speed....
eccentric millionaire financed by 'er indoors
Location:  Backworth Newcastle

Skype: chippiejnr

Offline philf

  • Hero Member
  • *****
  • Posts: 984
  • Country: gb
Re: DC motor for my lathe has low torque
« Reply #11 on: February 17, 2022, 12:48:09 PM »
Similar kind of issue, but a 3 phase motor controlled by a vfd is frequency controlled rather than voltage....

Secondary to the lack of torque at low speeds is the lack of cooling too....

For that very reason I fitted a mains powered fan to the 3ph motor on my CNC mill. It also has the advantage of being a lot quieter when the motor is running at 5,000 rpm.
Phil Fern
Location: Marple, Cheshire

Offline awemawson

  • Global Moderator
  • Hero Member
  • *****
  • Posts: 8651
  • Country: gb
  • East Sussex, UK
Re: DC motor for my lathe has low torque
« Reply #12 on: February 17, 2022, 01:12:13 PM »
But all these issues can be solved with a counter shaft / back gear arrangement - can that not be retro-fitted Steve?
Andrew Mawson
East Sussex

Offline vtsteam

  • Global Moderator
  • Hero Member
  • *****
  • Posts: 6349
  • Country: us
  • Republic of Vermont
Re: DC motor for my lathe has low torque
« Reply #13 on: February 17, 2022, 01:42:30 PM »
Thanks John for the cooling issues explanation. So there's no advantage then to going to 3ph/vfd motor combo re. low speed torque?

Yes Andrew, of course a back gear or jackshaft reduction can be added. I'm just trying to understand whether it is possible to get reasonable torque at low speeds via a motor, rather than going to gearing.

To those with them, how do the Asian mini-lathes stack up when trying to machine let's say a 4" diameter piece of cast iron with a carbide tool? Are they able to do that fairly well with their DC controllers and motors? Or do they have to take very light cuts?
I love it when a Plan B comes together!
Steve
https://www.youtube.com/watch?v=4sDubB0-REg

Offline John Rudd

  • Hero Member
  • *****
  • Posts: 2480
  • Country: gb
Re: DC motor for my lathe has low torque
« Reply #14 on: February 17, 2022, 02:14:18 PM »


To those with them, how do the Asian mini-lathes stack up when trying to machine let's say a 4" diameter piece of cast iron with a carbide tool? Are they able to do that fairly well with their DC controllers and motors? Or do they have to take very light cuts?

I remember the days of when I had a mini mill....that had a dc motor/controller....I found that I could only take light cuts when using a fly cutter...let alone carbide tipped tool....
The controller was set up such that if the motor was overloaded, the controller would just 'fold back' ultimately reducing the power to the motor which would stop...( my doing after reading the tech manual for the KB controller...) hence the machine had a better chance of survival without breaking anything...

Those folk with less experience that I've come across have unfortunately suffered with either a burnt out motor or they simply broke the controller because I suspect the controller wasnt set up properly at the factory....

My big lathe ( Warco WM290 lookalike...) was fitted with a 1.5kw dc motor and corresponding controller, but that didnt stay very long, I swapped it out for a 3ph 1.5kw motor, with a shaft speed of 2800 rpm ( a 2 pole motor rather than a 4 pole), winding the max frequency up on the vfd to around 72Hz gets me a spindle speed of around 1000rpm on the slow pulleys which is similar to the dc setup ( so that I got the same speed readout as before the changes..)

As it is now, I rarely use speeds in excess of the 1000rpm and have had no issues with slower speed for larger material...I made a riser block for my VMC mill which was 6 inches in diameter...No bother facing the ends with a carbide tool at slow speeds...
eccentric millionaire financed by 'er indoors
Location:  Backworth Newcastle

Skype: chippiejnr

Offline vtsteam

  • Global Moderator
  • Hero Member
  • *****
  • Posts: 6349
  • Country: us
  • Republic of Vermont
Re: DC motor for my lathe has low torque
« Reply #15 on: February 17, 2022, 02:40:07 PM »
Okay, thanks John, that puts it in perspective. I've never owned one of the mini-lathes or even seen one work, so I had no idea how much torque they had at low speeds. I just figured they must get better results than me with my homemade gear.

So the upshot is, I'm sending for another DC controller, in case this one is limiting the 1 hp motor in some way, and I'll see what needs to be done to add a jackshaft to change the present reduction from 5 to 1 to 10 to 1. The problem for me with that is it's sometimes problematic to find toothed belts in exactly the right length, plus pulleys, while I still need to maintain the 5 to1 ratio to encoder for the electronic feed screw.


Oh yes, and add a separate fan.
I love it when a Plan B comes together!
Steve
https://www.youtube.com/watch?v=4sDubB0-REg

Offline mc

  • Full Member
  • ***
  • Posts: 108
Re: DC motor for my lathe has low torque
« Reply #16 on: February 20, 2022, 06:35:07 PM »
Most electric motors are constant torque devices (until they reach rated speed, at which point they become a constant power device), so the amount of torque they produce is consistent over their speed range.
Current is directly related to the torque (load) applied, and speed is directly related to voltage. (this also applies to AC motors/VFDs, but you have the added complication of frequency controlling speed, but the basic current/torque and voltage/speed rules still apply).

That is the theory behind motors, but the actual speed/torque curves will not be perfect straight lines due to other factors, but they'll be near enough for basic calculations.

A good motor speed controller will always limit current to the motors rated current. You can try driving more current through by increasing voltage at a given speed, but that is when you're likely to damage the motor, as you're essentially pushing more current through the motor than it's designed for, which will likely lead to saturation (aka the motor magnets aren't physically capable of producing any more torque), and overheating.


This means that the OP's 900W (90V x 10A) motor with the maximum spindle speed of 2000rpm, gives a constant torque of 4.3Nm at the spindle.
At 100RPM it should still produce 4.3Nm, but as it's only doing 5% of it's rated speed, it will only be capable of producing 45W of power.

To put it context, you can probably get more than 4.3Nm turning a typical screwdriver handle.

Offline vtsteam

  • Global Moderator
  • Hero Member
  • *****
  • Posts: 6349
  • Country: us
  • Republic of Vermont
Re: DC motor for my lathe has low torque
« Reply #17 on: February 20, 2022, 08:40:41 PM »
Well mc you started me thinking....I'd given the motor specs in my second post only from (faulty) memory. Mainly because it had been quite a few years since it was added to the lathe, and the tag had been painted over. So I spent the last half hour looking for old pics of the lathe during construction, to try to see if there was one that included the motor label.

Finally I found one -- had to enlarge and enhance to read it, but I was fairly far off on the specs. Here it is:

And also note that there is a 5 to 1 reduction via toothed drive belt to the spindle as mentioned in the same post. An online motor calculator puts torque at 3.33 Nm direct drive, and I'm assuming that means 16.6 Nm after reduction. Is that correct?



I love it when a Plan B comes together!
Steve
https://www.youtube.com/watch?v=4sDubB0-REg

Offline vtsteam

  • Global Moderator
  • Hero Member
  • *****
  • Posts: 6349
  • Country: us
  • Republic of Vermont
Re: DC motor for my lathe has low torque
« Reply #18 on: February 20, 2022, 09:06:23 PM »
Also, the speed controller I have,as mentioned earlier is rated at 400 watts. The output is 90V not the 130 which the motor is rated for. I've also since read several purchasers saying that their measured output was under voltage (~70V).

If it's rated at 400 watts, and 90 V that must mean about 4.4 amps, is that correct? The actual motor is rated at 18 amps. So if the controller is limiting current to 4.4 and voltage to 90, seems like there's room for improvement with a better controller.

Right now I'm waiting to receive a treadmill DC motor controller, that would have been used with this motor, and I'm hoping that will give me better performance.
I love it when a Plan B comes together!
Steve
https://www.youtube.com/watch?v=4sDubB0-REg

Offline mc

  • Full Member
  • ***
  • Posts: 108
Re: DC motor for my lathe has low torque
« Reply #19 on: February 21, 2022, 11:53:42 AM »
There is a fair amount of more power to be had from that motor with a suitable controller.
4.4A sounds about right, so even upping to 15A will more than triple the available torque.

Unless you're aiming for maximum speed, not having full voltage available will only limit maximum speed. It won't limit torque at lower speeds.

As you've only got 90VDC, that means the motor will likely max out around 4600rpm.

Offline vtsteam

  • Global Moderator
  • Hero Member
  • *****
  • Posts: 6349
  • Country: us
  • Republic of Vermont
Re: DC motor for my lathe has low torque
« Reply #20 on: February 21, 2022, 01:47:02 PM »
Thanks mc, the new controller is rated at 130V, and 18 amps I believe. It's an M60.
I love it when a Plan B comes together!
Steve
https://www.youtube.com/watch?v=4sDubB0-REg

Offline vtsteam

  • Global Moderator
  • Hero Member
  • *****
  • Posts: 6349
  • Country: us
  • Republic of Vermont
Re: DC motor for my lathe has low torque
« Reply #21 on: February 22, 2022, 06:12:51 PM »
The M60 DC controller board arrived today, along with a choke that was part of the original installation in a treadmill. The choke goes in series with the motor.

I installed the new parts temporarily using jumpers on the new lathe, and then finished a project I had already started: turning the hot piston iron casting for the engine I'm building. That gave me an immediate and direct comparison between to old controller and the new.

The difference was gratifying!  :ddb:   The motor was much smoother and quieter, it it was able to run at an even slower minimum speed than it had with the other controller. And it had much better torque at low turning speeds. I may not need a countershaft, after all, though I'll know better after I've done a variety of work with the new controller. All around a great result, and I'm very glad it was worth the purchase! I was worried it wasn't going to make much of a difference.
I love it when a Plan B comes together!
Steve
https://www.youtube.com/watch?v=4sDubB0-REg