# Kenmore 158: Stepper Motor Max Speeds

Having a NEMA 23 stepper fit almost exactly into the spot vacated by the sewing machine’s AC motor was too good to pass up:

So I wired a power supply to an M542 stepper driver brick, connected the pulse output of a function generator to the brick’s STEP inputs, swapped motor leads until it turned the proper direction (CCW as seen from the shaft end), and turned the function generator knob:

The object was to find the step frequency where the motor stalls, for various winding currents and supply voltages. The motor won’t have enough torque to actually stitch anything near the dropout speed, but this will give an indication of what’s possible.

With a 24 V DC supply and 1/8 microstepping (40 k step/s = 1470 RPM):

• 1.00 A = 11 k step/s
• 1.91 A = 44 k/s
• 2.37 A = 66 k/s
• 3.31 A = 15 k/s

With a 36 V DC supply and 1/8 microstepping:

• 1.91 A = 70 k/s
• 3.31 A = 90 k/s

With a 36 V DC supply and 1/4 microstepping (40 k step/s = 2900 RPM):

• 1.91 A = 34 k/s
• 2.37 A = 47 k/s
• 2.84 A = 47 k/s
• 3.31 A = 48 k/s

The motor runs faster with a higher voltage supply, which is no surprise: V = L di/dt. A higher voltage across the winding drives a faster current change, so each step can be faster.

The top speed is about 3500 RPM; just under that speed, the motor stalls at the slightest touch. That’s less than half the AC motor’s top speed under a similarly light load and the AC motor still has plenty of torque to spare.

90 k step/s at 1/8 microstepping = 11 k full step/s = crazy fast. Crosscheck: 48 k step/s at 1/4 microstepping = 12 k full step/s. The usual dropout speed for NEMA 23 steppers seems to be well under 10 k full step/s, but I don’t have a datasheet for these motors and, in any event, the sewing machine shaft provides enough momentum to keep the motor cruising along.

One thing I didn’t expect: the stepper excites howling mechanical resonances throughout its entire speed range, because the adapter plate mounts firmly to the cast aluminum frame with absolutely no damping anywhere. Mary ventured into the Basement Laboratory to find out what I was doing, having heard the howls upstairs across the house.

She can also hear near-ultrasonic stepper current chopper subharmonics that lie far above my audible range, so even if the stepper could handle the speed and I could damp the mechanics, it’s a non-starter for this task.

Given that the AC motor runs on DC, perhaps a brute-force MOSFET “resistive” control would suffice as a replacement for the carbon disk rheostat in the foot pedal. It’d take some serious heatsinking, but 100 V (or less?) at something under 1 A and intermittent duty doesn’t pose much of a problem for even cheap surplus MOSFETs these days.

That would avoid all the electrical and acoustic noise associated with PWM speed control, which counts as a major win in this situation. Wrapping a speed control feedback loop around the motor should stiffen up its low end torque.

## 10 thoughts on “Kenmore 158: Stepper Motor Max Speeds”

1. Sam says:

Why not an AC motor with a VFD?

1. Ed says:

Or a brushless DC motor, hot from eBay?

I have the uneasy feeling that both rely on chopped currents smoothed by the winding inductance. I’ll never hear those harmonics, but Mary can, and that’s a dealbreaker.

Plus, a VFD can’t produce the super-slow motor speeds required for delicate maneuvers and can’t go much above the motor’s rated speed. A BLDC motor can produce crawling speeds, but their high end isn’t very high.

As nearly as I can tell, the sewing machine requires an order of magnitude speed variation, roughly 1k to 10k RPM, but without much torque on either end. Mary requires that the motor produce no acoustic noise beyond the hum of the OEM motor. Combine those two and it’s a pretty tight corner case.

Now, obviously, it’s a solvable problem: new sewing machines don’t produce annoying sounds. The trick is to find a non-mass-production solution, perhaps buying a defunct machine from eBay and cannibalizing its drive system.

1. Ed says:

You have excellent taste, sir, but … ah … that and a driver would put a serious dent in my toy budget. [wince]

Point taken, though. I was thinking of higher-torque, lower-speed motors; those little bitty miracle motors are a wonder to behold.

I’ll screw around with the OEM motor until I convince myself that I can’t make it work, which will surely require measuring torque / speed / load / current / whatever. Ought to keep me off the streets at night, anyway.

1. Ed says:

Donkey? [mutter]

Must. Measure. Torque.

2. david says:

How about a CVT? And maybe a steam engine; I mean, why take the easy way? :)