DC Motors and the Motor-Mind B

[jhoyoza]

Hello,

I’m a hobbyist and not quite experienced enough to understand all of the terminology.

I recently purchased a Motor-Mind B and simply love it! In fact it is so nice, I'm going to replace the ‘cheap motor’ that drives my toy-boat. I'm looking at the motor (below) to replace it with as it would be perfect as far as rpm, nominal voltage and size are concerned.

I would be helpful to actually understand the whys and nots, and to see if this particular motor is compatible with the Motor-Mind B. Additionally I may gain insight on just how to select a DC motor for the MMB and what to look for. (Or look out for.)
http://i145.photobucket.com/albums/r211/jhoyoza/motor.jpg
Specification :
Product information : Ball raced high-quality motor for use with 7 or 8 NC cells. Propeller size approx. 20 x 10 cm.
Nominal voltage 9.6 V
Operating voltage range 7.2 ... 12 V
No-load rpm 14300 min–1
No-load current drain 0.8 A
Current drain at max. efficiency 6.6 A
Current drain when stalled 55 A
Max. efficiency without gearbox 75 %

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&sspagename=STRK%3AMEWA%3AIT&viewitem=&item=5917169256&rd=1&rd=1


http://i145.photobucket.com/albums/r211/jhoyoza/MMB.jpg
MOTOR MIND B - Electrical Characteristics Characteristic (Pertinent)

Supply Voltage 6 - 36V VMOTOR voltage
Motor Current Load 6A peak / 1.75A continuous
Short circuit current limit 15 A Extended short-circuit could damage PCB traces, other components
Over-current trip point- 6 min 8 typ 10 max A The higher the temperature the lower the
trip point
Peak load current 6 A <100 ms pulsed at a period of 1S
Max continuous motor current 1.5 min 1.75 typ 2.0 max A Continuous current rating at room temp. 95% duty-cycle, resistive load.

----------------

If I’m reading this right the motor uses 6.6 Amp when at max efficiency? (And I’m not exactly sure what that means.) And the Motor-Mind B has a peak-load current of 6 Amps? And an over current trip point at potentially 6 Amps? (Also, pond weeds have been known to stall the motor.)

Is this a poor choice?

Thanks

Post Edited (jhoyoza) : 11/16/2006 7:41:07 AM GMT

[Zoot]

Stall current is the "peak current" in a sense -- it's the amount of current drawn by the motor when starting up from a dead stop (like trying to get your bicycle started -- it takes more oomph to get going then to keep going).

I think these motors are way too big of a current draw for the Motor Mind B. If you like your controller you might want to look for a less powerful motor, or you might want to look at beefier controllers (like the HB-25 from Parallax or the MD03 from Devantech).

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
When the going gets weird, the weird turn pro. -- HST

[jhoyoza]

Ah…I see, apparently the project boat I have now is built more for comfort than for speed. The motor shown above must be for racing boats, I bet cha‘.

Please correct me if I’m wrong - If I connect an ohm meter to a DC motor and slowly turn it until I can obtain the lowest possible value then apply ohms law I should get the approx stall-current or peak-current? So, any motor that measures near or less than 1.6 ohms is dangerous to a MMB.

For instance the motor in the boat now measures ~ 2.5 - 3 ohms at it‘s lowest reading. Thus, 9.6V / 2.5 ohms = 3.84 Amps being the approximate stall current? The Motor-mind B is rated for 6 Amps peak so I can safely assume the motor I’m now using is well within the rating of the MMB capabilities?

Also, what might the most efficient duty-cycle be for a DC motor be. For example, just as my car that can achieve 100MPH I still get my best gas mileage at about 60 MPH. Does this speed - verses - efficiency also exist for DC motors?

I thank you in advance.

[aquasapien]

I have a couple of motor minds. They are good. I had a bad expeirince once. They were each contorlling one of those tiny white geared DC motors you get from parallax and they worked well for many many weeks.

Then one night I turned on my stamp getting ready to use it and smoke came pouring out of my project box. I opened it up and both motor minds had popped the heatsink off and were dead and dead, for no apparent reason, although I will not deny somthing I did was the cause, I never figured it out.

I think I had paid close to 50 cdn for each of them so it was very annoying. I picked up two more but have never used them as I got annoyed and skitish with being limited to those units (I have to order them and that takes time) so I went the H-bridge way and have never looked back. There are always good TIP* transistors on junk circuit boards to be ripped out.

I am willing to bet there are many advanced feature with them but have never had to look for or use such features. Yes your motor looks like a ripper for sure.

If one was so inclined I wonder if 2 motor minds could be piggybacked to double the current ratings. hmmmm

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
The singularity is close at hand, Genetics, Nanotechnology, Robotics...and I get to be alive to see it.

[jhoyoza]

I’m A bit confused, my Motor-Mind B doesn’t have a heat-sink? Is it maybe a different version than yours? Or is there a heat sink available for it?

I did happened to notice something interesting:

I ran the NiCad’s until dead (about 60 mins w/no load) cycling different motor speeds the whole time using the existing boat-motor, and the MMB was barely warm to the touch?

So…I recharged the batteries and stuck the back end of the boat in the kitchen sink full of water and ran the motor again this time under load. And yes it did eventually trip the overheat protection switch. Everything was cool to the touch until full speed $ff was reached. And then within a few seconds tripped the switch. Thankfully it worked again as soon as it cooled.

Ok so it must be true higher speed require more fuel (current) and the load(s) on the motor also has a lot to do with current draw. Apparently a DC motor subject to enough load is apt to ‘heat up’ any controller board?

I think I will purchase and use a HB-25. Weight is a concern for me however the HB-25 is less than twice as heavy as a MMB and should be acceptable. (Backordered until 12/15/06...maybe I should be thinking of an ice-boat!…lol)

I am still curious about motor selection as the specs appear not to be standardized and in many cases or not-specified or replaced with marketing jargon. I suspect ‘over-kill’ is a good thing when it comes to DC motor controllers and perhaps experience has a lot to do with it?

Thanks

-J

[aquasapien]

The heat sinks on mine came with them are the same size as the whole board and are pasted to the front. I saw the pic on the parallax site it they dont have the heat sink.

Maybe that is why mine were 50 instead of 29 bucks.

I think it is the load on the motor that will most directly affect the heat in the controllor.

If surfaceice is concern for your boat build a sub and stick a camera in it. That would be fun.

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
The singularity is close at hand, Genetics, Nanotechnology, Robotics...and I get to be alive to see it.

[jhoyoza]

Thanks aquasapien,

It may end up performing like a submarine after all, by the time I’m through with it! (Even though I'm trying desperately for that not to happen…lol.) For now I would like to keep things dry. Perhaps a wireless camera below the water line would be great idea, or perhaps in tow underwater behind it?

Now you got me thinking!

[I would like to give a special thanks to Charlie Knox for contacting me and providing me with the pair of brand-new HB-25 motor controllers, as they are still backordered for a month otherwise. (At less than cost too!) They work like a charm, and solved my problem just as Zoot suggested.]

Thanks to everyone, Charlie, Zoot and especially this forum!

-J http://forums.parallax.com/images/smilies/turn.gif

Post Edited (jhoyoza) : 11/23/2006 2:33:46 PM GMT