I think you make a good point here Kevin.
However, I think most Mosfets would fail shorted first.
However, if one of the legs failed shorted, AC would cease to happen and the
leg would become a permanent magnet for an instant, and the motor would
likely stop or really slow down a lot, until the mosfet over heated and
burned out to an open. Those batteries have a lot of juice.
No matter what, on an pulse width system, if you stop or alter the pulse
width, which the mosfets are responsible for, the motor would not turn very
good.
Any interruption of the proper pulse width would not make the motor turn
faster.
Bob Noble
http://www.sonic.
------------
From: "Kevin Pemberton" <pembertonkevin@
Sent: Saturday, February 13, 2010 09:23
To: <electricboats@
Subject: Re: [Electric Boats] (A Little) More On Electrical Safety...
> Hey Eric,
>
> As I understand standard Transistors failing with thermal overload and
> lock closed. Mosfets fail open.
>
> AC motors require alternating current cycles to rotate. If the system
> were to lock open in the H bridge, the motor rotor would lock as if it
> were a brake. You are right in assuming the motor would not go full bore.
> You might note that brushless motors are in fact a form of AC motor.
>
> All said there is a hazard. A full open circuit may cause a fire
> without proper circuit protection. Fuses vs. breakers? Breakers leave
> the users hands out of harms way if installed correctly. Fuses installed
> in accordance to the NEC will have a main disconnect before the fuse in
> higher voltages, and a good practice for all circuits. Most Machinery
> has fuses at the machine. A disconnect in sight of the machine. And
> breaker protecting the circuit. Fuses can be purchased in slow blow, or
> standard. Motors because of the start load typically use slow blow so
> their rating can be a little lower to protect better.
>
> I hope this helps in your designs, don't cut corners to save a buck, but
> there is no need to over engineer.
>
> Kevin
>
>
>
> Eric wrote:
>>
>>
>> Is this potential problem of runaway controller limited to DC
>> controllers? Should this be a consideration for a system using the
>> open frame Mars motor? It seems like a DC to AC convertor won't fail
>> in a full on condition.
>>
>> I can understand the issue with a DC system which is typical in EV cars.
>>
>> Eric
>> Marina del Rey
>>
>
>
No comments:
Post a Comment