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Date: Fri, 06 Jun 2014 10:49:17 -0400
From: "Stephen R. Besch" <sbesch AT buffalo DOT edu>
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To: geda-user AT delorie DOT com
Subject: Re: [geda-user] Seized gear motor ~ steering questions and advice
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Reply-To: geda-user AT delorie DOT com

Rob,

I can't be sure from the photo if this is brushless or not, but you 
should be able to tell from the manufacturer's part number (not given at 
ebay). It does help to know that this is a brand new motor. Since its 
2-wires and reversible, the probability of it being brush commutated 
goes way up. The motor itself is relatively small - probably a rather 
high RPM, with a really high gear ratio - so I still can't connect the 
dots to failure. The spec sheet gives running torque, not starting 
torque. What kind of likelihood would you place on the possibility of 
impulse damage to the gearbox from too-rapid application of current 
under load? (Personally I don't put much stock in this scenario). The 
summary: This is still somewhat of a mystery.

Now, about the stepping motor. Here's some answers to some of your 
questions. The higher torque with bipolar operation is predicted from 
the nature of the stepper. Magnetizing each sequential coil pair in 
essence pulls the armature ahead to the next pole. If you use a bipolar 
scheme, the lagging pole will push while the leading pole will pull - 
thus twice the force. With a unipolar scheme, you either have the 
leading pole pulling or the lagging pole pushing - not both.  The full 
bridge drive electronics is more complex, but worth the effort if you 
need the torque. The other bit is that you don't control speed by 
varying applied voltage but by the rate of switching the coils on and 
off. Since the driver transistors operate in saturation, transistor 
dissipation is minimized. The PWM controller attempts the same thing 
with the brush commutated motor, but you've already had a bad experience 
with that scheme. Best of luck with the stepper.

Steve Besch

On 06/06/2014 04:25 AM, RichardBown wrote:
> On Fri, 06 Jun 2014 09:30:12 +0200
> onetmt <onetmt AT gmail DOT com> wrote:
>
>> Il 06/06/2014 08:08, Rob Butts ha scritto:
>>> I'll try and take it apart tomorrow. In the meantime this is the
>>> link to the motor:
>>>
>>> http://www.ebay.com/itm/30RPM-High-Torque-Gear-Box-Electric-Motor-12V-DC-37MM-/291106511233?pt=LH_DefaultDomain_0&hash=item43c74cd981
>>>
>>> Maybe you can tell what kind of a motor it is.
>>>
>>> Also, I've found information on a Nema 23 stepping motor which seems
>>> like what I should be using. It is a 1.8° 200 steps/revolution. It
>>> is a six wire connection uni-polar or a four wire connection
>>> bipolar with pinouts: A, A COM, A-, B, B COM, B- but I'm having a
>>> hard time finding information on how to drive this motor.
>>>
>>>   
>> For this kind of stuff you should start studying stepper drivers, like
>> L6474 from STMicroelectronics for instance.
>>
> are you sure this is a stepping motor ?, this looks as if it only has 2
> connections .
> Have a look at how much current is taken when you apply some volts to
> it,
> Thats a high gear ratio , it may be the gear box may have lost a spur
> gear tooth.
>
> If its taking some current you can rule out brush problems, and if you
> connect a scope across the connection and apply power, if the armature
> is OK you will see the back emf as you disconnect power.
> It will give a clue where the problem lies.
>
> Good luck if you decide to strip down the gear box
>


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