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Old 08-09-2022, 08:24 AM
moespeeds moespeeds is offline
Join Date: Apr 2021
Location: Cherry Hill, NJ
Posts: 181
Default Current replacement for the blue "Slo-Syn" stepper AP servos?

Hi guys,

My 8 has the older blue "Slo-Syn" stepper motors. I've been having trouble with my AP holding wings level in any kind of turbulence or gusts. I changed out my old Tru-Trak for a new Xcruze 100 to no avail.

At Oshkosh last week I spoke in person to Stephan, the brain behind the Xcruze system, and in describing my problem he pointed out that my servo is most likely shot.

I have a whole new Garmin panel, AP, and servos on order, but who knows how many months that will take. Garmin is projecting 6-8 months. In the interim, I'd like to get this servo swapped out so I have a reliable AP. So what do I replace this thing with?
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Moe Colontonio
Cherry Hill, NJ
RV-8 N495T
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Old 08-09-2022, 10:00 AM
idubrov idubrov is offline
Join Date: Mar 2021
Location: San Antonio, TX
Posts: 61

Originally Posted by moespeeds View Post
Hi guys,
My 8 has the older blue "Slo-Syn" stepper motors. I've been having trouble with my AP holding wings level in any kind of turbulence or gusts.

In the interim, I'd like to get this servo swapped out so I have a reliable AP. So what do I replace this thing with?
Based on the specifications, looks like a regular NEMA 23 stepper motor to me, the ones which are used in CNC equipment of all kinds. Very common in hobby-level projecs (like 3d printers or routers).

Disclaimer: everything below is based on my experience with CNC hobby projects, I have absolutely zero knowledge how stepper motors are used for autopilots.

If it was a CNC project I was doing, I would just buy NEMA 23 motor with at least the same current / torque rating (higher is better, but motor could be larger / longer). Something like 23HS22 I've just found on eBay. It is 1A/181oz-in, close to KML061F02 1.05A / 170 oz-in (bipolar configuration; since KML061F02 has 4 leads, it must be used in a bipolar configuration).

The way stepper motors are controlled (again, in a hobby CNC world) is by feeding them the specified current via the stepper motor driver (which usually takes input in the form of two control signals, "direction" and "make a step").

So, if motor is 7A motor, it does not mean it would require 7A, but it means you can configure your stepper motor driver to provide up to 7A of current through it (more current means more holding torque). In a given NEMA package motors could be of different length with different specs (typically, the longer the motor the more holding torque it can provide).

The only thing that was relevant for my projects were the current and the torque (I want to configure my driver for the maximum current the motor is capable of handling so I get the maximum holding torque). However, I never had to worry about inductance or resistance of these motors, which might be important for a specific stepper motor driver you have in your autopilot. The hobby CNC stepper motor drivers are used are pretty universal for non-critical it-can-fail-and-nobody-cares applications.

P.S. The stepper motors have two windings, A and B. Some has 4 leads (2 for each winding), some has 6 leads (2 for each winding + 1 center leads for each winding). The 6 lead motors could be wired in different configurations, but the easiest is to treat them as a 4 lead motor (find two leads with largest resistance, and ignore the center lead).

P.P.S. Also, might worth doing a basic motor check. Measure resistance between the wires to see if it is the same for both windings A and B and also you don't get any connection ("infinite" resistance) between leads of A and B windings.

P.P.P.S. Looking at my collection of motors, there are also with 8 leads which are two halves of two windings. The most flexible ones, you can connect halves in serial, in parallel, use only one half.

P.P.P.P.S. Seems like CNC motors tend to have higher current / lower resistance and impedance, for the same torque (so they work better at higher speeds). Without knowing the details about the actual driver (which, as I understand, is a part of the servo module), hard to tell the range of acceptable spec differences if the motor is replaced with a different model.
Ivan Dubrov #75069
Building an RV-7
San Antonio, TX (near 1TT8 Bulverde Airpark)
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Last edited by idubrov : 08-09-2022 at 11:41 AM.
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