DC Motors

[SLM]

This may not be the best section for this query, but here goes…

I’m toying with the idea of switching my 240v AC motor from my pillar drill, with a DC one. It’s as much about the project itself, as the objective usefulness! So, as long as I don’t impede the performance, I’m happy to experiment.

So my question is, how do I go about working out the required motor specification for the new DC one?

Cheers!

 

[Myfordman]

Use one with the same power output would seem to be the way to go.
More to the point can you share why you want to do this?

 

[fleyh]

See if this helps:

 

[fleyh]

More to the point can you share why you want to do this?

My guess would be - to be able to control motor speed with a knob (electronically) rather than moving the belt.

 

[Myfordman]

A specialised controller will be needed to run the dc motor. That cost will be better spent on an inverter to run a three phase motor. Lots of detail in my motor paper link in the footer.

 

[SLM]

Fleyh hit the nail on the head; speed control, reversibility and, I’m lead to believe, better torque.

I appreciate that I can achieve these things with a vfd. But it’s an interesting experiment.

 

[SLM]

A specialised controller will be needed to run the dc motor. That cost will be better spent on an inverter to run a three phase motor. Lots of detail in my motor paper link in the footer.

Are you saying that hands down, you think that’s a better solution than a DC motor?

 

[sploo]

3 phase motor + inverter better than a DC motor? For this application - probably yes. The 3PH motor + inverter will be very quiet and very smooth (though you'd want an "inverter rated" motor that was happy to be run across a range of speeds). For decent 3PH induction motors 20% up to 200% is feasible (10Hz to 100Hz, on a 50Hz supply); though you will lose torque as the speed drops, so if your pillar drill has belts/pulleys for speed control then do be prepared to use them for really low speed/high torque work.

An equivalent power DC motor may be smaller (and therefore more portable) but in my experience, will be a lot louder. How the torque curve varies with rpm I'm not so sure.

 

[SLM]

Thanks Sploo,

That’s interesting; I hadn’t considered noise. The drill is floor standing and 180kg, so size and weight of the motor are not really considerations.

 

[sploo]

I have a tiny milling machine with a DC motor. The low down torque is OK, but if it were large enough to take the size and weight of an AC induction motor of the equivalent power I'd definitely go in that direction. A 180kg pillar drill should (IMHO) definitely have an induction motor not a DC.

For an inverter, Huanyang VFDs are pretty good for the price, and with a simple addition of a thermal switch to the internal fan it'll be nice and quiet too.

 

[SLM]

I have a Seimens VFD on my table saw, so I’d probably use another of those, as I’m beginning to get my head around the way that works!

 

[SLM]

Which brings me to my other option:

https://www.ukworkshop.co.uk/threads/meddings-s68-motor.138380/

 

[Myfordman]

Are you saying that hands down, you think that’s a better solution than a DC motor?

Yes everyday of the week. I started off with a DC motor on my mill and lathe in the earoly 80 as inverters were very new and expensive. I run inverters on most machines now as being able to nudge the speed up or down is very useful indeed.
Have a read of my paper and if you have questions let me know.
A vector inverter would be a better type to use for best torque over a range of speeds.

 

[Spectric]

Why make things more complicated than they need be, an Ac motor is very basic and gives long service compared to a brushed Dc motor with plenty of options available. Yes you could go for a Dc motor but how are you going to power it? Most Dc applications I have worked on have been with Ac motors and electronic commutation for linear motion control and specialised applications, for general industrial use then the workhorse is the three phase Ac induction motor and has been for years.

 

[Sandyn]

Yes you could go for a Dc motor but how are you going to power it?

I would use batteries with enough capacity to run the drill for a day and charge when required.

 

[Inspector]

I have seen people use treadmill motors to power small lathes and 2x72 belt sander/grinders etc. The issue is finding suitable power for them. Some use the electronics with modification from the treadmill and others order stuff to make or a ready made unit. Can't tell you the best/cheapest/simplest way to go. You also have to make your own motor mount.

I would also go the three phase motor and VFD route myself.

Pete

 

[SLM]

Yes, I’m swinging back toward VFD + 3ph motor. Thanks for the input!

 

[Sandyn]

There is scope for some really innovative design if you wanted to go down the DC motor route for a project. There are many brushless DC motors available ~500W for motor scooters and the like. I would also be looking at electric bike technology. An interesting project, but I agree that the simplest route is just the three phase motor and VFD.

 

[SLM]

There is scope for some really innovative design if you wanted to go down the DC motor route for a project. There are many brushless DC motors available ~500W for motor scooters and the like. I would also be looking at electric bike technology. An interesting project, but I agree that the simplest route is just the three phase motor and VFD.

Yeah, that was my thinking. I may yet swing back to DC; It’s probably going to come down to how much time I have, and other projects demanding the drill.

 

[sawtooth-9]

Going DC seems like a lot of trouble for not much benefit over a 3 phase AC with VFD.
I have never found it necessary to have the ability for small changes in speed for a drill.
Because of torque limitations at low frequency and motor specs at the high end - I usually only run VFD between 25 and 80 Hz
And yes, DC will be noisier.