[RCaffin]

There have been some vague references here and there to the differences between round steppers and square steppers. The clearest comments I have been able to find were from Marcus Freimanis of GeckoDrive (a very helpful guy). His comments were to the effect that round steppers are an old design using ferrite magnets which lose their strength over the years, while the modern square ones use rare-earth metals and keep their magnetism. Well ... OK, but ...

My problem was that I have a drawer full of old round stepper motors in the NEMA 23 size, plus a couple of larger ones, mostly made by North American Philips. These were very fine when we bought them in the 80s or early 90s. Surely they must still be of some use today? So I put one into a new build and tested it, just to see. I thought it might be worth while spelling out what I found.

The round motor is a Philips 9904 115 23101, 1.8 degrees, 380 mN.m, 1 A MB09. It has a NEMA 23 front end and a 1/4" shaft. It has 6 wires. The driver I used is a Gecko 203V Vampire drive. (Rugged beast.)

When we first bought them we could make them scream like a jet engine. They went fast. Today, not so good. If I ran the per-phase windings in series at 1 A the motor got stinking hot. Oops - I should be using only half of each phase. I fixed that and temperature was OK, although I am not sure the frame like the high frequency switched-mode current: it got hotter than I would have expected.

But I could only get about 450 pulse/second into it before it started to stall or hiccup. Oh Dear: very sedate.
In addition, I could stop the shaft from spinning with my fingers.
More of the Oh Dear. Maybe Marcus is right?

So I bought, at vast expense of course, a new square stepper motor (NEMA 23 of course) from Homann Designs (I am in Oz).
The model number is 23H256-28-4B, torque rating is 155 oz.in, current is 2.8 A, and the length was about the same (size 57). It's called an SM-06. The manufacturer is unknown. Being an NEMA 23, it just plugged into the mounting.

It ran fine at the old speed but with the current control set to 2 A instead of 1 A, so I upped the peak pulse rate ... and upped the peak pulse rate again ... and again. In the end it was still running just fine at 4,500 pulse/second. Yes, TEN times faster, and much more powerfully too, at 2 A rather than its rated 2.8 A. I can't stall that one with my fingers!
Actually, it was running at over 5000 pps with 0.1 second acceleration time, but I scaled it back a bit for safety.

Power
Now 2.8 A sounds a lot of power off a 50 V supply (140 W), but the current meter showed nothing like that at all. Hum? Ah, of course - switch-mode current conversion. Yes, the motor runs at 2.8 A, but the effective voltage on the motor is only a few volts (maybe 3 V?) So the current drain from the 50 V supply is probably around 2.8 * 3/50 = 0.17 A. No, the motor is not dissipating 140 W!

Conclusion
My stack of old round stepper motors may still be used, one day, to wave a flag or something. But they have indeed lost their guts and are unlikely to find a home in my CNC.

HTH
Cheers
Roger Caffin

[James Newton]

The old round motors typically are made for /slower/ but higher torque per amp applications. They usually have a higher inductance, which means you will get more torque at lower current, but it won't go as fast. Your testing didn't involve loads, so you only saw the bad side of it. And you are comparing a 0.5 amp motor with a 1 amp motor. Set that square stepper at 0.5 amp drive, and run a load on both motors... You gotta compare apples to apples man.

[RCaffin]

Hi James

Permit me to disagree with you on some points. Yes, the designs are different, but my experimental findings hold.

First of all, both motors were tested free-wheeling and under torque-limiting load, and I did mention this.
Round motor: "In addition, I could stop the shaft from spinning with my fingers. "
Square motor: "I can't stall that one with my fingers!"

Second, the top speed of the round motors had degraded badly:
"When we first bought them we could make them scream like a jet engine. They went fast. Today, not so good."

Thirdly, actual winding current has very little to do with power.
"And you are comparing a 0.5 amp motor with a 1 amp motor."
This is irrelevant. One motor winding might have 100 turns and take 1 A for 100 A.turns. The second motor might have 50 turns and take 2 A, again giving 100 A.turns. Naturally, the second motor will have about half the impedance of the first motor, so they end up taking the same power - and power is what matters here.

Yes, I had already seen your website - and read application notes back in the 80s and 90s saying pretty much the same things.

I have been using steppers for about 30+ years. I think I was comparing apples with apples.

Cheers
Roger