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Use with Series Wound Motors

Our controllers are principally designed for Permanent Magnet motors, but they can be made to work with series wound motor with some modifications to the wiring.

To control speed and direction of a Permanent Magnet motor you need to control the armature only, by using two wires and by reversing the polarity of current flow between these two wires to change motor direction. Although this will control the speed of a series wound motor, you cannot reverse it by simply using two wires. You must reverse the direction of the armature current without reversing the field current.

There are two ways of connecting a series-wound motor to do this.

  1. Battery line connection
  2. Full bridge field rectifier

1. Battery line connection

We’ve used this method on a number of series wound projects now and it works well. Although the drawing is for the 4QD-300 this scheme will work with any 4QD controller apart from the Porters.

To properly reverse a series wound motor, the field winding should be connected in series with the battery positive lead, and the armature connected as normally used with a PM motor.

series wound

There is one caution; the field inductance can cause the main decoupling capacitors to work a lot harder. Depending on motor type and load these can overheat so keep a close eye on them during initial tests.

Power diode

To reduce this problem, a power diode (shown on the diagram) must be fitted : this is across the field winding and it allows field current to circulated independently of the armature current.

This diode is quite difficult to specify accurately. At full load it is doing nothing, and also at no load it is doing nothing, but at half load they will carry the full motor current so they may get hot and will need some heatsinking.

Suitable Shottky power diodes are:

120NQ035 35v 120A
240NQ045 45v 240A
203CNQ080 80v 200A
QA250FA20 200v 250A

A high voltage device is not required : the field is unlikely ever to have more voltage across it than half the battery voltage, but this is dependent on the relative resistance of field to armature.

We stock the 250A diode – our part number is 39-06.

2. Full bridge field rectifier

A bridge rectifier (of suitable rating) is used to ensure field current always flows in the same direction and only the armature current reverses.


Whilst this method is the simplest, note that the full motor current flows through the rectifier. For the high currents that the 4QD series can handle this does mean using a substantial power rectifier with appropriate cooling.