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Why did my motor reverse direction?


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#1 abarthmike

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Posted 16 June 2017 - 11:05 PM

I posted this on SF but this is a motor specific section  :good:

 

question.gif

MB Slot Krahs. FC-130 Wasn't running well. Slow and smelt of escaping electricity (smoke)

 

Took the endbell off (wiring still attached to the chassis)

Removed the arm/endbell as a unit

Removed arm.from endbell

Cleaned between comm segments with a toothpick

Cleaned up comm a bit with 1500 grit wet'n dry

Put arm back into endbell

Put endbell/arm back in motor

 

Put the motor back into the car and it ran backwards.ohmy.png  Had to reverse the lead wires at the guide! 

 
Now motor has lost low speed "creap" car starts off quite fast, relative to before.
 
Haven't taken motor apart again , yet.

Mike Rust




#2 slotcarone

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Posted 16 June 2017 - 11:07 PM

You must have put the can on upside down. This will make it run backwards.


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#3 Zippity

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Posted 16 June 2017 - 11:30 PM

Yep, you inadvertently swapped the magnets when you reassembled the can and endbell :(

 

An easy mistake to make :)


Ron Thornton

#4 abarthmike

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Posted 17 June 2017 - 12:06 AM

Cheers.

 

Unlike most? S can motors the MB slot on does not have an "index" slot in the can. Most? other can only be assembled one way.


Mike Rust

#5 Cheater

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Posted 17 June 2017 - 07:22 AM

Which is why most motor rebuilders mark the cans and endbells before disassembly so they do go back together the same way every time...


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#6 Ramcatlarry

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Posted 17 June 2017 - 08:47 AM

On many motors, when you remove the armature from the field magnet, they drop some strength. This is quite true with old Alnico magnets and less so with newer ceramic and rare earth magnets. 

 

Did you test the motor before disassembly to see if it ran better one rotation direction or the other? With inline drive cars, simply flipping the crown to the other side make a car run better.  

 

Time for a fresh motor, anyway. If you let out the smoke, it is now a fishing line weight.


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Larry D. Kelley, MA
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racing  around Chicago-land

 

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#7 Bill from NH

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Posted 17 June 2017 - 10:43 AM

Which is why most motor rebuilders mark the cans and endbells before disassembly so they do go back together the same way every time...

 

I don't consider myself a motor rebuilder, but all motor I've ever built have had an off-center score  with a Dremel cutoff disc for correct assembly. The off-centeredness prevents mixup of endbells if working on more than one motor at the same time.


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#8 Zippity

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Posted 17 June 2017 - 01:30 PM

Greg,

 

I like your idea of an off centre score.

 

Sometimes a simple compass is a slot racer's best friend :)


Ron Thornton

#9 Dennis David

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Posted 17 June 2017 - 05:36 PM

"Sometimes a simple compass is a slot racer's best friend"

... next to a hammer
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#10 Mike Patterson

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Posted 17 June 2017 - 09:04 PM

If you can't fix it with a hammer, it must be an electrical problem. :D


I am not a doctor, but I played one as a child with the girl next door.


#11 Dennis David

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Posted 17 June 2017 - 09:44 PM

I use hammer for those issues as well.

Dennis David
    
 


#12 abarthmike

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Posted 18 June 2017 - 12:17 AM

Can someone point me to some info as to why rotation the can 180 degrees reverses the motor direction. Is it that the brushes are not actually on the centreline or something. Mechanically the axis is the same and the arm would rotate anyway when power is applied. 

 

It'll end up with the three fingers of my hand pointing the wrong way?


Mike Rust

#13 Zippity

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Posted 18 June 2017 - 12:56 AM

Mike,

 

It is all to do with the magnets.

 

If you reverse the can when reassembling the motor, you have inadvertently reversed the polarity - positive and negative, or some such crap :)

 

 

Trust me - it happens :)


Ron Thornton

#14 SpeedyNH

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Posted 18 June 2017 - 08:40 AM

exactly. I always scribe an off-center reference mark across the can-endbell joint (or otherwise identify it) when I build or take apart a motor.

 

summary of why it can go the other way:  at some point in the arm's rotation, some pole will be attracted to the N magnet and repelled from the south. if you swap the magnetic field from side-to-side with respect to the electrical polarity of the brushes (which would happen if you either swapped the magnets or put the can or endbell back on 180° out, either deliberately or accidentally), the armature will then turn in the opposite direction.  it's the same effect as swapping the wires, which you could also then do to nullify the effect.

precision-built motors are usually sensitive to can-to-endbell orientation for reasons of internal alignment, so in that case one always wants them put together the same way.

 

speedy  


Steve Lang

#15 Zippity

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Posted 18 June 2017 - 01:46 PM

I always place my South magnet closest to the rear axle - don't know why - just read about it a long time ago :)


Ron Thornton

#16 Bill from NH

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Posted 18 June 2017 - 02:44 PM

I marked the poles on my zapper N & S. I zap my motors so the leadwires run straight (not crossed). This doesn;t make the motors run any faster, but it prevents wiring motors in that run backwards.


Bill Fernald
 
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#17 Bazzie

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Posted 19 June 2017 - 05:20 AM

Say the positive terminal was on the north magnet pole side, if you rotate the endbell through 180 deg, now the positive will be at the south pole side, so you're reversing the direction of the current


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