Electric motorbike racing! - hackerspace_brisbane

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Message from discussion Electric motorbike racing!
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From: Triffid Hunter <triffid.hun...@gmail.com>
Date: Thu, 14 Jun 2012 11:23:55 +1000
Message-ID: <CACL3eKAJJaJYmmfbzL-TqbJmithE+2zhhJsM1qO=Kb+xtXz...@mail.gmail.com>
Subject: Re: Electric motorbike racing!
To: hackerspace_brisbane@googlegroups.com
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On Thu, Jun 14, 2012 at 10:02 AM, Jimmy Bowler <denomina...@gmail.com>wrote:

> Yeah you could use a bunch of mosfets if you want but remember that it is
> a really bad idea as you will get a mismatch between the transistors and if
> you start to blow a couple of them you may have a catastrophic failure. if
> you get a dead short what happens when you are going flat out around the
> corner and your back wheel locks up from a dynamic breaking effect of your
> speed controller fault.


this is why we run the numbers and make sure the mosfets can handle 4x the
stall torque at 80c ambient ;)

unlike BJTs (and probably IGBTs too), mosfets will share power fairly
happily.. their Rds(on) increases with temperature, so if one heats up, it
shifts some of its share of the load to its colder buddies.

by this same effect they can go into thermal runaway, but to avoid that we
simply make sure that if the silicon is at 175c, with worst case load
they'll cool down.. then they'll never approach 175c except during faults

always have a kill switch of course, and some huge fuses!

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<div class=3D"gmail_quote">On Thu, Jun 14, 2012 at 10:02 AM, Jimmy Bowler <=
span dir=3D"ltr">&lt;<a href=3D"mailto:denomina...@gmail.com" target=3D"_bl=
ank">denomina...@gmail.com</a>&gt;</span> wrote:<br><blockquote class=3D"gm=
ail_quote" style=3D"margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-le=
ft:1ex">

Yeah you could use a bunch of mosfets if you want but remember that it is a=
 really bad idea as you will get a mismatch between the transistors and if =
you start to blow a couple of them you may have a catastrophic failure. if =
you get a dead short what happens when you are going flat out around the co=
rner and your back wheel locks up from a dynamic breaking effect of your sp=
eed controller fault.=A0</blockquote>

<div><br></div><div>this is why we run the numbers and make sure the mosfet=
s can handle 4x the stall torque at 80c ambient ;)</div><div><br></div><div=
>unlike BJTs (and probably IGBTs too), mosfets will share power fairly happ=
ily.. their Rds(on) increases with temperature, so if one heats up, it shif=
ts some of its share of the load to its colder buddies.</div>

<div><br></div><div>by this same effect they can go into thermal runaway, b=
ut to avoid that we simply make sure that if the silicon is at 175c, with w=
orst case load they&#39;ll cool down.. then they&#39;ll never approach 175c=
 except during faults</div>

<div><br></div><div>always have a kill switch of course, and some huge fuse=
s!</div></div>

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