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4QD Motor Speed Controller
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Half Bridge

All of our controllers use state-of-the art MOSFET circuitry switching at around 20kHz - that is 20,000 times per second. In the half bridge two sets of MOSFETS are connected across the battery, as shown in the diagram below.

hbr

The drive MOSFET switches rapidly on and off in a series of pulses (the 20kHz bit). When it is on, current flows from the battery, through the motor, through the drive MOSFET and back to the battery (path 'A'). The Drive MOSFET switches off - but the motor has inductance (inductance is, to electricity the same as Mass is in mechanics) so the motor current cannot stop suddenly. Most other controllers use a power diode (where the diagram shows the 'flywheel' MOSFET) connected across the motor so the motor current can circulate when the drive MOSFET is off - a sort of 'flywheel' to smooth out what would otherwise be electrical surges caused by the sudden changes in electrical flow. However - power diodes get hot and waste a lot of heat. For this reason 4QD use a MOSFET which is a much better device since they don't get so hot. Moreover, depending on the method used to switch the MOSFET, the controller can give regenerative braking. To get regen braking, the top MOSFET is switched on when the bottom one is off. Our 1QD series doesn't give regen braking because they switch the MOSFET synchronously with the motor current - this is a system invented by 4QD and causes our 'standard' golf caddy (or golf trolley) controller (the Eagle) to safely give 40 amps when many other caddy controllers are limited to about 25!

If you are technical enough to understand circuit diagrams, there is a much more detailed description in our circuits archive.


MOSFETs Switching frequency

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Last modified: Mon, 11 Aug 2014 10:57:01 BST
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Page's Author: Richard Torrens