When you over tighten a belt ...

[wimvb]

... and combine that with temperature changes, you have a recipe for metal fatigue.

A few days ago, my AZ-EQ6 started tracking erratically, and guiding was all over the place. I narrowed the cause down to the RA motor (it made a strange noise when slewing with the hand controller).

Today I removed the motor and opened it up ...

I've ordered a replacement motor from Pierro Astro, but these are quite expensive (140 €). As a backup, I'll see if a standard NEMA17 0.5 Nm stepper will do.

Btw, does anyone know if this is a 0.9 or a 1.8 degrees/step motor?

[vlaiv]

Don't waste money on replacement motor - just use 1.8 degree Nema 17 stepper.

Any idea of how much current is delivered per stepper? What is total consumption of the mount? About 2A when slewing (I'm guessing). This means 1A per motor at most.

Something like this should do the trick:

https://www.omc-stepperonline.com/nema-17-bipolar-1-8deg-44ncm-62-3oz-in-0-85a-5-3v-42x42x48mm-4-wires-17hs19-0854s

You can also measure length of body of stepper to narrow it down. I just guessed that it is 48mm, but looking at your image - it might be less like 34mm

(at least it won't cost you much to try).

[wimvb]

53 minutes ago, vlaiv said:

48mm

It is 48 mm (haven't measured it, though). The removed back makes it look shorter than it is. The front measures 42×42 mm, and it's slightly longer than that. 0.85A/phase is also plausible, the mount draws about 2.5 A while slewing. But I also have to make sure that the shaft is long enough. The aluminium block in the background (right side) sits between the motor and the belt. I ordered the replacement because I need something fast. I don't want to miss galaxy season.

[pipnina]

21 hours ago, wimvb said:

Btw, does anyone know if this is a 0.9 or a 1.8 degrees/step motor?

The number of bars on the rotor shows you the step angle. A 1.8 degree motor will have 100 bars on the stator and 98(IIRC) on the rotor. A 0.9 deg motor will have twice as many.

Yours certainly looks like an 1.8 to me

[vlaiv]

2 minutes ago, pipnina said:

The number of bars on the rotor shows you the step angle. A 1.8 degree motor will have 100 bars on the stator and 98(IIRC) on the rotor. A 0.9 deg motor will have twice as many.

Yours certainly looks like an 1.8 to me

Yep, two more things might give a clue. AZ-EQ6 was in production before 0.9 degree steppers were common place, and this as well:

EQ mod prerequisites page shows that worm is 180:1, then total reduction is 720:1 (belted part is 4:1). There is 51200 micro steps per worm revolution. That includes 4:1 belt reduction and steps from stepper motor.

51200 / 4 = 12800 micro steps
AZ EQ6 uses 64 micro steps

12800 / 64 = 200 and that is of course 1.8 degrees since 360/200 = 1.8.

[wimvb]

23 minutes ago, pipnina said:

The number of bars on the rotor shows you the step angle. A 1.8 degree motor will have 100 bars on the stator and 98(IIRC) on the rotor. A 0.9 deg motor will have twice as many.

Yours certainly looks like an 1.8 to me

6 bars per coil on the stator, which has 8 coils, and 2 x 50 on the rotor

15 minutes ago, vlaiv said:

Yep, two more things might give a clue. AZ-EQ6 was in production before 0.9 degree steppers were common place, and this as well:

EQ mod prerequisites page shows that worm is 180:1, then total reduction is 720:1 (belted part is 4:1). There is 51200 micro steps per worm revolution. That includes 4:1 belt reduction and steps from stepper motor.

51200 / 4 = 12800 micro steps
AZ EQ6 uses 64 micro steps

12800 / 64 = 200 and that is of course 1.8 degrees since 360/200 = 1.8.

Thanks, both.

Vlaiv, I got the same information from the EQ5/EQ6 group on facebook.

Besides the number of steps and the torque, there is also a difference in the phase resistance, and with that the current draw. There are low current versions and high current versions of most stepper motors, as well as 5V and 12V versions. The motor in the mount is 12V, but since a model number is missing, it may be either low resistance (high current) or high resistance (low current). I measured a resistance between connections of about 6 - 8 Ohms. Not very conclusive, since the phase resistance also depends on how the phases are wired internally.

[vlaiv]

1 hour ago, wimvb said:

There are low current versions and high current versions of most stepper motors, as well as 5V and 12V versions. The motor in the mount is 12V, but since a model number is missing, it may be either low resistance (high current) or high resistance (low current). I measured a resistance between connections of about 6 - 8 Ohms. Not very conclusive, since the phase resistance also depends on how the phases are wired internally.

I don't know much about this, but from what I've gathered, rated voltage of stepper is not important as it is used. This I've gathered from 3d printing forums as steppers are used to move the mechanics of 3d printers.

In any case, stepper can be driven in two different ways, if I'm not mistaken, and most modern stepper drivers are current drivers. They just pump enough current (depending on settings) to get the motor running. I think that max voltage has something to do with holding torque in that case - if you want more holding torque (or is it max speed?) - you need to provide higher voltage. Some stepper drivers work with 48V - and you can use those drivers with 3.5V motors for example.

With current drivers - you really don't care about declared voltage.

See this answer for more details:

https://electronics.stackexchange.com/questions/200324/how-to-interpret-the-stepper-motor-voltage-requirement

[pipnina]

7 minutes ago, vlaiv said:

I don't know much about this, but from what I've gathered, rated voltage of stepper is not important as it is used. This I've gathered from 3d printing forums as steppers are used to move the mechanics of 3d printers.

In any case, stepper can be driven in two different ways, if I'm not mistaken, and most modern stepper drivers are current drivers. They just pump enough current (depending on settings) to get the motor running. I think that max voltage has something to do with holding torque in that case - if you want more holding torque (or is it max speed?) - you need to provide higher voltage. Some stepper drivers work with 48V - and you can use those drivers with 3.5V motors for example.

With current drivers - you really don't care about declared voltage.

See this answer for more details:

https://electronics.stackexchange.com/questions/200324/how-to-interpret-the-stepper-motor-voltage-requirement

I only have a cursory knowledge of electrics in motors in general, but I think as a motor spins faster the magnetic back force increases which means you need yet higher voltage to move the motor. I'd guess if voltage correlates to max speed it's because of that. Holding torque I'd guess would be current based as the strength of the magnetic field is proportional to current and not voltage.

[vlaiv]

26 minutes ago, pipnina said:

I only have a cursory knowledge of electrics in motors in general, but I think as a motor spins faster the magnetic back force increases which means you need yet higher voltage to move the motor. I'd guess if voltage correlates to max speed it's because of that. Holding torque I'd guess would be current based as the strength of the magnetic field is proportional to current and not voltage.

I think you are right! Same thing is mentioned in the answer that I linked.

[wimvb]

That may very well all be, but I don't want to blow out my mount's motherboard, testing. Better safe than sorry.

[wimvb]

in the mean time, it's a clear night 😏

[gorann]

On 17/03/2024 at 20:02, wimvb said:

in the mean time, it's a clear night 😏

But then the moon is out, so you do not miss any high quality data. I hope the motor arrive before the new moon !

[wimvb]

4 minutes ago, gorann said:

I hope the motor arrive before the new moon !

Will be delivered on wednesday, so all is good.