I have two motors in series that are connected to a 36v speed controller. They are identical motors, 12vdc, 30000RPM.

My problem is when applying power, 1 motor slowly drops in voltage until it reaches zero. At the same time, the other motor slowly speeds up.

If I apply full power, the motor that went to zero will then spin but no where near the same speed of the other motor. My DMM confirms the smaller voltage at full power on the slower motor.

I've also tried another motor to replace the slow one but still same result.

What could be happening here?

Could it be that the faster motor is actually different in some way?


I've now wired the motors in parallel and they work perfectly but now I have another thing I'm confused about...

In parallel, the motors should now see the full voltage of the battery which is around 31.5v (8s lipo at 50% charge) but both see half that?? what is going on now?


Attached is a crude drawing of the current setup. enter image description here For clarification, both motors now work at the same speed and the shifter works perfectly in so far as i can change the polarity with a flip of the switch. The problem now is both motors only see half the voltage each from the battery.

I've confirmed that when disconnecting 1 motor, the other motor reaches the correct voltage....as soon as I reconnect the other motor, both motors go back to a voltage of 15.5. could this be a current limiting issue?

The controller can handle around 25a and the battery, 90a. Both relays are also rated at 40a @ 12vdc. At 31.5v it should handle around 15a. Both motors were pulling around 4a @ 15.5v.


sorry for all the edits, I'm learning more crap all the time!

I've found another symptom/issue...Not only does the voltage half for each motor but it slowly drops!? Now i'm more confused than ever!. i've uploaded 2 videos showing this issue. 1 video shows 2 motors running together and the other, just 1 motor running. DMM is connected to the same motor on both videos.

https://www.youtube.com/watch?v=4M-1r9IBKuU (parallel motors) https://www.youtube.com/watch?v=uO_2VKqxTaA (single motor)

  • 4
    \$\begingroup\$ That's what motors in series do. Don't connect motors in series. \$\endgroup\$
    – user16324
    Apr 26, 2020 at 10:55
  • \$\begingroup\$ Different mechanical loads. \$\endgroup\$
    – Andy aka
    Apr 26, 2020 at 11:05
  • \$\begingroup\$ Motor A + Motor B, then Motor A + Motor C. How about Motor B + Motor C? Nevermind. It will probably be the same = Do not connect motors in series. \$\endgroup\$ Apr 26, 2020 at 16:52
  • \$\begingroup\$ "What is going on now?" Show a photo. You've done something wrong. \$\endgroup\$
    – Transistor
    Apr 27, 2020 at 9:39
  • \$\begingroup\$ ha, clearly lol. I think i may have an idea why. I have a shifter to reverse polarity via 2 relays if i need to go backwards. I'll make a little diagram of the setup and upload it asap \$\endgroup\$
    – Sal
    Apr 27, 2020 at 9:40

2 Answers 2


Could it be that the faster motor is actually different in some way?

Yes. Due to manufacturing tolerances and process variations, no two motors are identical. If one motor draws slightly more current for any reason (higher brush or bearing friction, greater load etc.) the other motor will speed up and produce more voltage, causing the 'greedier' motor to slow down.

It can work if the motors are well matched and the loads increase greatly with rpm (eg. propellers), or they share a common load (eg. coupled to the same gearbox, individual wheels on a road vehicle).

  • \$\begingroup\$ Thanks for the info. I'm gonna try and remove the "faster" motor and replace that with the spare one to see if it's better matched. Failing that, is there a better way to connect the motors? \$\endgroup\$
    – Sal
    Apr 26, 2020 at 12:09
  • 1
    \$\begingroup\$ @Sal Swapping out motors is silly when the problem is with the flawed principle of what you are trying to do. Connect the motors in parallel and (if the speed controller cannot efficiently operate at a low PWM) reduce the supply voltage. Or better yet, give each motor its own controller. \$\endgroup\$ Apr 26, 2020 at 17:33
  • \$\begingroup\$ I didn't mention this, but the motors are inside gearboxes and I've just looked inside both gearboxes and they slightly differ. They're meant to be identical! I'll try parallel first and check the results but I'll then have to restrict the voltage as I'm already slightly over supplying them as it is and dont want to change battery or controller if I can help it. \$\endgroup\$
    – Sal
    Apr 26, 2020 at 20:00
  • \$\begingroup\$ What are the motors driving? \$\endgroup\$ Apr 26, 2020 at 20:31
  • \$\begingroup\$ an electric car, 1 gearbox per rear wheel. the test was with the rear wheels off the ground and made sure both gearboxes were able to spin with the same amount of hand force just to check nothing was impeding either gearbox \$\endgroup\$
    – Sal
    Apr 26, 2020 at 20:38

it seems I may have the answer...The speed controller is rated at 36v with a low voltage cutoff of 31.5v. These controller aren't made with lipo batteries in mind! I charged the batteries last night. Fully charged they're at 33.6v. Now both motors run at around 25v which is pretty close considering voltage drop under load.

As the batteries were reaching below that threshold, although the controller should have simply switched off, it didn't (it's a cheap one) but clearly limited the voltage going out! In conclusion, I need a Li-ion battery which I've actually ordered 2 weeks ago anyway.

Any thoughts are more than welcome and thanks for all the feedback and help!


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