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Challenge: Multiple Motors in Series

  1. May 17, 2009 #1
    I have an electrical engineering question for all you electrical engineers!

    If I have multiple motors in series, there will be a voltage drop due to the internal resistance of the motors. What will this look like for the motors? Will each motor experience a different voltage or will the voltage they receive be identical? In other words, will they all act identically (RPM, torque, etc.)?

    For each of the motors in series to act identically to a single motor, what would have to change about the battery or controller input to the motors (in terms of the current, voltage, etc.)?

    If anyone could give some insight on this matter I really appreciate it!

  2. jcsd
  3. May 17, 2009 #2
    You don't say what kind of motors you are talking about, and there are many different kinds. Each will respond somewhat differently, particularly depending on the loads on the motors.

    All that said, this does not sound like a great idea. Why do you want to put them in series rather than in parallel?
  4. May 17, 2009 #3
    Because you said "battery", you are discussing dc motors. So if they act identically to a single motor, they are running at the same RPM; i.e., on the same shaft or belted with the same gear ratio. If they are running at the same stator voltage, then identical rotor currents will provide identical torques. So my guess would be to use two shunt-wound motors, connect the two stators in series and the two rotors in series. Do not bridge across the middle. Fuse each separately. Control the pair with a single stator control and single rotor control (with twice the voltage rating). (warning: I are not an EE)
  5. May 17, 2009 #4
    I am not sure exactly what sort of motors I have. I know they are DC but I have no clue about brushes, etc. I have a spec sheet which is attached if that helps at all.

    Also, the only reason why I am considering this is I am wondering what will be the most energy conservative. I figured I would have less electrical consumption if I draw only 8.2 amps rather than multiple times that.

    Any thoughts on this would be appreciated! I obviously don't know too much about this stuff.

    Attached Files:

  6. May 17, 2009 #5


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    When a DC motor slows down due to increased load, the back EMF of the motor also drops and this causes the current in the motor to increase and give more torque.

    If there is another motor in series with the one getting extra load, this will get more voltage across it and tend to speed up increasing its back EMF.
    The first motor has less voltage across it so it will not get as much extra current as it would without the extra motor in series with it.
    So the speed regulation of both motors will be poor.

    With this interaction between motors and dubious torque performance, it seems like a bad idea to put them in series unless you knew they were just being used as fans or something like that with a constant loading.

    The motor current will only be at a maximum under heavy load, so it might be a lot less under normal conditions.

    If you wanted to save power, you could arrange to feed the power to the motors as Pulse Width Modulation. You could have the motors in parallel but have each draw less power.

    It even looks like the motors have speed control on them already. Is that what the "High, Medium and Low" wires are for?
  7. May 17, 2009 #6
    I believe the only difference between the High Medium and Low is they change the internal resistance so that the amperage is different.

    I am still a little confused by this though, why does putting multiple motors in series affect the load of each one if I keep the voltage up? Why couldn't I just increase the voltage to lets say 30V and then run all of these motors in series so that they each had a load of 24 volts (if that makes any sense). I guess what I didn't/don't know is, will running motors in series put a different load on each one? I thought that they would all have an equal load but it sounds like I am wrong.

    I guess it might help for me to tell what the project I am doing is, so here is a link to a quick blurb I have on another forum http://www.diyelectriccar.com/forums/showthread.php/reverse-trike-31611.html

    It sounds like the thing to do is to run these motors in parallel, so unless what I said needs even more clarification OR if you guys can think of some way I could better engineer these motors for this application (which I would REALLY appreciate) I guess my problems are solved.

    Thanks to all that responded and I look forward to any other thoughts you guys might have. Oh, and I am new to this forum so if you guys think I should re-post my project to the forum then I can definitely do that :smile:
  8. May 17, 2009 #7


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    Because of the slight differences between each of the motors, you're almost guaranteed not to have motors that have the same output rotation for a given input current. They also won't have the same torque, nor will they have the same impedance and resistance (they'll all be close, however).

    I once ran two 12V fans from a single 24V rail, but I considered this to be acceptable because they provided the airflow needed (and I didn't want to put another large 12V battery, nor drain one half of my 24V battery faster than the other).

    But the bigger question is whether or not you do, in fact, have 48V available for running your motors in series? If not, it's a moot point. You're better off running several motors in parallel. And if you're running, say, two motors in series off of your 24V supply, you may not get any motion at all (this becomes even more problematic the more motors you run in series).
  9. May 17, 2009 #8
    All of what you said is really helpful. I did not really realize what sort of things could come into play if I run them in series but now it makes a bit more sense, thanks a ton. Problem solved! Now the actual work begins..
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