Motors in Series and Parallel

Motor controllers have no way of knowing how many motors they are driving, it’s the total load that matters [see our motor current calculator ]. So you can use several motors, but you then have the choice of connecting the motors in series or in parallel.

ser+par/png For example, consider two 12v motors connected in series using a 24v battery with a controller that can supply up to 150 amps. The full output current of the controller will flow through both motors but the 24v maximum output will be shared so each motor will only see 12v (at up to 150 amps).

If the motors are parallel connected then each motor will see the full 24v (so speed will be doubled) but they will share the current. The motors will now have 24v at up to 75 amps each.

75 amps may be a reasonable peak current for many commercial motors, but 150 amps is going to cause 4 times as much heating in the motors (Power = I2 x R ).

Incidentally, this series-parallel connection gives the possibility of a ‘gear change’, where the series arrangement will give high torque but slow speed and the parallel arrangement will give low torque and high speeds.


It is also possible to connect lots of small motors, for instance some garden railways use a collection of motors (4, 6 and 8 being common), perhaps 12v car fan motors, with a 24v battery.

The drawing shows four motors connected as two series pairs and 6 motors connected as three series pairs, the pairs all being in parallel.

Note: one customer of ours has experienced problems when using 2 x 12V Graupner Speed 900BB motors in series with a 24v supply. Although they would run when under no load, this arrangement would not provide any significant torque. I suspect that this may be due to the brush timing of this particular motor.