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A permanent magnet motor is reversed simply by interchanging its two armature wires. This can of course be done with any controller by using a suitable, high current, double pole changeover switch. However reversing should never be done while the motor is rotating: not only will that cause a mechanical jolt – but large currents could flow which could easily damage controller and/or motor.

So reversing controllers are available: these will do the reversing safely, even if the switch is operated at full speed. A reversing controller is especially called for where the inexperienced may operate the machine!

Reversing Controllers

4QDs reversing controllers are listed (the links take you to the specifications):

  • DNO series
  • Pro-100 / 160 / 360
  • Pro-150 Programmable controller
  • Pro-120
  • 4QD series


This is the standard system used on 4QD controllers. Is the controller is operating (even at full speed) when the reversing switch is changed, then the controller automatically slows down at a rate determined by the decel ramp adjustment to zero speed. The controller can then reverse the motor (this may require the speed to be turned to zero) and accelerate to full speed, rate determined by the acceleration ramp.


In this system, reversing whilst the controller is operating has no effect. But if the reverse switch is operated and the speed is manually reduced to zero, then the controller will start up in the pre-selected direction. VTX Mk2 and Pro-120Mk 2 can be changed on board to give pre-select reversing if required.

Half speed

In some vehicles, it can be disconcerting to travel backwards as fast as forwards, so all reversing controllers have a half speed reverse feature, where the top speed, when reversing, is reduced. This feature can be disabled by removing a pluggable link on the circuit board (when full reverse speed is the same as full forward speed).

It is also possible to change one resistor to alter the ‘half speed’ factor, though this is seldom done in practise.

Reversing switch

All controllers made by 4QD are operated by a voltage input to a terminal: if the reversing input is low, they go forwards, if it is connected to a suitable voltage (usually the battery, but the actual threshold is usually around 5-7 volts), then reverse is selected.

The reverse input is high impedance so is suitable for operating from an electronic front end, e.g. a transistor driven from a microcontroller.


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