Could someone advise on this motor please ?

Hi,

I have a stepper motor from an old inkjet printer that I would like to use in an Arduino project. The motor is this one:- http://www.eminebea.com/en/product/rotary/steppingmotor/pm/standard/pm55l-048.shtml

The L293D motor shields available say they can only supply up to 600mA. This motor’s specs mention 800mA in the ‘Bipolar Chopper’ column, but nothing at all in the ‘Unipolar Const Voltage’ column on that page. As all this is new to me, I don’t really understand what this means. Is it possible to control a motor in different ways, and each way might draw a different current ? …or is the manufacturer’s page just covering several sub-versions of the motor and detailing the specs in different columns ? The motor I have has six wires coming out of it.

Basically, what I’m trying to establish is whether an L293D motor shield is up to the job, or whether I need something beefier - and if so… what ?

Any help is very much appreciated !

Cheers !

That link describes two motors - what is the winding resistance of your motor?

How about beefing it up by using mosfets and just use the driver to trigger those mosfets?

MarkT: That link describes two motors - what is the winding resistance of your motor?

Hi,

The wires come out of the motor case in a block of six - three each side like this:-

Gn Br Or Bk Ye Rd

There doesn't appear to be any connections between the groups on each side, and after a bit of googling I think the middle wires on each side are centre-taps (Or & Bk). The resistance between Gn & Ye (and Br & Rd) is 25.9 Ohms. Between Gn & Or (and Or/Ye, Br/Bk & Bk/Rd), it's 13.1 Ohms. So.... I'm guessing the motor I have must be the one in the left column of the manufacturer's page. Which is also the one without a current listed :~

Soehave: How about beefing it up by using mosfets and just use the driver to trigger those mosfets?

That sounds like a good idea. I will investigate this further !

You can use it as unipolar, Or and Bk are common, drive the others in an order like Gn/Br/Ye/Rd.

Or bipolar with Gn/Ye and Br/Rd as the two windings, other wires open circuit.

The windings don't match that datasheet so its a bit of a guess as to the current rating. For a given size of motor the power rating ought to be the same, but that datasheet looks suspicious to me - if 0.8A is the bipolar rating for 5.5 ohm windings, then the unipolar version should be 160ohm, I suspect they menat to say 12V for the unpolar version.

With 13 ohm windings I'd guess 0.35A bipolar (chopper drive).

You've lost me on some of the terminology, but it's nothing I can't google :) Are you saying that this motor can run at less than the 600mA limit imposed by the L293D motor shield ? Is there a way I can measure it before buying a shield ?

I just want to be able to run the motor at very slow speeds, with the high torque the specs say it has, using the cheapest motor shield possible.

You can't measure the specifications easily - you need to correct datasheet for that, and these motors come in many versions for different applications, rarely publically documented.

The best you can do is estimate the power handling from specs for the same physical size of motor (here its the same motor apart from windings), then assume the windings on this one can run at the same power without overheating. power = V^2/R = I^2.R, hence estimate winding current/voltage.

This isn't the whole story as the magnetic field also depends on the current and winding. If you know the specified torque you can try measuring the torque/current relationship to estimate the nominal current better.

If it is 0.35A, then you need 4.5V+4.5V plus the 2.5V or so loss in the L293, suggesting about 12V supply for an L293D motor shield operating it as bipolar. Or ~8V supply using darlingtons in unipolar mode (you typically use 1.4 times as much current in one half-winding, leading to the same dissipation)

Its not going to be miles off I think - that datasheet does suggest the motor is about 3.5W (from the bipolar column, 0.8A at 5.5 ohms)

:astonished:

Sorry - you've gone waaaaay over my head with that. I'm still a beginner with this. I don't really understand what you have said. I think maybe I should just build myself a motor controller with the higher capacity chip (298?) to be on the safe side.

You never have to drive it full current. You'll just get a little less torque, and even then you can get that torque back by just reducing the rotational speed of the stepper (the faster a stepper moves the less torque it has).

The number one rule is that if the motor gets too hot to hold your hand on then you're using too much current. Datasheets be damned; just start with a low current and keep slowly increasing it until it gets hot. ;)

As far as drivers go you'll be much happier with a driver based on the A4988 (~$10 at pololu.com) rather than the L293/L298.

Unfortunately, I saw your reply about two hours after I'd ordered an L298N controller! Never mind ! I'm intrigued by your comment about not driving it at full current. How do you do that then ? I'm still learning but until now I'd thought it was just a case of sending 12V (or whatever) to the wires or not sending 12V. How do you control the amount of current ?

Use a PWM signal to the L298 instead of just a full on or off.

I'd suggest finding a small DC motor to use with the L298 and practice controlling the speed of the motor.