Stepper motors

[Weldingartisan]

Do any of you clever electromagician types know to equate the power of a stepper motor to good old fashioned horse power?
I built myself a welding turntable using a geared down DC motor and a mofset(?) speed controller, but could not get enough torque at a slow enough speed. I have thought about a worm drive reduction unit but the almost infinate speed adjustment (plus meddling in something I know nothing about!) of a stepper motor appeals to me.W/A

 

[Luke]

The data sheet for the motor should give you a torque figure in Nm. Torque x speed = power in watts, which you can convert to horse power. Bear in mind that torque will drop off as you speed up, but power should remain relatively constant. A decent data sheet will give you the torque curve of the motor.

Do you have a stepper drive to hand? they're move involved than a DC motor driver.

 

[Weldingartisan]

No, I don't have a motor to hand, but when I made equiries to a company a couple of years ago, soon realised there will be more controls (and expense!) than first thought. The stumbling block then as now was the size of the motor required. So with your formulae I shall do me sums, Thankyou. W/A

 

[Luke]

you need speed in rad/s for the maths to be correct.

 

[Weldingartisan]

rad/s? Sorry I'm old school (or just old) RPM, ft/min, etc. W/A

 

[Luke]

radians per second. 2 pi radians in 360 degrees.

 

[shenion]

Stepper motors are low torque and very inefficient..

If you want better control, look at brushless DC control. More like an advanced stepper motor. More torque and less wasted heat.

Steppers are best for CNC apps where you need absolute positioning with low cost. Brushless are better for variable-speed apps.

I have used brushless gearmotors with integrated brushless controllers. It has a 0-5V input you can connect a potentiometer to (or control from a microcontroller) to set the speed and a low-current direction line. The also have an encoder output to allow speed sensing. Worked well. My app used them to raise/lower a 100lb tray of equipment via cables.

 

[Luke]

I agree that steppers are poor compared to a DC (or AC) servo, but they are significantly cheaper.

Personally, having not used a welding rotating table, I wonder if positioning is a requirement at all. I suspect speed holding is all that is required, as such, an induction motor and inverter is going to be cheaper and simpler than a servo.

I can't help thinking that this is straying away from a simple/cheap/DIY solution though. I think that in the same position I would be experimenting with things like wiper motor/gearboxes before coming to this sort of discussion.

 

[Weldingartisan]

Thanks shenion, could a brushless gearmotor be controlled with a mosfet motor controller (I already have one)? W/A

 

[Luke]

Nope, sorry.

 

[snowcat]

Depends on the size of the table but stepper wouldn't really be the way to go, CNCs use them as they move exactly the same amount with each pulse.... around 21 NM of torque is the most your pull from the biggest motor, your also need to build a D/C rectifier, run a driver board and control via something....

We used a D/C motor for a Friends manipulator, copied design from a bode however it was luck we found a motor suitable S/H.... as they were £££ new.... the manipulator was to carry around 2T on the bed..

on that size table if I was to build another I would most likely use a 3ph A/C with a gearbox and run a VFD, this would be the cheapest and easiest option many of the mfrs have gone down this route to chop the cost of big D/C motors.....Its also possible to pulse your stops and starts via the welding plant through the VFD.....

Luke think were both on the same wavelength on this.....

 

[the snooper]

could you just use a motor and gear it with pulleys and belts to reduce speed tiny pulley on motor big on table obviously you would need the rpm of the motor then work out the gearing ratio you could use more then 1 belt and 2 pulleys making a gear box

just a thought

 

[shenion]

Check ebay for geared motors. Here's one I picked up recently:

This will be used to drive the feed screw on my lathe. It is 24V @ 120 RPM. Has a good amount of torque. Just need to build a 0-24V supply to drive it.

Have picked up others when I made a wire feeder. These are mega $$ new, but can be got cheap surplus. The above was something like $35 for 3 of them.

 

[Weldingartisan]

I agree that steppers are poor compared to a DC (or AC) servo, but they are significantly cheaper.

Personally, having not used a welding rotating table, I wonder if positioning is a requirement at all. I suspect speed holding is all that is required, as such, an induction motor and inverter is going to be cheaper and simpler than a servo.

I can't help thinking that this is straying away from a simple/cheap/DIY solution though. I think that in the same position I would be experimenting with things like wiper motor/gearboxes before coming to this sort of discussion.

Thanks for the replies but yes, we have just gone full circle - my turntable is presently driven by a geared down wiper motor!
This project came into being after having a spare 4QD Vortex 75 motor controller left over from a robot build and I seem to have gone to great lengths to engineer a load of poo based on this concept! It is stable down to about 2RPM but that is still a bit too fast for me to TIG around 100mm diameter.
Shenion - I did look on the bay of e yesterday and found a small geared motor at 4RPM which may be OK if it works with the cotroller, I shall probably give it a punt and see for 10quid or so. W/A
Methinks a total rethink is in order.

 

[Luke]

Can you go for something like a ring gear? I don't know the dia of your table but if you could drive it on it's circumference you should be able to get a decent/improved ratio.

 

[graffian]

Can you go for something like a ring gear? I don't know the dia of your table but if you could drive it on it's circumference you should be able to get a decent/improved ratio.

I wanted something similar, I was going to use a flywheel[car] and then get a worm to mesh with it, easy ~100:1 reduction ratio, I was going to drive it with an old beetle dynamo.
I used a large stepper to power a rotary table, stepper came from the table of a matchmaker mill in my local scrapyard and the drive was curtesy of a laptop firing a row of mosfets in a suitable order.