[DeTomaso] Just whe I thought it was safe to go back in thewater...

[Christopher Kimball]

Dear Steve,

Thanks for the email.  I think you may be right--I have two relays (brand 
new) from Dakota Digital, one for each of the fans that are in the car now.  
By piggy-backing the third fan on one of the others, I probably did overload 
the circuit; but it was the 30 amp fuses that melted (along with one of the 
holders).

I'm thinking I can simply add a third relay, triggered by one of the fan 
wires.

Sincerely,

Chris


>From: "Steve Hawkins" <shawkins6 at houston.rr.com>
>To: <chrisvkimball at msn.com>
>CC: <detomaso at realbig.com>
>Subject: RE: [DeTomaso] Just whe I thought it was safe to go back in 
>thewater...
>Date: Sat, 7 Jul 2007 22:48:07 -0500
>
>Chris,
>
>Relays have specific current ratings.  You need one that will support the
>current of 2 fans.  Depending on what fans you have you could easily be
>using 40 amps.
>
>-----Original Message-----
>From: detomaso-bounces at realbig.com [mailto:detomaso-bounces at realbig.com] On
>Behalf Of SOBill at aol.com
>Sent: Saturday, July 07, 2007 7:07 PM
>To: chrisvkimball at msn.com
>Cc: detomaso at realbig.com
>Subject: Re: [DeTomaso] Just whe I thought it was safe to go back in
>thewater...
>
>
>Chris,
>
>A possible problem is that one or more of the fans is mechanically  jammed.
>The most likely suspect is the new one. Be sure you can move the fan  
>blades
>
>freely with all power off.
>
>If all the fans rotate freely:
>
>When DC motors are not rotating and power is applied, the fan looks  very
>much like a short circuit and large current will flow.
>
>The "in rush" current can easily be 5 times the normal running  current.
>
>As the fan accelerates up to normal running speed, the armature of  the 
>fan,
>
>while cutting thu the magnetic field of the permanent magnet, will  
>generate
>
>"back EMF" which reduces the current flowing into the fan. This
>acceleration
>continues until the fan reaches normal speed, the "back EMF"  reaches a
>maximum
>value, and the current into the fan combined with the applied  voltage is
>just enough to perform the mechanical work  required.
>
>Relays should be no short term part of the problem. You need to  have fuses
>which can support the inrush current of the three fans. Normal fuses  are
>rated
>to blow with a specified current over a specified period of time. This  is
>to
>handle in rush current or transient overloads.
>
>Hope this helps.
>
>Have fun,
>
>
>SOBill  Taylor
>sobill at aol.com
>
>
>
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