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HARDWARE QUESTION: Maximum AC voltage VESC can tolerate?

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danvel
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HARDWARE QUESTION: Maximum AC voltage VESC can tolerate?

Mi application is as follows: The VESC uses a 28V battery to drive a very low kv (80) motor to crank start a 110cc internal combustion engine in an RC plane.

After that, the VESC should be completely unactive (high impedance state, no driving, no regeneration).

The problem is, that at full throttle, the combustion engine can reach 7000rpm, which means 88v at electric motor terminals.

My question is: Can the hardware of VESC tolerate this voltage without being damaged?

If not: Can it be modified in any manner (mosfet change, etc) so that it can tolerate this voltage.

Thank you in advance.

arvidb
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No and no.

As soon as the motor voltage reaches over the battery voltage, the MOSFET body diodes will start to conduct - so your motor will act as a generator even if the VESC does nothing. This will try to limit the rpm to (28 V+2*0.7V)*88 rpm/V ≈ 2600 rpm until either the ICE stops or something fries.

The only solution I can see at a glance is to use a higher Kv motor - 270 rpm/V will let you reach 7500 rpm on 28 V. The VESC will have to handle three times the motor current for the same torque though (280/88). But the battery current and motor losses will stay the same.

danvel
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OK. Thank you very much for your fast and accurate answer.

The problem here is that cranking torque is high and irregular (it fluctuates every revolution with piston compression). A 270 kv motor will probably be unable to overcome the compression force.

Right now I am using a 100A conventional esc for starting and I disconnect the 3AC terminals from the ESC with mechanical relays right after start but this is bulky and I'd prefer an all-solid-state solution.

 

arvidb
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A 270 kv motor will probably be unable to overcome the compression force.

It will if you give it three times the current! And it will do so with the same losses as the lower Kv motor since it will have thicker and shorter windings. The question is, will the VESC handle it? What is the max current of the 80 rpm/V motor during ICE start?

Roger Wolff
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Well... he's using an 100A esc, so "on the order of 100A" sounds about right. This means that three times that is going to be above the VESC capabilities for sure. 

 

On the other hand, leaving half the "range" unused is also not a good strategy. So... When you have say 56V fo battery voltage, and then only a 1.5 times higher KV, maybe it can work?
Switch the motor from delta to star configuration? Wait! NO it shoul be the other way around. 

 

Barry E. Palmer
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