Series wound DC motors in series


Not really Arduino related yet, but any suggestions are welcome.

I’m planning on building an electric UTV of sorts. I have some 24V 133A (continuous duty) series wound DC motors and I would like to mechanically and electrically link two of these motors together, to get enough horsepower.

I’m thinking of connecting them in series with a DPDT contactor between the armatures and the fields for reverse. Attached, is a simple circuit. I still have to figure out the specific mosfets and how many in parallel, drivers etc.

I’m thinking that series connection is better than parallel, as I don’t risk one motor trying to outperform the other, and also wire gauge is more manageable.

  • Is the circuit correct (not taking into account, the simplified mosfet circuit)?
  • Do I need flyback diodes, or is the diodes in the mosfets sufficient?
  • Where would be optimal to place noise suppression capacitors?

Thank You

[edit] Just realized that the mosfet body diode can not be used in this configuration, so the question should be: “where is the optimal placement of flyback diodes in this circuit?”

In that diagram those motors are not series connected, they are shunt connected. In a series wound motor the field windings and armature receive the same current, and they limit each other's current. Shunt connecting a series wound motor could overload the field windings drastically - with such big motors that's death and destruction to every component pretty much from batteries to contactor.

You could connect all 4 windings in series I think, but it matters whether they are mechnically coupled or not.

Get things wrong with a series motor and they can overspeed and explode, in particular they must not be run unloaded. With two in series I would worry about unequal power/voltage between them - I suspect its OK with the shafts coupled since they will have the same field current the torques should also balance, but don't take my word for this, this is something for the experts to checkout.

I don't know about PWMing series wound motors, its not something I've seen described

Hi Mark, thank you for your reply. I intend to have the shaft coupled, and also to have an over-speed shutdown and over-current shutdown.

I'm not sure I understand, why the motors are shunt connected? The current goes from 48V source through armature 1 through armature 2 through reversing contactor through field 2 through field 1 through reversing contactor through mosfet to ground (or actually the other way round, if I remember my physics teacher correct). Or have I made a mistake in the diagram?

I was thinking, that this way, I would turn two series wound 24V motors into a single 48V motor, with the same current capability. I.e. instead of two motors with 0.1ohm armatures and 0.1ohm fields, I essentially have a single motor with 0.2 ohm armature and 0.2ohm field.

However I'm probably not knowledgeable enough to make those assumptions, as I don't understand all the concepts with motors, such as EMF, flux, induction, saturation etc.

I think PWMing series wound motors should be okay, as I've seen commercial controllers such as Kelly controllers, where the same controller is used for permanent magnet and series motors.

Thank You again, Peter

Yes, that schematic does show series wound motors. I've never tried connecting series wound motors in series but, assuming that they are identical and mechanically linked, they should act the same as a single motor.

As far as the protection diodes are concerned, I would put one across the pair of armature connections and another from motor 2 A2 to the FET drain.


Hi Russell, Thank you for your reply. I’ve updated the schematic with the diodes, where I think you suggested them to be. Is that correct? I’ve also added a capacitor, where I think it’s most likely to work best.

Thank you, Peter

Looks OK to me. I trust you will only be operating the reversing relay when the motors are stationary?


Yes, I'm going to monitor rpm, voltage, current, throttle, brakes, safety switches etc. with Arduino, and set conditions for enabling main contactor and reversing contactor. So only enable reversing contactor with zero rpm, zero power and zero throttle.

Thank you again, Peter

Well that's a very confusing diagram - its conventional for current to flow down so confusion is avoided.

Hi Mark. You're absolutely right - my apologies. Positive should have been top and ground bottom, and I can clearly see, that my diagram looks like a parallel connection now. Thanks for the tip, I would like present my ideas clearly, and tips like this help. Peter