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'[EE] reversing DC motor EMF issue'
2007\12\18@113507 by alan smith

picon face
Have a board that is driving a couple of relays (with diode suppression across the coil) where the relays are used to engage a 12V DC motor, and the other is used to reverse it (swap the leads).  There are 3 smaller motors also in the same circuit..ie...driven off the same board so anything happening on the 12V rail tends to be seen by everything...it essentially turns on these motors when the others are activated.
 
 So, first thought is to put some sort of suppression on the larger motors being driven from the relays.  Since we are reversing the direction, a diode isnt going to work....what about a varistor? Or...what, across the motor leads?
 
 I'm going to have him disconnect the bigger motors to see if it affects the smaller motors so at least I can identify where the problem is originating from.

     
---------------------------------
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2007\12\18@115637 by M. Adam Davis

face picon face
The supply to the relays that goes to the motor does not switch
polarity, put the diode there, and use large gauge wire to connect the
diode, relays, and motor.

It's not ideal, but it's very close to what you need.

-Adam

On 12/18/07, alan smith <spam_OUTmicro_eng2TakeThisOuTspamyahoo.com> wrote:
> Have a board that is driving a couple of relays (with diode suppression across the coil) where the relays are used to engage a 12V DC motor, and the other is used to reverse it (swap the leads).  There are 3 smaller motors also in the same circuit..ie...driven off the same board so anything happening on the 12V rail tends to be seen by everything...it essentially turns on these motors when the others are activated.
>
>  So, first thought is to put some sort of suppression on the larger motors being driven from the relays.  Since we are reversing the direction, a diode isnt going to work....what about a varistor? Or...what, across the motor leads?
>
>  I'm going to have him disconnect the bigger motors to see if it affects the smaller motors so at least I can identify where the problem is originating from.
>
>
> ---------------------------------
> Never miss a thing.   Make Yahoo your homepage.
> -

2007\12\18@122210 by Thomas C. Sefranek

face picon face
part 1 1621 bytes content-type:text/plain; (decoded 7bit)

If you put your suppression or flyback diode at the feed of the polarity
switching relay contacts, you have clamping no matter what the motor does.

 *
 |  __O    Thomas C. Sefranek  .....WA1RHPKILLspamspam@spam@ARRL.NET
 |_-\<,_   Amateur Radio Operator: WA1RHP
 (*)/ (*)  Bicycle mobile on 145.41MHz PL74.4

ARRL Instructor, Technical Specialist, VE Contact.
hamradio.cmcorp.com/inventory/Inventory.html
http://www.harvardrepeater.org
{Original Message removed}

2007\12\18@123239 by Dr Skip

picon face
It sounds like you have 2 relays with the wiring on the terminals reversed so
that if one is on, it's +- and if the other, -+, correct?

If so, that's not good. A delay, stuck terminal, etc can cause both to be
active at the same time, shorting it out. It would be better to use a DPDT
relay for switching, and another with terminals in series with that one for
power on/off. You still get disconnect but with no chance of shorting.

Wire the DPDT so common is to motor(s) and the terminal sets are reversed.


alan smith wrote:
> Have a board that is driving a couple of relays (with diode suppression across the coil) where the relays are used to engage a 12V DC motor, and the other is used to reverse it (swap the leads).  There are 3 smaller motors also in the same circuit..ie...driven off the same board so anything happening on the 12V rail tends to be seen by everything...it essentially turns on these motors when the others are activated.
>    
>   So, first thought is to put some sort of suppression on the larger motors being driven from the relays.  Since we are reversing the direction, a diode isnt going to work....what about a varistor? Or...what, across the motor leads?
>    
>   I'm going to have him disconnect the bigger motors to see if it affects the smaller motors so at least I can identify where the problem is originating from.
>
>        
> ---------------------------------
> Never miss a thing.   Make Yahoo your homepage.

2007\12\18@125923 by alan smith

picon face
Thats how its wired.  One is used to swap the polarity, the other is to turn on the power to the motor,  So, set direction, then turn on the other to activate the motor.  Never can you have them shorting.
 
 Thanks on the diode....duh....should have been obvious but I was looking at snubbers, etc to add.  This is 'consult by phone' with them, so they are going to disconnect the motors and see if it is that thats getting back into the system (I suspect it is).

Dr Skip <drskipspamKILLspamgmail.com> wrote:
 It sounds like you have 2 relays with the wiring on the terminals reversed so
that if one is on, it's +- and if the other, -+, correct?

If so, that's not good. A delay, stuck terminal, etc can cause both to be
active at the same time, shorting it out. It would be better to use a DPDT
relay for switching, and another with terminals in series with that one for
power on/off. You still get disconnect but with no chance of shorting.

Wire the DPDT so common is to motor(s) and the terminal sets are reversed.


alan smith wrote:
> Have a board that is driving a couple of relays (with diode suppression across the coil) where the relays are used to engage a 12V DC motor, and the other is used to reverse it (swap the leads). There are 3 smaller motors also in the same circuit..ie...driven off the same board so anything happening on the 12V rail tends to be seen by everything...it essentially turns on these motors when the others are activated.
>
> So, first thought is to put some sort of suppression on the larger motors being driven from the relays. Since we are reversing the direction, a diode isnt going to work....what about a varistor? Or...what, across the motor leads?
>
> I'm going to have him disconnect the bigger motors to see if it affects the smaller motors so at least I can identify where the problem is originating from.
>
>
> ---------------------------------
> Never miss a thing. Make Yahoo your homepage.

2007\12\18@131531 by Apptech

face
flavicon
face
Other advice good. But, you don't actually spell out what
"the problem " is.
This may help.


       Russell



> Have a board that is driving a couple of relays (with
> diode suppression across the coil) ....


2007\12\18@132321 by Apptech

face
flavicon
face
>  Thanks on the diode....duh....should have been obvious
> but I was looking at snubbers, etc to add.  This is
> 'consult by phone' with them, so they are going to
> disconnect the motors and see if it is that thats getting
> back into the system (I suspect it is).


Still don't know what's actually wrong.
Is there electronics that crashes?
Did it work before?
What changed?

If electronics:

- Do diode fixes

- Tap 12V feed for electronics closest to battery / supply
(or AT supply and have own power leads (so no "common mode
drop" in leads is shared with electronics and motor.

- Keep motor current loops areas as small as possible
(radiation).

- Run electronics off own supply temporarily to check for
radiated signal.



       Russell


2007\12\18@150750 by Jinx

face picon face
> Since we are reversing the direction, a diode isnt going to work....

You could try head-to-head Zeners at the motor. ie diode
cathodes are together and the two anodes to the motor terminals.
Also ceramic caps, 0u1 or 0u01 - one across the terminals, one
each from a terminal to the case. Possibly even ferrites or chokes
on the leads and/or screened cable

2007\12\18@161724 by M. Adam Davis

face picon face
Also note that if you apply the diode fix before the relay, you'll
still end up with severe arcing due to back EMF as the contacts
switch.  Changing the arrangement and timing the closures of the
relays appropriately would be appropriate, as well as looking at
snubbers.

For instance:

Power --> Power relay --> Diode --> Reverse relay --> motor

Consider switching the power relay off and then waiting for back EMF
to dissipate before changing the reverse relay.  Consider fitting high
voltage zeners (or movs or other transient surge suppressor) to cut
arcs short, and/or appropriately sized snubbers.

If all else fails, you can always switch to an H-Bridge, where you'll
have a bit more control and the mosfets already have diodes.

-Adam

On 12/18/07, alan smith <.....micro_eng2KILLspamspam.....yahoo.com> wrote:
> Have a board that is driving a couple of relays (with diode suppression across the coil) where the relays are used to engage a 12V DC motor, and the other is used to reverse it (swap the leads).  There are 3 smaller motors also in the same circuit..ie...driven off the same board so anything happening on the 12V rail tends to be seen by everything...it essentially turns on these motors when the others are activated.
>
>  So, first thought is to put some sort of suppression on the larger motors being driven from the relays.  Since we are reversing the direction, a diode isnt going to work....what about a varistor? Or...what, across the motor leads?
>
>  I'm going to have him disconnect the bigger motors to see if it affects the smaller motors so at least I can identify where the problem is originating from.
>
>
> ---------------------------------
> Never miss a thing.   Make Yahoo your homepage.
> -

2007\12\19@124235 by alan smith

picon face
Sorry...the problem is, the original design (before i got there).  They are using 2N3904's to switch ground for various devices....LEDs (fine) and small 12VDC motors (not so fine).  I redesigned the board (still doing that) to use relay/motor drivers (ON semi NUD series) but since I have to use the existing boards for this, I added the relays to drive the larger motors and the smaller motors still run off the transistors.  What happens is when the relays kick on, the smaller motors (that are on the same 12V plane of the board) engage...they get enough of a surge to kick start them (I guess?) and they just free run at that point till they get unplugged from the board.

Apptech <EraseMEapptechspam_OUTspamTakeThisOuTparadise.net.nz> wrote:  > Thanks on the diode....duh....should have been obvious
> but I was looking at snubbers, etc to add. This is
> 'consult by phone' with them, so they are going to
> disconnect the motors and see if it is that thats getting
> back into the system (I suspect it is).


Still don't know what's actually wrong.
Is there electronics that crashes?
Did it work before?
What changed?

If electronics:

- Do diode fixes

- Tap 12V feed for electronics closest to battery / supply
(or AT supply and have own power leads (so no "common mode
drop" in leads is shared with electronics and motor.

- Keep motor current loops areas as small as possible
(radiation).

- Run electronics off own supply temporarily to check for
radiated signal.



Russell


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