Reverse brushless controller

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Reverse brushless controller

Post by Andrew_Hibberd »

I have been throwing an idea arround on paper for an invertable spinner. However the disc spins down when it is upside down. So I have been looking into reversing the direction of the brushless controller, so this comes under an itelligent system. The lightest one i have found with this function avaliable on the market is the pheonix 25, which is far to big. Has anyone else seen anything smaller?

I have been looking into swapping two motor wires to reverse the direction after i have stopped the motor and starting it up again. I have been looking at using a mosfet H bridge to reverse the two motor wires. However i am concerned what happens when you put a reverse voltage onto a mosfet due to the diode between source and drain. I have thought of two options which might work. Either a H bridge of n channel mosfets with no integrated diodes. Or using a rectifier arround the mosfet so the voltage doesn't reverse during the pwm control. The last concern is the internal resistance of these fets reducing the power of the motor. Relays would be easy to use but would be far to heavy to use.

Has anyone got any ideas on swapping two of the wires to the brushless motor?

Thanks in advance

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Post by BeligerAnt »

Swpping two phases over should reverse the motor, but I don't think you can do it with an H-bridge (at least not easily).

I think the problem comes at the point where one of the phases that you want to swap has to be tri-state or (effectively) open circuit. As you rightly surmise, you cannot turn a MOSFET "off" in the reverse-biassed condition. The parasitic diode will ensure that it always conducts from source to drain regardless of the gate-source voltage (assuming an N-channel MOSFET).

An H-bridge usually expects its inputs to be "high" or "low" but in this scheme one or other input will often go to the third state, high-Z or open circuit. I'm not sure an H-bridge can easily handle this condition.

To get an "off" condition with MOSFETs you need to use an N-channel and a P-channel in series. It's what they do in Li-ion protection circuits. You do need to be careful of the power dissipation in the diodes though, as one or other is usually conducting.

The internal resistance of the FET needn't be a problem. It depends on what size FETs you can afford. On resistance of a few mili-Ohms is not unusual these days.

A better solution would be to re-arrange the drive signals to the existing power stages, but it does involve some serious digging around in the innards of the controller :o
Gary, Team BeligerAnt

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