Thread: Weld rotator

How hot they get will depend on the voltage they're being driven by. A higher voltage will increase the power (and holding torque, though that's not relevant here), while a lower voltage will decrease both and also the temperature. They're just a motor at the end of the day, and if a small DC motor is driven hard it will get hot. Unlike conventional motors, I have yet to see a stepper with cooling on it, so I guess there's a clue right there

HF interference could certainly be a consideration and would need appropriate shielding etc, but one way or the other you need to be able to control the motor speed. Unless the intention is to go completely old school and control it with a rheostat or similar, the chances are it will be some form of electronic controller. Once you go down that path, and unless you already have a DC motor, you may as well just use a stepper. An electronic DC motor controller (much like the much loved VFDs here) is really just a digital controller made to behave like an analogue one by switching the ons 'n' offs really quickly.

The two main reasons I suggest them is because the speed can be determined regardless of load, that could be a factor with a table that is anything other than horizontal. If you provide 200 pulses per minute to it, the motor will either do precisely the 1 rpm as in the example above, or it will sit there and stall, there's very little in between. The other is they are almost ubiquitous these days, so there's just so much information out there on how to use, wire, and control them, there's no need to reinvent the wheel and it's a little like assembling an electronics Meccano Set. Just find a source that you like the look of and shove the wires in the holes they'll tell you.

In the good old days we'd probably wander down to the local wrecker, get handed some tools and shown the yard, and come back with a windscreen wiper motor or externally bolted on window motor from a dodgy Hong Kong import if you got real lucky. Unfortunately in most areas of Australia those days are well and truly over and even something as simple as an electric antenna will cost you and arm and a leg ... if you can even find a good used one! Almost certainly not for 10 bucks delivered anyway

Hi Guys,

Pete F wrote

main reasons I suggest them is because the speed can be determined regardless of load,

Not actually true !
Overloaded stepper motors will skip steps long before stopping! Until you actually need accuracy of count, you may never even know about it. The situation becomes worse when micro stepping.

Originally Posted by BaronJ

Hi Guys,

Pete F wrote

Not actually true !
Overloaded stepper motors will skip steps long before stopping! Until you actually need accuracy of count, you may never even know about it. The situation becomes worse when micro stepping.

What I went on to say was: "The two main reasons I suggest them is because the speed can be determined regardless of load, that could be a factor with a table that is anything other than horizontal. If you provide 200 pulses per minute to it, the motor will either do precisely the 1 rpm as in the example above, or it will sit there and stall, there's very little in between." What you've chosen to highlight is the "in between".

I'm trying to keep this reasonably simple for those who have never used steppers before to show they really aren't that scary and generally don't need feedback as some people who have never used them may try to suggest. I was comparing to DC motors (the alternative proffered from the outset), where the speed will vary depending on load. In the hundreds of hours I've run steppers (ie billions of steps) I've never had one skip a step, even once (and yes I will know as I will see the skipped step in the result) other than when the motor was being overloaded. When comparing to a DC motor what I said was absolutely correct. A stepper will maintain a constant speed regardless of load, right up to the point of overload. There's a thin margin there where it may begin skipping steps, then if the load continues to increase it will stall outright. For critical applications you will use a servo feedback to confirm position and no skipped steps, but they are relatively expensive, needlessly muddies the waters for those trying to get their head around all this, and both are completely unnecessary in this application. All anyone needs to remember is if you provide x number of pulses, you will get y angle of rotation. If you provide x number of pulses per second, you will get Y rpm.

How much load is too much for the type of stepper being referred to in this thread? I've never used a welding rotary table so I'm guessing here, but if one were to accidentally drop a hand onto the rotating table, would the stepper stall?

Originally Posted by jack620

How much load is too much for the type of stepper being referred to in this thread? I've never used a welding rotary table so I'm guessing here, but if one were to accidentally drop a hand onto the rotating table, would the stepper stall?

It depends on the size/power of the motor used and what load the table is already carrying.
If the motor is too small and the table is already loaded up then a hand might stop it.
The motors come in a very wide range of sizes and power/torque.
The physical sizes are given by their NEMA rating, "NEMA 11" is a 1.1" x 1.1", "NEMA 23" is a 2.3 x 2.3" size, etc the physical size is a rough indication of the torque they can develop.
Small physical size Low V, low current motors like a NEMA 11 might have about 0.02Nm of torque, whereas a NEMA23, 24V motor might have around 100+ times more torque, through to the bigger industrial sizes.
For a small table with light loads a NEMA 17, 12V, 0.5Nm motor would probably be sufficient.
The are very popular for use in 3D printing and cost about $12 on ebay.
For a bigger table with heavier loads then a NEMA 23 may be a better option.

A stepper and a DC motor are essentially the same in regard an overload condition, and why I didn't want to get too much into missed steps, stalling, etc. If a DC motor is loaded it will slow down, as the load increases eventually it will stall. If a stepper is loaded down it won't slow down, it will maintain the same speed right up to the point of overload, at that point it will begin missing steps and very shortly thereafter will stall. AC synchronous motors and steppers are very similar in this regard, and something familiar to anyone running equipment through a VFD.

Using steppers and a suitable driver would give you the possibility of using a microcontroller and a simple pause/play forward/rev store/clear and speed pot interface to store a sequence that could be replayed if doing any repetition work.

Here's a good introductory video by This Old Tony on how to implement a stepper drive.

https://www.youtube.com/watch?v=K0XfRPi_h2M

Graham.