Powering stepper motors

Hi , (I did search Google and the only result that was not commercial was this Other results were forum questions where people had datasheets . I don't ) I decided to buy a power source for 3x steppers , 2 of them 6 wired and 1 of them salvaged from a DVD writer (bipolar) The problem is that I'm in doubt about powering them . The general V=IR I=V/R does not work , as I = variable V = (in doubt but I suppose 4) and R = 110 per 2 wires . 4/100 = 0.04 , really , 0.04 amps ?! This is wrong . I thought I didn't measure it correctly and maybe even I didn't read it correctly but on the motors sticker I could read 110 Ohms . Maybe I should use a secondary formula ? I even calculated W and then used the other formula (don't remember it) to calculate I but got similar results . (Just read the formula somewhere) Am I doing something wrong (of course I am) ? I think 100 is the motor itself , and 100 / 4 = 20 but still 4 / 20 = 0.2 amps ?? :'( Thanks in advance ..

Ohm's Law works to describe the steady state current in a motor winding. Since you have different motors, measure the winding resistance for each of them and tell us what that is.

You really need proper stepper motor driver boards - one for each motor - such as the Pololu A4988. The driver boards allow the motors to be driven with a high voltage (up to 35v for the A4988) and they have an adjuster to set the max current to suit the motor. Obviously you need to choose a stepper driver that can comfortably meet the current required by the motor.

Six-wire motors can usually be used as bipolar motors by ignoring the connections to the centres of the coils.

…R

Hi thanks , yes I'm going to buy the 'EasyDriver' . Problem is that I'm in doubt wether A4988 (polulu) can be used with GRBL as everyone use Easydrivers (I think anything with Step/Dir works , right ?) This A4988 says it can work up to 2A , don't know about the easydrivers . The 2 similar motors are 110 Ohms a pair . The little one has a protective cover over the weird wires and the solders on the mini board . I'll remove it now .(and update post) Thanks .

The BigEasydriver uses the same A4988 as the Pololu A4988 except that the BED defaults to microstepping and the Pololu defaults to single stepping. The "ordinary" Easydriver has a much lower current capability - 750mA IIRC. The Pololu DRV8825 can manage a little more current than the A4988.

I believe that it is not realistic to expect 2 amps from an A4988 unless you add a heatsink and what I have seen referred to as "aggressive cooling".

...R

So I guess I can use the A4988 with grbl . I will add 2-3 fans for 3 drivers and I might not even use the 2 A maximum . I should see how much current I need . Thanks . Edit : I'll have to test the motors with ULN2003 before actually buying drivers . Found a cool instructable for uln2003 s .

Arman5592: Edit : I'll have to test the motors with ULN2003 before actually buying drivers . Found a cool instructable for uln2003 s .

Only waste your time with that if you need to verify that the motors actually work.

...R

So I'd test it with an H bridge ... Actually , a weird thing happened . After I soldered the 6th wire on the mini board (it was 5-wired , disconnected the 2 connected ones and soldered the 6th wire) all the resistance disappeared ! Now it shows 0 in any combination of 2 wires !! Same thing happened with the third little motor , because the mini board had protective layer on the solders , I re-soldered them ,now it's 0 Ohms in any combination . Every weird accident happens to me :astonished:

Arman5592: all the resistance disappeared ! Now it shows 0 in any combination of 2 wires

Do you mean that it shows a short circuit (zero ohms) or an open circuit (infinite ohms) - the latter seem more likely.

If you had a reading of 110 ohms previously it can't just disappear.

...R

I guess I understood what happened . The meter wouldn't show results instantly . I found the pairs and the 2 common wires , pairs have 113 Ohms resistance . But I= E/R , 5/100 is 0.05 Amps . But even an LED wont work at 0.05 A ... I am wondering how much current it needs ? Thanks .

Arman5592: But I= E/R , 5/100 is 0.05 Amps . But even an LED wont work at 0.05 A

Maybe it is designed for a higher voltage ?

...R

Motors from a DVD writer will be for 12V, most likely.

Pictures of the nameplates of each motor would be handy, whatever information you have.

Robin2:
Maybe it is designed for a higher voltage ?

…R

Thanks . Didn’t think about that , but I hope not . My problem right now is with the power sources as they have high amps and low voltages , my current choices are like 12v 5A 12v 10A . Perhaps I should buy one of those in computers . They are cool stuff .
However , it would need 5A at 500 v . 0.5 A at 50 v . This is odd .

MarkT:
Motors from a DVD writer will be for 12V, most likely.

Pictures of the nameplates of each motor would be handy, whatever information you have.

Hi , thanks , attached a picture .

Arman5592:
My problem right now is with the power sources as they have high amps and low voltages , my current choices are like 12v 5A 12v 10A . Perhaps I should buy one of those in computers . They are cool stuff .
However , it would need 5A at 500 v . 0.5 A at 50 v . This is odd

Can you rewrite this so that I can understand it, please.

…R

When using stepper motors use a chopping microstepping driver like the Easy Driver with a as high as possible power supply voltage. Then adjust the the current on the driver to get as smooth operation as possible

Robin2: Can you rewrite this so that I can understand it, please.

...R

:D The 'power supplies' aka 'power switching' or 'super ac/dc converters' as I call them , they come in some specific standards (BUT variable voltage) and the general choices I've seen around me would be : 12 V - 5 A , 12 V - 10 A and like that . I've even seen 5 V 40 A . Just A increases . Better to say V is the same and R is decreased . But unfortunately , I've never seen something like 36v 3 A . Usual power sources would also grow at A when they get more V .

Perhaps I should buy one of those in computers . They are cool stuff .

Yes . They are much better choices and I've found some versions that are cheaper than the 12v 10 A I mentioned . Their advantages : 1- Voltage already divided , I can give exact similar power to each driver individually with minimum effort . 2- Power is regulated to different amounts . Great when I have different motors for different E & I .

However , it would need 5A at 500 v . 0.5 A at 50 v . This is odd .

Well ,

Maybe it is designed for a higher voltage ?

So it requires 0.05 A when powered with 5v . So if I give it 50 v , it should require 0.5 A . It requires 500v so it draws 5 A ? This IS good , to have a motor that needs minimum current but it's odd . Steppers are known for drawing high current , so this is quite weird . The fact that I'm making a mistake is beyond doubt . I think maybe I made a mistake . Perhaps with the formula .

It may be a problem where you live but there are plenty of suitable power supplies for sale on Ebay specially for stepper motors. And you could always use 2x12v supplies to give 24v.

Arman5592: So it requires 0.05 A when powered with 5v . So if I give it 50 v , it should require 0.5 A . It requires 500v so it draws 5 A ?

This makes no sense. It is essential to know the maximum current that the motor can take - and I appreciate it may be hard to find that out. My guess is that a motor with a 113 Ohm coil is designed for very low currents - such as you may get with 12v. Remember the magnetic force depends on the current and the number of windings. That coil is likely to have a lot of windings.

High current stepper motors generally have low resistance and few windings to minimise the inductance.

Either experiment very cautiously with the motors you have or buy some motors for which you can get the full specifications.

...R

Robin2: It may be a problem where you live but there are plenty of suitable power supplies for sale on Ebay specially for stepper motors. And you could always use 2x12v supplies to give 24v.

This makes no sense. It is essential to know the maximum current that the motor can take - and I appreciate it may be hard to find that out. My guess is that a motor with a 113 Ohm coil is designed for very low currents - such as you may get with 12v. Remember the magnetic force depends on the current and the number of windings. That coil is likely to have a lot of windings.

High current stepper motors generally have low resistance and few windings to minimise the inductance.

Either experiment very cautiously with the motors you have or buy some motors for which you can get the full specifications.

...R

Hi , thanks . I think I'd buy a computer power supply . They have lots of outputs , I could use one for the fan (for drivers) and other stuff . Maybe LED lights ;D I'll test with low current first and see if it gets hot . So (just to be sure(sorry for asking again)) does the Polulu A4988 work with GRBL ? People usually use Easydrivers and I'm worried :( Thanks for helping .

Arman5592: So (just to be sure(sorry for asking again)) does the Polulu A4988 work with GRBL ? People usually use Easydrivers and I'm worried :(

I don't know. The BigEasydriver uses the same Allegro A4988 chip as the Pololu A4988 so I presume the Pololu boards can be used. However the Pololu defaults to single stepping and the BED defaults to microstepping so GRBL would probably need to be properly configured.

You need to ask this question on a GRBL forum.

And I guess you have not tried to Google "grbl pololu a4988" ? ? ?

...R

Hi , thanks . I had done that before , but lead to CNC shields and totally different problems , but I found some reasons that prove A4988 works with GRBL . The first one is that the GRBL shield has pin headers for Polulu A4988 , which is a reason . The second and the more reliable one is that ... people have already made CNCs with A4988 :grin: Thanks for helping .