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I want to add a "reverse" function to a brushless motor ESC - in theory, it can be done by simply switching any two of the three output wires - I would like do to it via solid state relays, but I am not sure how to configure them - basically I want to be able to change which wires go where, so for example:

A --> B C --> D

switches into

A --> D C --> B

when I energize/de-energize the circuit

I am new to using solid state relays, however I would like to use them if possible. I believe that they do cross at 0, but at different time intervals, so I am not sure if I can rely on that to occur (I don't want to cause a short circuit by A --> C or vice versa) or how to ensure a "break before make" functionality such that both paths break before any new paths are made.

other possibly important specs: these will be handling 20A or so of current at ~14 volts

I am looking for guidance as to how I should wire this up or where to begin, even what specs I should look for in the relays.

P.S. - I would prefer an answer involving solid state relays.. I don't really want to use an H bridge that would need to support ~20A or so, nor drop ~1.4 volts, as that would waste a ton of power

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The circuit is simple enough:- just wire the inputs of each pair of SSRs in series, and power them through opposite sides of a toggle switch. A single resistor will set the LED current of either pair. The toggle switch provides automatic 'break-before-make' action.

DPDT SSR switch

However this may be a bad idea for several reasons:-

  1. You need AC relays. AC SSRs generally use SCRs or TRAICs, which have output attributes that make them unsuitable for your application - including high voltage drop, turning off at every current reversal, requiring several volts to turn on, and slow switching speed (not good at high rpm or when using PWM speed control).

  2. PhotoMOS SSRs have lower voltage drop, and stay on continuously (so switching speed isn't a factor), but are primarily DC devices. To handle AC requires wiring two FETs back-to-back in series, which doubles voltage drop and power loss.

  3. High current PhotoMOS relays are expensive!

  4. Even the best PhotoMOS relay will drop some voltage, causing an imbalance between the two switched phases and the third un-switched phase. A sensorless controller relies on symmetrical back-emf waveforms to get accurate commutation timing. Distorted back-emf may cause rough running, lower efficiency and loss of sync at high loading (this isn't a problem for a sensored motor and ESC, but then you need to also swap the sensor inputs to the ESC).

  5. You must ensure that the motor direction cannot be switched while it is running. Unlike a reversing controller which knows how to change direction, your ESC will not automatically slow down to a stop and restart when you switch the wires!

An H-Bridge won't do the job either. The best (and obvious) solution is to use a bidirectional brushless ESC.

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