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motor load sharing

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strantor

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If 2 permanent magnet brushed DC motors of the same brand and model are to be coupled shaft-to-shaft to drive a load, what should be done to ensure that they contribute equally to drive the load?

What I am referring to is, in applications I have seen before, the 2 motors were simple electrically paralleled but due to different brush resistances, different lengths of cable run, etc. one motor took the bulk of the electrical power and burned up.

I am trying to prevent that.
1. should they be given the same amount of current or the same amount of voltage?
2. if motor A were to apply less force to the load than motor B, would motor A actually be a burden on motor B, or would it still be helping motor B, but to a lesser extent than it could if it were equal?
 
They should share the load automatically if they are the same. However, if you have different resistances due to the stated variables, the same current will ensure they both do the same work. I'm assuming they are coupled and will run at the same rpm.

Mike.
 
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Assuming you connect them in series. Then you at least ensure that the current through both is equal.

When loaded, torq will be a function of input current.

However, a motor that is twice the size usually have better performance/loss caracteristic than two equal motors.
 
Consider post #3. Two "identical" motors might not operate identically, despite having the same current. They must operate independently of each other. You have two choices: either measure the torque of each motor and use the measurement to feedback to a controller to insure niether motor is excessively loaded, or use a mechanical coupling device that combines each motor's input in an independent way. Such a coupling device would be a mechaincal differential, for example.
 
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Have never thaught of using a mechanical differential in the opposite way. That way, both motors should have the same torque all time.

Having that said, when speaking about mechanical differential I personally think that it might be a solution for large motors. For small motors in toy cars, I guess that such a mechanical coupling will eat a great part of the motors output effect because of poor effiency of small gears.

That is a ting OP haven't mentioned yet.

I stay my ground in this question. I cannot see why one motor should be damaged by the other. Well of course, the shaft shouldn't be rigid all the way.
 
If that question was meant for me, then yes, that's exactly what I meant ( by mechanical differential ) Lego mindstorm actually uses small differentials and they don't seem to eat up much of the available power. As for your last comment, I'm not the expert, but I can see that slight differences in otherwise "identical" motors would put strain on them if rigidly connected. I could be worng, but if it were me, I'd carefully consider that possibility, and probably use some form of isolation.

PS: On a second reading of your post, you didn't actually ask a question. I mis-read, sorry.
 
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First off let me say thank you for all the responses! I would have been back sooner, but I never recieved an email notification. I didn't think I had any responses.
They should share the load automatically if they are the same. However, if you have different resistances due to the stated variables, the same current will ensure they both do the same work. I'm assuming they are coupled and will run at the same rpm.

Mike.
Yes, they should run at the same RPM being as they will be rigidly couple together at the shafts.
You are overloading the motors if a slight imbalance damages one motor.
yes, but consider that you have 2 400A motors in parallel, and your controller is sets the current limit at 800A, a slight difference in brush resistance or brush timing could cause one of the 2 motors to draw a lot more than 50% of those 800A.
Assuming you connect them in series. Then you at least ensure that the current through both is equal.

When loaded, torq will be a function of input current.

However, a motor that is twice the size usually have better performance/loss caracteristic than two equal motors.
As far as I have seen, this is currently the best way to do it. people who are having success with dual motor setups are running in series, but it necessitates having double the voltage, and that means more batteries, more weight.
Consider post #3. Two "identical" motors might not operate identically, despite having the same current. They must operate independently of each other. You have two choices: either measure the torque of each motor and use the measurement to feedback to a controller to insure niether motor is excessively loaded, or use a mechanical coupling device that combines each motor's input in an independent way. Such a coupling device would be a mechaincal differential, for example.
that's an out of the box idea. I will need to improve my understanding of differentials and see if I can apply it.
Have never thaught of using a mechanical differential in the opposite way. That way, both motors should have the same torque all time.

Having that said, when speaking about mechanical differential I personally think that it might be a solution for large motors. For small motors in toy cars, I guess that such a mechanical coupling will eat a great part of the motors output effect because of poor effiency of small gears.

That is a ting OP haven't mentioned yet.

I stay my ground in this question. I cannot see why one motor should be damaged by the other. Well of course, the shaft shouldn't be rigid all the way.
This is not a toy, it is for a go cart. the motors I am considering are the **broken link removed**or **broken link removed**

I am brainstorming to build a controller with dual outputs for 2 motors for load sharing, which would solve a lot of problems in trying to parallel motors.
 
strantor said:
This is not a toy, it is for a go cart. the motors I am considering are the Etek motors or Mars motors

Then the answer is easy. One motor per driven wheel. No need to combine the motors' outputs. A controller can sense feedback from the motors, turning angle, etc, and send correct drive to each motor.
 
Then the answer is easy. One motor per driven wheel. No need to combine the motors' outputs. A controller can sense feedback from the motors, turning angle, etc, and send correct drive to each motor.
You know, that idea had crossed my mind a while back but I decided against it for some (probably stupid) reason. don't remember what that reason was, but I was less informed then than I am now, thanks to you guys. So maybe I could put an encoder on each motor for feedback, so they are both speed controlled. That would be a good solution
Thanks BrownOut!
 
So maybe I could put an encoder on each motor for feedback, so they are both speed controlled. That would be a good solution
Remember you need to deal with speed differentials when turning. Thats why it is easier to work with one big motor and a macanical differential.
Andy
 
Why is it easier? A controller can't do a simple differential calculation? I find that impossible to believe.
 
all this is making me think 3 wheeler (3rd wheel being a swivel) with electronic differential steering control and no steering wheel, just a joystick. but maybe I'm getting ahead of myself....
 
Why is it easier? A controller can't do a simple differential calculation? I find that impossible to believe.
Because there are so many variables. A simple differential calculation could not do it all.
Lets start with a 4 wheel go cart going strait but one tire has more air than the other and a larger circumference, one wheel will be working harder than the other.
Next there is steering, how more sensors would that need.
lastly you need it to fail safe so it will not go out of control if a wire brakes or the battery dies.
Lots and lots of stuff to consiter on a four wheel cart and two motors.
Andy
 
all this is making me think 3 wheeler (3rd wheel being a swivel) with electronic differential steering control and no steering wheel, just a joystick. but maybe I'm getting ahead of myself....
Power wheels use two motors and hard tires so one can slip. If you ran it in a circle all the time one motor would ware out.
Make a tank or a six wheel with tank control.
Andy
 
Because there are so many variables. A simple differential calculation could not do it all.
Lets start with a 4 wheel go cart going strait but one tire has more air than the other and a larger circumference, one wheel will be working harder than the other.
Next there is steering, how more sensors would that need.
lastly you need it to fail safe so it will not go out of control if a wire brakes or the battery dies.
Lots and lots of stuff to consiter on a four wheel cart and two motors.
Andy

Sure lots of sutff to consider. But there are many controllers made for just such a thing, so it can't be all that hard.
 
all this is making me think 3 wheeler (3rd wheel being a swivel) with electronic differential steering control and no steering wheel, just a joystick. but maybe I'm getting ahead of myself....

Actually not a bad idea. Steering can be accomplished by slowing down one wheel. Use a damper on the front wheel to prevent the "shopping cart" effect, where the swivel wheel oscillates back and forth.
 
Sure lots of sutff to consider. But there are many controllers made for just such a thing, so it can't be all that hard.

Got any links for one of these controllers? I've never been able to find one with e-diff capability (doesn't mean one doesn't exist, though). I've also never seen any open-source e-diff systems, either; for my project (which is based on a PowerWheels H2 platform), I was actually thinking about implementing an e-diff control (likely after I mounted some pneumatic tires/wheels on it - slip of the wheels wouldn't work well, then).
 
Got any links for one of these controllers?
I have never seen one made spicificly for differential drive ether, but I suppose one could be adapted.
 
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