Mechanical Engineer needs a Small Motor 3

[FredRosse]

Built a John Fitch steamboat model for the museum, and need to get a proper drive motor. The boat is 7 feet long, a replica of the first commercial steamer in the world, 1790 vintage. The model is powered by live steam or electric power, with radio control. The boat can be seen at:

http://www.youtube.com/watch?v=u_G5U7wcWQo

A slow speed steam engine drives the rear paddle mechanism, and a small 8VDC 5A, 2000 RPM motor below deck (about 2 in dia x 4 in long, 1/4 in dia shaft) is connected to the engine crankshaft via a small (1/4 inch wide) timing belt at 10:1 ratio. The timing belt drive has very little friction.

The PM DC motor has ordinary DC switching, proportional forward and reverse, and when running in electric mode the motor drives the paddle mechanism and continuously cranks the steam engine. This all works very well.

When running on steam power, the concept is to use the de-energized motor only as a flywheel, remaining connected to the steam engine, through the timing belt, and turning ten times as fast as the steam engine. The problem is the “notchy” nature of the de-energized motor, and it takes considerable torque to turn over the de-energized motor. I would like to find a similar motor that would spin freely when de-energized, but have no idea if this type of motor exists, and if indeed these physical characteristics are even within the realm of being obtained.

I want to avoid installing a clutch between the engine and motor if possible. Any suggestions from the EE community?

Thanks in advance for any help.

 

[benta]

Have you tried disconnecting the motor completely from the controller? Your problem might be the diodes in the power stage partially short-circuiting the motor.

Benta.

 

[ScottyUK]

'Cogging' is common in DC machines designed for high specific output because the magnetic design will likely have a number of 'sweet spots' where the magnetic circuit formed by stator and rotor has minimum reluctance. The rotor will naturally rest in one of these positions. The 'bumpy' rotation you notice as you turn the motor by hand is felt when the motor moves from one preferred position to the next. They type of motors used in cordless drills and model RC cars often suffer badly from this problem because they are required to deliver maximum output from minimum size, and other requirements are secondary to power output and efficiency.

Some motors sacrifice a little power output to get smoother operation by skewing the slots of the rotor or stator. This means that there is no 'perfect' position where the magnetic poles of rotor and stator align along their full length, so the cogging is less pronounced.

Specialised motors from the likes of Portescap have no iron in the stator which means that they do not suffer from the problem at all. They are expensive and typically reserved for servo applications. Wound field machines typically have little to no cogging, although finding one small enough for your application might be a little harder these days.

 

[waross]

To add to Scotty's information:
The motor may be oversized for your application. Measure the current draw when the motor is working normally. For example; if the current is only 1/4 of rated current the motor may be oversized by a factor of 4. If you can use a smaller motor the "cogging" may be reduced to something that you can live with.
You may check Benta's suggestion by disconnecting the motor from the controller to see if it makes a difference.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[mikekilroy]

Since you are multiplying the torque ripple from the magnetic detent of the magnets attracting to the steel in the rotor by a factor of 10, I doubt if skewed magnet design would get you where you want to be.

You need a motor called "ironless rotor" design: they can be designed to have absolutely 0 cogging torque so would solve your problem. since it is for a museum I would be happy to offer one to you from my old stock if it would fit. Kollmorgen PMI pancake motor, see typical specs here:

http://www.kollmorgen.com/en-us/products/motors/brush-dc/servo-disc/

they typically are similar 2-3000rpm with 8-12v at 4-8amps input. but they are larger diam and very short. I assume also you do not need a tach on it?

Let me know if one will fit.

 

[ScottyUK]

Failing Mike's generous offer, there's a Portescap motor on British ebay for £6, which would barely buy you the box it originally came in.

http://www.ebay.co.uk/itm/PORTESCAP...=UK_Trains_Railway_Models&hash=item4d0804faf9

 

[FredRosse]

Mikekilroy, thanks very much for your offer. It appears that the S6M4 motor woild fit well, at 3.39 inch diameter, if available, or something equivalent.

 

[waross]

A nice Christmas gesture Mike. Also a good reflection on Eng-Tips.
Yours
Bill
p.s. lps

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[mikekilroy]

Email sent fred, with offer of part no I see here at old office (home if too big I gotta run across town to new office to see stock.

 

[itsmoked]

Wow what a hideous drive mechanism! What a shock to see that when you expect to see the conventional paddle wheel. It's really clever though. Nice work FredRosse.

Nice offer too Mike.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Skogsgurra]

A Quadriped Duck!
Beams are cheaper than wheels. Like it.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[mikekilroy]

Fred,

We UPSed you the U9 motor early last week. In the meantime one of my contacts came thru with a smaller version - about 2.5" dia (I emailed you specs and drwing) as you wanted for this boat. It should be here this week. I sent you a couple emails but maybe your or my spam filter ate them? Let me know if you want the smaller motor also.