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Author: Subject: AC voltage amplification with DC battery
elementcollector1
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AC voltage amplification with DC battery

I've been trying to solve this all day, with no success. Essentially, I need a component or circuit that takes an AC input and amplifies it to an AC output using the power of a DC battery. More specifically, I'm trying to increase the power output of a 3-phase ESC (electronic speed control) circuit for a 3-phase motor, so that it can run the motor at its maximum power rating (the ESC is rated for much less than this, but it is cheap).

The battery is 11.1 volts at a total of 70 amp-hours.

The ESC is rated for 6-12 volts DC input, and outputs three AC sine waves 120 degrees apart in phase, with peak voltage for each apparently being equal to the battery voltage (what little I found of this on other forums was very unclear on what the output even was).

The motor's desired input is 18.5 volts at 33.8 amps (each), and a total of eight motors makes this 18.5 volts at 270.4 amps. The amperage pulled by the motor really depends on what kind of load it's trying to spin, but I would want the circuit to be able to transfer 270.4 amps.

Therefore, in summary, I want to use 11.1 volts DC battery input and an 11.1 volts AC 'gate' to amplify to 18.5 volts AC output for each of the three outputs. I've considered single supply op-amp circuits, but I'm pretty sure they don't make them with those kind of power outputs. MOSFETs were a marginally better idea, as those can handle much higher power supplies, but as far as I know MOSFETs can't handle AC current. Does anyone have any ideas for how I might do this?

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macckone
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The ESC is probably incapable of handling the amperage you need but
it could be useful as a sine wave tracker assuming it outputs a true sine
wave. Converting DC to true sine wave output takes a bit of circuitry.
If you want to do it at that amperage it is going to be a challenge.
Over driving your ESC will likely destroy it.
If your motor can handle pulsed DC then it makes life a lot easier.
phlogiston
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You could have the ESC drive a set of FETs that have a higher current rating than the one integrated into the ESC.

However, this would be a lot more work and not very cost-effective compared to just buying an ESC rated for higher currents. For what you get, these things are amazingly cheap these days.

So, unless you want to pursue this for the learning experience, I'd recommended just spending a bit more on an ESC that can drive the motor you want.

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elementcollector1
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 Quote: Originally posted by phlogiston You could have the ESC drive a set of FETs that have a higher current rating than the one integrated into the ESC.

I thought of this initially, but the problem is that the ESC outputs AC sine waves, which MOSFETs can't handle (at least, not at the gate). As a FET transitions between fully on and fully off, it loses power as heat. This isn't a problem for DC, where the gate voltage is usually constant and over the threshold, but for AC it spends most of its time in between and thus loses a lot of power to heat.

 Quote: Originally posted by phlogiston However, this would be a lot more work and not very cost-effective compared to just buying an ESC rated for higher currents. For what you get, these things are amazingly cheap these days. So, unless you want to pursue this for the learning experience, I'd recommended just spending a bit more on an ESC that can drive the motor you want.

You'd think so, but most 300-amp ESCs are at least \$200, compared to this modification costing me just about nothing (as I have access to a university circuits lab).

I've since turned to modifying the ESC circuit itself, which appears to mostly be 3 half H-bridges run by 25V/30A N-channel MOSFETs. These appear to be what is limiting the current, so I thought it might be possible to 'stack' more MOSFETs on top of the ones soldered to the board, decreasing the on-state resistance and increasing the maximum current/voltage capabilities (as each added parallel MOSFET only has to handle a proportional fraction of voltage and current). Is this correct circuit reasoning, or am I about to set something on fire?

[Edited on 7/4/2017 by elementcollector1]

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Twospoons
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 Quote: Originally posted by elementcollector1 I've since turned to modifying the ESC circuit itself, which appears to mostly be 3 half H-bridges run by 25V/30A N-channel MOSFETs. These appear to be what is limiting the current, so I thought it might be possible to 'stack' more MOSFETs on top of the ones soldered to the board, decreasing the on-state resistance and increasing the maximum current/voltage capabilities (as each added parallel MOSFET only has to handle a proportional fraction of voltage and current). Is this correct circuit reasoning, or am I about to set something on fire? [Edited on 7/4/2017 by elementcollector1]

The limiting factor here will be the gate drive capability in the ESC. You will need to consider the total gate charge requirement of your paralleled MOSFETs - if the gate drive is inadequate the on/off transitions will be slower and the power loss in the MOSFETs will rise.
With paralleled MOSFETs there is also the possibility of parasitic oscillation - something which can be dealt with but you need to be aware of.

Better solution would be to keep the control side of the ESC and build a new bridge with big fat MOSFETs and appropriate gate drive.
You are also going to have to tinker with whatever current sensing is used by the control circuit.

All things considered if it was me I'd just buy a bigger ESC.

I'd also be a bit wary of trying to drive 8 motors from one controller - I suspect some very weird things could happen if the motor loads/speeds are not identical.

Oh, and you are not going to get 18V out with an 11V battery. Not without a lot of complexity.

On a final note : I would be extremely surprised if the ESC was delivering sine waves. Most brushless motor controllers for model use ( this is what this ESC is, right?) are hard switched 3-phase square wave outputs, usually PWM'd for current control.

[Edited on 4-7-2017 by Twospoons]

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Varmint
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You can't take the outputs and drive any external amplifier to achieve your goal. This is because there is feedback circuitry at each output that the controller uses to sense the motor dynamics in real time. This means those outputs must be connected directly to the motor to be controlled, any non-motor load will result in a controller error.
macckone
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It sounds like your ESC is not a true sine wave model.
In that case you are better off scraping it and starting from
scratch for the circuit. Although you could use the ESC
circuit as a basis for a new circuit.
Photonic
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Hey Element -- Take a look at "Half Bridge Drivers (or Controllers)" and "Discrete Half Bridge".

Realistically speaking Chinese ESCs are way cheaper than even the components in the US.

Also, do you need sine wave drive? If it's square wave drive you can probably do this really easily using 2 FETs and a driver circuit per channel. Just make sure to get low Rds-on FETs and use proper heatsinking. Faster driving = less heat dissipated (hence drivers).

I've got some decent lab equipment here that I'm happy to lend a hand if you need help testing, and I can give you a hand with PCB design basics if you go that route. Send a U2U if I can be of any help.

Also, Attached I've included the datasheet for a 3-Phase driver. They have some good reference designs for it, too. It was one of those projects I have three quarters done but left it on the backburner. My application was a BLDC servo controller though which is slightly different from your application.

Attachment: DRV8305 - Three Phase Gate Driver With Current Shunt Amplifiers and Voltage Regulator.pdf (1.2MB)