EQ5/4 stepper motors

[luka]

Hi,

Does anyone have any info on the steppermotors used in the EQ4/EQ5 dual axis controller kit ($200 at Andrews). The only number on the motors is 42PM48L. Google does not reveal much :(
I want to modify the controller to enable faster slewing and I have to know the max torque, current/voltage of the steppers so I don't destroy them.


Also can anyone with the GOTO version of EQ5 (or EQ4) mount tell me what model their motors are (the number should be written on the motor). These motors should allow up to 800x slewing speeds :D
The current controller allows max of 8x :mad2:

Thanks,
Luka

[AdrianF]

Would be interested in info on these stepper motors aswell. I have a controlled and stepper motors I would like to motorise my 10" GSO with but I don't have info on how to connect the controller to the motors.

Adrian

[luka]

Adrian,
This page (http://telescope.marford.me.uk/Projects/PICcontroller.htm) has details how to construct the controller which may be of help. No specs on the motors though.

[bojan]

Hi Luka,
Most likely those motors are 200s/rev, NEMA 17 size, rated at 12V, producing close to 300Nm torque or less
Steppers are not easy to burn: the only way to do so is to use them at much higher voltage, then they will draw excessive current and overheat, but you can very easily detect that condition (when they become too hot to handle).
The maximum speed depends on coil inductance, and some other mechanical parameters, but very rarely they can be run much faster than 1000steps/sec. If you try to run them faster, they will simply stall and vibrate around one position and that will be it.

[luka]

Thanks Bojan.
The controller that the motors came with uses only 6V (4 D-size batteries) and I think that this is way below the motor rating (6V/17Ohm=350mA per coil) considering their size. The original controller only allows stepping of up to 8xR.A. speed which is about 80 steps/s.
I managed to step the motor at about 200 steps/sec (full stepping) which is very slow for slewing from one part of the sky to another. I was hoping to run them at their real rated voltage/current to be able to run them faster but I am not sure what the real ratings are.

I was also hoping that the motors might be the same as the motors that come with EQ5-GOTO kit. Hence my question about the specs/models of the EQ5-GOTO kit motors.

Regarding determining the maximum current/voltage of a steppermotor by monitoring the temperature, I managed to kill one stepper several weeks ago (at work) and it was just running slightly warm, not hot at all. Hence I am being overly carefull now.

P.S.
Also you probably ment 350Nmm or Ncm and not 350Nm - this would be extremely high for such small(ish) motor.
By the way, if anyone is buying stepper motors from Farnell, they have a typo on their website showing Nm instead of Ncm for quite a few steppers.

[bojan]

Yes, that was Ncm sorry :ashamed:

And if your power supply is 6V then motors are probably rated at 6V...
As to your motor, perhaps it had a bad solder joint or short? Perhaps it can be repaired.. I even managed to convert motors from 5 1/4 floppies having 5 leads to 6 leads, by opening them and performing some cutting and re-arrangement of existing wiring...

[luka]

I replaced the damaged motor with an identical one and it worked perfectly.
The damaged motor actually was not fully dead but lacking torque and skipping steps so it may be repairable. Opening it is a good idea, I might try that tomorrow. Thanks.

[Bassnut]

Do some home work 1st. Steppers are awefull, search for what the price is for regular DC motors and a servo drive.

[bojan]

Servos are more expensive.. and not easy to use (because of much more complicated drivers)
Good source for steppers are old floppy's and printers. Sometimes they can be found in bulk on ebay...
For my bartelized dobson I am using steppers from ancient 8" floppies..

Luka, your stepper probably lost one phase, due to a broken wire.. it would have happen even without your treatment sooner or later, I am sure :-) Maybe the problem is in the connector even.. take the Ohmmeter in your hands and measure resistance of coils and you will know (after determining what type of motor is it)

[luka]

The broken stepper motor is from work so I am not too fussed about it :D And it was only $50ish.
But I will measure resistance and try opening it today as Bojan suggested. At least I might learn something new. Will let you know how it goes.
The main problem was the mis-advertised torque on the Farnell website which I could not get and in the end, to increase torque I gave it a higher voltage/current... it was running fine for a week and then started giving problems.

With all the lessons learned, now I am more cautious with my telescope mount motors but I cannot seem to find any info on them at all. Hence this thread.

[snowyskiesau]

Do the motors look anything like these (http://www.ms-motor.com/pm_stepper_motor/pm_stepper_motor_42.html)?

The specs here state a step angle of 7.8 degrees which is common for this type of motor. The NEMA series would be 1.8 degree step angle (or for lower power motors, .9 degree step angle.)

[Glenhuon]

A good source for steppers is the real old type of flatbed scanners. The ones I've got from them were pretty solid pieces of work. Often have gearing associated with them that can come in quite handy too.

Bill

[luka]

First of all I would like to thank everybody for their comments and suggestions :thumbsup:
I think I finally know what the motors are and I also learned few new things.

Regarding the broken motor, Bojan you were right. One of the coils does not work properly, the resistance is very different compared to the other coil and also to the healthy motor of the same type.


Geoff, I think you have found it. The motors look very similar (ignoring the gearbox on the top of mine and the mounting brackets). The coil resistance in mine is 17 Ohms which lead me to believe that I have 42BY48L03 (15 Ohms). However, the model number was different (BY instead PM) but then I tried adding 03 to my model number and searching google - it came up with this webpage (http://www.motionking.com/Products/PM_Stepper_Motors/42PM_Stepper_Motor.htm). The motors listed there have the same specs as the ones you have found so now I know that my 42PM48L is just a rebadged 42BY48L. Interestingly the BY motor is called PM motor on the webpage but has BY in the model number.
Also I noticed that my motor has the centre wire removed from the coils to make it bipolar only.

So my suspicions were right, the original controller is "under-powering" the stepper motors. The motors are rated at 19V, 1.27A and they are running at 6V, 0.35A only. I suppose that with all the gearing there is enough torque to rotate the mount at 6V and this will save the battery power. The controller was not designed for fast rotations.

Hopefully I'll have time to try testing things tomorrow :D
I will post the results here.
And if my little project works the full details ;)

I have also attached a photo of my motor for anyone else who might be looking for the same info. This is the standard EQ4/EQ5 dual drive from Andrews.
55604

[snowyskiesau]

The limited current the controller is putting through the motor will reduce the torque well below the motors rating to about 27% of maximum (.35/1.27) The gearing would both help with the torque and also with the limited number of steps per revolution - 48.

You should be able to run the motor comfortably off 12V. Increasing the current will probably not buy you much except to flatten the battery more quickly.

[luka]

Yes, I've learned this the hard way. And I got really disapointed about the cheap design and motors :mad2:

Basically I was increasing the driving voltage and changing the stepping time on the microcontroller to see how fast I could step the motor before it starts skipping.
6V - 200 steps/s
12V - 250 steps/s
18V - 330 steps/s
(all values are approximate).

So I gained a little bit of speed but at the cost of much higher power consumption (also the motors were running warm). In the end I decided that it was not worth the trouble and that I would need new motors for faster slewing :doh:

The main lesson I learned is that these motors are cheap low torque 7.5 degrees per step motors. There is a gearbox (with gearing of about 100 times) which increases the torque and reduces the step size to give the mount small enough movement step for acceptable tracking. The gearing limits the maximum speed at which the mount can be moved so fast slewing is out of question.
On the other hand the more expensive GOTO motors utilise smaller step size motors (usually 1.8 deg/step) and use microstepping instead of very high gearing (they may use gearing as well but not with such high ratios). Hence it is possible to move the mount at a much higher rate once the microstepping is off.

Oh well, at least it was fun playing with the motors and now I know where I stand, or better said how slow the mount can move.
Again, thanks for everyone's help :thumbsup:

[MrB]

You can still increase your stepping rate even further than the 18V rate without over-heating the motors, but you will use more power overall.
Bump the voltage up even further, say 40volts, but use a series resistor to limit the current draw.
For example, if you decide to limit current to 1A, with a 17 Ohm winding you need 17V supply.
For a higher voltage at the same current, say 40v, you will need a 23 Ohm resistor in series with the winding... but it will need to be a pretty chunky resistor(s) to dissipate 23 Watts of heat.
This will increase torque at all step rates, so will also allow you to ramp (accelerate) faster, but most noticable is at the higher step rates.

You may be wondering how this works if the motor still see's 17V at 1A and all the extra power supplied is just being wasted by heating up a resistor?
Trust me, it works.
Let me know if you would like to know why.

[luka]

Simon,
I can't really see how this should work. The motor will get the same current/voltage (in your example 1 A/17 V) if you apply 17 V without a resistor or 40 V with a 23 Ohm resistor in series (due to voltage drop across the resistor).
The torque of the stepper motor depends on the electrical current flowing through the coils. If we do not increase the current, the torque won't increase.

[MrB]

It has to do with the inductance of the windings resisting the current flow, will add more later, just cooking a late dinner :)

[luka]

I completely forgot about the inductance in the motors (which affects the rate of change of current in the coils). It is all clear now.

This is actually a quite interesting, although probably not the way to go due to a large power waste which will kill my batteries in no time.

Thank you and enjoy your dinner :)

[MrB]

Hi luka, you are correct, there is the same amount of power available to the motor, and as you say, torque is proportional to current.
The windings in the motor form an inductor and inductors resists any change in current.
When power is applied to an inductor, the current takes time to increase to a steady level, this is called the Time Constant and is given by:
T= L/R
or Time = Inductance/Resistance
This gives the time in seconds that it takes for the current to reach 63.2% of full rated current. It takes 5 time constants to reach 98.5% of full current flow.

http://www.tpub.com/neets/book2/32NE0144.GIF

The pic above should give an idea of where stepper motors fall over.
The "Decay" side is the gradual reduction in current flow when power is removed.
If the step rate is low, there would be a certain amount of 100% time between the "Growth" and "Decay" sides.
Even if there is enough time for the coil to reach 100%, as in the pic, the current flow is compromised and torque suffers.
When operating at high step rates, the coils are being switched on and off so rapidly that the current doesn't reach 100% before it is switched off again.

Using T=L/R, if we double the resistance, we halve the time it takes for the inductor to reach 63.2%, and therefore 100%
Ofcourse, to double the series resistance and maintain the same current flow we have to double the voltage...

By having a higher voltage appear across the motor while the current is increasing to its rated level, the current is getting more "push" and the time constant is reduced.

Hope that makes sense?

[MrB]

Heh, sorry, didn't refresh before posting.
Yeah there is a lot of wasted energy.
I've heard of people running stepper motors up around 100v :eyepop:

Thats some serious power loss, fortunatelly there are some great stepper drivers around these days... chopper drives eliminate a lot of the waste heat through the series resistor by eliminating it altogether.
They supply the full voltage across the motor coil, 48v or whatever you desire, and then use pulse width modulation to limit the current flow.