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DC Motor Problem? Answered

Hi , to whoever is reading this .I wired 2  3v dc toy motors in series and I used about 4.5v as the power source. As I powered it up one motor spun fast and the other spun slower , so I thought the one running slower was defective an so I bought another motor and wired it in series with the  that was spinning fast and thought their speeds would be equalled, however the new one I bought started running faster and the old I one I had that spun faster began spinning slower than the new one. I tried the new one with the old one that had been spinning slower and arrived at the same result.. Could this be because I wired the motors in series ? I  Really need help s o please help me and thank you in advance and for taking the time the read this.


The situation you are experiencing is caused by the rotating commutator and brushes in the motors. The variability between the different motors is due to manufacturing variances.

If you do an internet search for "commutator" you will see that it is described as a rotating switch. As the motor rotates the commutator switches the current off in one coil and on in another causing the motor to rotate, Depending on the motor there many be several coils being switched on and off. In toy motors there are usually only two or three coils and this makes the problem you are having worse. During this rapid switching there can be brief periods when there is no current flowing. Add to this the BEMF that commenter rickharris mentioned and it is a wonder you can get either motor to run when wired in series.

As for a circuit that will allow you to operate the two toy DC motors in series, I would suggest that you get two high value electrolytic capacitors and put one across the terminals of each of the motors. I would recommend capacitors of at least a couple of thousand MFDs. Also be sure to use capacitors rated at least twice the voltage feeding the circuit and observe proper polarity when hooking them up.The idea is that the capacitor will act like a small battery that will charge up when the current is flowing to the motor and discharge into the motor to keep it powered when the current is not flowing. This should help even out the motor speeds.

If this doesn't work then you will have no other option than to power them in parallel as another commenter suggested. Good luck.

When a permanent magnet motor spins the cols are moving through a magnetic field. This creates a current flow in the coil (in this case called Back Electro motive force BEMF) because this current opposes the driving current.

This current will also reduce the current through the whole circuit.

As your second motor now has a reduced current it turns slower. There will be small differences in the motors that allow one to pick up faster and so become the controller.

IS there any way i can use something like a divider to divide equal amounts of volts and current between them and how would i build one?

Your best bet is run them in parallel using the correct voltage - This may mean you have to regulate the voltage you have.

You don't say whet the application is so that isn't helping us devise a solution.

You could mechanically like the motors together that will make them run at the same speed.

You could put a speed controller (PWM) on one or both or them - this will require them running in parallel.


4 years ago

  • Toy DC motors running in series are running on the same current
  • And SHOULD be turning the same speed RPM..
  • Give them time to wear in the commutator bars and brushes and bearings.
  • Tap the slower motor with a small wooden mallet it may speed up.
  • SKILL saw production lines had end of line mallet adjustment of tolerances man that would strike hand drills for proper speed as they tightened the 4 screws in the aluminum housings of those days.
  • What can you expect of toy machines snapped together by children in a 3d world shop.  

That is of course if they are the same motor, made the same way, in a 3rd world shop.

A drop of oil on the bushings might help on the old motors.


Can I use something like a L293D motor controller?
Would that work?

yes, but you will get a runaway situation rapidly, exactly analagous to the case of two parallel bjts, without emitter degeneration.

Most likely it is the fact they are wired in series there are small differences in the motors even if they are the same motor.

To tell for sure wire them in parallel or with separate power source and see if they run at different speeds to begin with.