[EVDL List]

[quote]EVDL Administrator wrote:
> I've converted home space heaters for EVs by replacing the AC fan motor with
> a DC motor and adding a substantial contactor for the heating element.
>
Don't forget that you can't use the stock power switch, thermostat, or
safety overtemperature fuse. They will fail on high voltage DC!
> Another motor solution would be to use a small, low cost inverter to power
> the AC motor from your 12v house battery or source.
That's a clever idea. It may cost more than replacing the 120vac fan
motor with a DC fan motor, but it's also a lot easier.

--
Ring the bells that still can ring
Forget the perfect offering
There is a crack in everything
That's how the light gets in -- Leonard Cohen
--
Lee A. Hart, 814 8th Ave N, Sartell MN 56377, leeahart_at_earthlink.net

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[quote]On 19 Aug 2012 at 14:22, Lee Hart wrote:

> Don't forget that you can't use the stock power switch, thermostat, or
> safety overtemperature fuse. They will fail on high voltage DC!

Thanks. I should have mentioned that. The thermostats (room temperature
and thermal limit) have to be rewired so they control the contactor. If you
leave them in series with the element, they'll weld themselves the first
time they try to open.

This is relatively easy with the room temperature thermostat. It's a little
more challenging with the limit thermo, which is usually crimped right onto
the element itself.

You'll also need a DC rated replacement for the thermal fuse (backup to the
limit thermo). I confess that when I did it, I sort of let this slide, but
you shouldn't.

David Roden
EVDL Administrator
http://www.evdl.org/


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[EVDL List]

There exist two relatively simple solutions
that avoid having problems with breaking
high voltage DC:

1. the limit and overtemp (thermal fuse) are
wired into the DC that engages the contactor,
so when one opens then the contactor loses its
coil drive and opens.

2. the overtemp (thermal fuse) can be wired to a
"crowbar" which will short circuit the heater
so it blows the (DC rated!) HV fuse. This means
that the overtemp does not need to break DC, only
be able to withstand the DC as long as it is not
activated and carry the full current that blows
the DC fuse to protect the heater.


Cor van de Water
Chief Scientist
Proxim Wireless Corporation http://www.proxim.com
Email: xxx@xxx.xxx Private: http://www.cvandewater.com
Skype: cor_van_de_water XoIP: +31877841130
Tel: +1 408 383 7626 Tel: +91 (040)23117400 x203

-----Original Message-----
From: xxx@xxx.xxx.edu [mailto:xxx@xxx.xxx.edu] On
Behalf Of EVDL Administrator
Sent: Sunday, August 19, 2012 12:54 PM
To: Electric Vehicle Discussion List
Subject: Re: [EVDL] Heating Core Relay Tackled

[quote]On 19 Aug 2012 at 14:22, Lee Hart wrote:

> Don't forget that you can't use the stock power switch, thermostat, or

> safety overtemperature fuse. They will fail on high voltage DC!

Thanks. I should have mentioned that. The thermostats (room
temperature and thermal limit) have to be rewired so they control the
contactor. If you leave them in series with the element, they'll weld
themselves the first time they try to open.

This is relatively easy with the room temperature thermostat. It's a
little more challenging with the limit thermo, which is usually crimped
right onto the element itself.

You'll also need a DC rated replacement for the thermal fuse (backup to
the limit thermo). I confess that when I did it, I sort of let this
slide, but you shouldn't.

David Roden
EVDL Administrator
http://www.evdl.org/


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[EVDL List]

[quote]Cor van de Water wrote:
> There exist two relatively simple solutions
> that avoid having problems with breaking
> high voltage DC:
>
> 1. the limit and overtemp (thermal fuse) are
> wired into the DC that engages the contactor,
> so when one opens then the contactor loses its
> coil drive and opens.
>
> 2. the overtemp (thermal fuse) can be wired to a
> "crowbar" which will short circuit the heater
> so it blows the (DC rated!) HV fuse. This means
> that the overtemp does not need to break DC, only
> be able to withstand the DC as long as it is not
> activated and carry the full current that blows
> the DC fuse to protect the heater.
>
I like it. :-) #1 uses the contactor to turn off the heater. But the
contactor itself can fail, with its contacts welded closed. So #2 uses a
different method to turn off the heater.

The crowbar would consist of something like an SCR with the thermal fuse
connected from gate to cathode, and a resistor that supplies enough
current to trigger it "on" if the thermal fuse opens. The anode of the
SCR then has a resistor with a low enough resistance to force the HV
fuse to blow.

You need to pick the SCR and resistor to be able to carry the crowbar
current for long enough to blow the fuse. For example, if the heater
normally draws 10 amps, you could use a 20amp fuse (so it won't open by
mistake). Suppose the fuse specs a 4 second maximum time to blow at 200%
or 40 amps. Then your SCR and resistor need to be good for 40 amps for 5
seconds.

--
Ring the bells that still can ring
Forget the perfect offering
There is a crack in everything
That's how the light gets in -- Leonard Cohen
--
Lee A. Hart, 814 8th Ave N, Sartell MN 56377, leeahart_at_earthlink.net

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