[ausev] can I do this without blowing myself up...?

[Erik Bigelow]

Hi Mike,

I thought your email address looked familiar. I don't know if you
remember but we met about 5 years ago when I was at UT working on a
Human Powered Vehicle team. We came to check out the Bike-e I think. 

>From what I remember seeing in the electronics catalogs the switches
that had DC ratings on them were around an order of magnitude less than
the AC, but I can look at that tomorrow. It's my understanding that it
is the high voltages that many EV contactors have to break that leads to
trouble, and not as much the current. I think around 36V there isn't as
much to worry about, but I'm not an expert!

I'd recommend trying it a couple of times and listen for arcing and look
at the contacts if you can see them without too much trouble. 

Another thing to consider is that at start up your 2x500W motors may be
pulling much more than 35A*36V=1260W. I think my 8" ADC is rated around
20 hp and can work at about 3 times that for reasonably short times. Can
you check if the 500W is a peak or continuous rating?

Erik

-----Original Message-----
From: ausev-bounces at austinev.org [mailto:ausev-bounces at austinev.org] On
Behalf Of Christopher Robison
Sent: Sunday, March 12, 2006 9:43 AM
To: ausev at austinev.org
Subject: Re: [ausev] can I do this without blowing myself up...?

If current can be split between the two contacts simultaneously (i.e.
they throw together) then it sounds like this switch is DPST, not SPDT.
(SPDT would have a common connection, and then two contacts that are
connected interchangeably depending on the position of the switch.)

Also, you have to derate for current significantly for a DC-rated
application, if you're only given an AC rating. DC is harder to switch
than AC given that AC crosses through zero 120 times a second. Maybe
others can weigh in here but it sounds like this switch won't
necessarily be safe.

  --chris


On Sun, 2006-03-12 at 07:49 -0600, MLAB wrote:
> I am making a key switch for electric load-hauler, using a relay
through 
> which all the battery power will pass on its way to the controller. 
> This will be a 36V system running 2x 500 watt motors, so I expect the 
> peak amp draw to be about 35 amps.
> 
> I have a big relay here (an Omron G7L-2A-TJ-CB), which is a
single-pole 
> double throw model, so it has two sets of contact, each rated 25 amps 
> each.  (Granted, it says 25A 277VAC, but I figure it can handle 25A 
> 36VDC, or am I just cruising for bruising?)
> 
> Anyway, would it be safe to split the battery wire into two (full
size) 
> wires and run the current through both sets of contacts?  These would
be 
> simultaneously switched by the key switch.  Presumably, each contact 
> would carry max 18 amps, or half of battery current.
> 
> Is there something I am missing?  Or would this be a safe arrangement?
> 
> 

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