The 4QD series controllers are designed for Permanent Magnet motors, so they control the armature only, by using two wires and by to 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 them 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-would motor to the 4QD controllers.

1. Full bridge field rectifier
2. Battery line connection

Full bridge field rectifier

Battery line connection

Some time ago a user of our connected his series wound motor as below and found that it worked fine. However - be warned that this is not a connection we have tried and it is given here without any guarantees. Furthermore. as it's not our idea and is untested, we regret we are unlikely to be able to answer any questions it may raise.

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

The diagram shows this.

We have illustrated it with the 300amp version, but the method is the same with the smaller controllers.

There are two cautions:

1. Capacitor heating
   In using the controller thus is that the field inductance causes the (two or four) main decoupling capacitors to work a lot harder. Depending on motor type and load these can overheat and, in the extreme situation, vent.
2. Reduced Field current
   The armature current is continuous, the field is chopped, averaged by the main capacitors. So the field will be reduced compared with normal operation and starting torque will be reduced.

Flywheel diode

To reduce both of these problems, a 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 as it is conducting pulses of field current when the armature is re-circulating. At full load it is doing nothing, and also at no load it is doing nothing.!

Suitable power diodes might be:

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

These are Shottky diodes which are low dissipation types, even so they may get hot and will need some heatsinking.

The currents quoted are continuous current ratings, transient currents handling abilities are enormous (26 thousand amps). A high voltage device is not required : the field is unlikely ever to have more voltage across it than half the battery voltage, but of course this is dependant on the relative resistance of field to armature.

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