Motors in Series or Parallel
Ordinarily a motor should be driven from a voltage source that matches its specification, but a number of our customers have asked about connecting two 12V motors in series in order to run them from a 24V battery. Our 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 need to consider whether to connect the motors in series or parallel.
Lets consider the two cases above, both using a 24V battery, and with 12V motors that are each rated at 240 Watts and which will therefore draw 20 Amps from a 12V battery.
In the first case with the motors in series, when the controller is giving full power the current from the controller will flow through both motors but the 24v output will be shared so each motor will only see 12V. In this case each motor will be operating at it’s specified rating and will be giving 240W output power.
However…….the brushes and commutator in an electric motor don’t always react in the way you might expect. Some motors have the brushes offset slightly from the neutral position, this is known as brush timing and is done to alter the performance characteristics of the motor. We haven’t done the maths on this but the effect is to restrict the ability of current to flow through the combination. More than one customer has experienced problems with using 12V Graupner Speed 900BB motors in series. Although the motors would run under no load, this arrangement would not provide any significant torque.
So onto the 2nd case, if the motors are in parallel then when the controller is giving full power each motor will see the full 24V, now as Power = V2/R and we haven’t changed R this will cause each motor to give 4 x the power i.e. 960W. Whilst this is an excellent way to get more performance it probably won’t do too much for the reliability.
But remember we have a controller in the circuit, so all you need to do is to restrict the maximum output from the controller so that it only gives 50% output [12V], and normal service is resumed.
For details on how the controller reduces the power see this page on how PWM works.