BiPolar or Unipolar.

[Tandum]

I've heard that one of these motors (http://catalog.orientalmotor.com/item/all-categories/pk-series-stepping-motors/pk223pa-sg36?cid=1002&plpver=11&categid=100&prodid=3001048&itemid=11923&origin=groupdetail&by=prod&filter=0&grpid=7249&backtoname=Compatible%20Motors%20(s old%20separately))will fit in my mount.

What does bipolar and unipolar mean. What I'm worried about is that the existing motor is 2 ohms. This new one is 2.8 ohms unipolar which is pretty close but 5.6ohms bipolar. We don't want to burn out the control box, just swap motors.

I'm guessing there are twin windings in the motor and how you wire those windings up is either uni or bi. Correct?

[bojan]

Robin,
2.8ohm or 5.6 ohms will not burn the control box. It just mean, that the resistance of motor coil is higher.

Unipolar motors are driven with 4 switches (transistors), while bipolars need H-bridge.

However, those motors at the link you supplied are VERY expensive, and I wander if what you are trying to do is worth it.
What kind of mount do you have? This type of motor is very unusual for any kind of mount that I know of.

BTW, they have 6-wires, so they are unipolar.

To use them as bipolar motors, you have two choices: to use the entire coils (and leave the middle terminal unconnected, this will result in 5.6ohms per coil) or to use half of each coil (middle and one of the end terminals, the other one is then left unconnected) . Either choice is OK and safe to use (as your original motor is 2 ohms), however, you may notice the difference in holding torque, and in 5.6ohms case the maximum speed will be lower (because of higher inductance of the coil). This will probably not be an issue as the motor has gearbox (36 :1) , so the overall torque will be way higher than really needed anyway.

http://en.wikipedia.org/wiki/Stepper_motor

[rcheshire]

http://www.divms.uiowa.edu/~jones/step/types.html

Thanks Bojan. We posted almost simultaneously:lol:l

[bojan]

Yep.. it seems we both can't sleep ;)

[2stroke]

Sorry if i'am hijacking your thread op, but are there any cheap sources in our country to cheap reduction steppers? chasing a 300:1 but even a 200:1 would do for a eq platform :)

[Tandum]

Cheers Guys, I got the picture now. Bojan the P43HG (http://translate.google.com.au/translate?hl=en&sl=ja&u=http://www.sanryusha.co.jp/motor/stepping/p43g.html&ei=mmR-T_mmD4iviQfav_nWBA&sa=X&oi=translate&ct=result&resnum=1&ved=0CCMQ7gEwAA&prev=/search%3Fq%3Dp43hg%26hl%3Den%26biw% 3D1920%26bih%3D955%26prmd%3Dimvns) dec motor in my em-200 has plastic gears in it's gearbox and they always end up with lots of backlash.
I got the model of this new motor from this Japanese site (http://translate.google.com.au/translate?hl=en&sl=ja&u=http://home.e-catv.ne.jp/okadaf/telescope.html&ei=7WELT6u9CKyQiQfdmriBBg&sa=X&oi=translate&ct=result&resnum=1&ved=0CDMQ7gEwAA&prev=/search%3Fq%3Dem-200%2Btemma%2B2%2Bjr%2Bmotor%26hl%3 Den%26lr%3Dlang_ja%26tbo%3D1%26biw% 3D1920%26bih%3D955%26tbs%3Dlr:lang_ 1ja%26prmd%3Dimvns) and it seems to have solved his problems although it is a little hard to read :)
You may need to hit translate again to get the bottom half of the page readable.
It's actually cheaper than one from tak.

[bojan]

OK, now I see what it is.. Still too expensive, though (IMHO)
Try here:
http://smartautomation.en.made-in-china.com/product/KMnEbecJHSVr/China-Stepper-Motor-and-Driver.html

However, this guy has his own controller/driver - so the motor type and gearhead reduction ratio was not the problem for him.
Are you doing your own, or you are using the original controller? If you are just replacing the motors, they may not be suitable because of wrong reduction ratio.


According to my understanding of the translated text, those two motors doesn't have the same ratio - first motor has 7000steps/rev total (with gearbox), the other one (orientalmotor) has 7200s/rev..
You will have to configure your control box somehow to fit the new gear ratio.

[Tandum]

I'm using the original, I didn't realise he had his own. What difference will 200 steps make? Both RA and DEC have encoders on the worm shaft.
It might be easier to just try and get another RA motor (http://translate.googleusercontent.com/translate_c?hl=en&rurl=translate.google.com.au&sl=ja&tl=en&u=http://www.sanryusha.co.jp/motor/stepping/p43eag.html&usg=ALkJrhg9gsFRYBGhNICLLjw5pX7tb8f 6cA) to fit in. They use the same motor but have a more solid gearbox.

[bojan]

The encoder on the worm shaft will make the 7000 or 7200 steps on the motor side irrelevant (otherwise it is a huge issue for GoTo).

However, then the backlash in gearbox is irrelevant as well. Why do you do this ?

But, if you really want new DEC motor, then yes, another one like RA is better idea, I think. Cost will be lower, and the result will be the same (less backlash in DEC motor transmission).

[Tandum]

Guiding, the longer the focal length the harder it gets.

[bojan]

Fair enough... Did you have your own, bad practical experience?
Because, I am not entirely convinced the backlash is a problem with encoders on the worm shaft.
However, the meshing between worm and worm gear will be the problem, which can't be solved with new motor gearbox.

[Tandum]

The backlash is in the gearbox, you can see it in the motor output shaft and when you pull the gearbox apart you can see it in the plastic gears.
Encoders won't do anything to correct/improve dec guiding, guiding corrections are supposed to be tiny.
Thanks for the help, I'll get a price for an RA motor and mock one up to see if 2 will actually fit before deciding which way to jump.