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

[Steve Hawkins]

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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