In a message dated 7/21/99 1:22:25 PM Eastern Daylight Time,
alfapete@pacbell.net writes:
> Certainly the first function is to protect the wiring. I don't dispute
> that. But devices such as a wiper motor which requires a fairly high
> current draw / high amp fuse has internal motor windings which are smaller
> than the wiring going to it. If the wiper is stalled in one position (by
> snow or something placed above the wipers) the amp draw goes up until
> someting gives. Wiring, with its length will disipate heat up before
> finally melting. The windings in the motor will cook and you ruin it -
> potentionaly faster than damaging the wire. If you put in a fuse which is
> rated higher than normal you certainly could/will ruin the motor before the
> wiring has any ill effects. IMHO.
Peter,
I understand what you're saying, but I must still respectfully disagree with
you. The wiper motors in our Triumphs only pull 1.2 amps under normal usage.
Even if the current is doubled under locked rotor conditions, it would still
only be 2.4 amps, and I think the motor windings can safely carry that for a
reasonably long period of time.
Larger motors that can be damaged by locked rotor current are usually
protected against damage from long term operation in this condition by
thermal breakers rather than overcurrent trip devices. That is, they are
tripped on high winding temperature rather than over current. There is a good
reason for this. Most motors will, from time to time, experience brief
moments of locked rotor current during the course of normal operation. For
example, a temporary jam on a conveyor belt. Hopefully, the jam will break up
by itself, and the operation can continue uninterupted. If not, the thermal
breakers will trip after a period of time before any damage is done to the
motor. If over current devices were to trip the motor on overcurrent, it
would be an instantaneous trip, and it would be a big nuisance having to
reset the breaker or install a new fuse unnecessarily for a temporary jammed
condition that would have cleared by itself if left alone.
Taking the wipers and snow example you gave, you can see where this would be
a nuisance also. If you turned on the wipers, hoping it was a soft snow and
not hard ice, the wiper motor would instantly blow the fuse, and you would be
faced not only with having to clear the snow to get the wipers freed up, you
would also have the task of getting the snow and ice cleared from the hood so
you could open it and replace the fuse.
Another thing to consider is how you would need to fuse it. If you put a two
amp fuse on it, it would have to be the only thing on that fuse. As presently
configured (Triumphs), the fuse feeding the wiper motor also has about 15
amps of other loads on it - heater fan, gauges, turn signals, etc. A two amp
fuse just wouldn't carry the normal load of the wipers and ANY thing else.
You may be right, the wiper motor may burn up if left on in a stuck condition
for an extended period of time, but I'm sure it would take much longer than
the time the average driver would leave the switch on if the wiper blades
were not moving. If a driver leaves the switch on that long, as far as I'm
concerned that is the same as putting aluminum foil in the fuse holder to
replace a blown fuse. Not something to design for. I know in the times I've
had wiper blades stuck from ice and snow (I grew up in Indiana, where winters
were a bit harsher than here in Tennessee), no damage was done to the motor
even after several tries, hoping the wipers would break free on their own and
I wouldn't have to get out and remove the ice.
There are some situations where the fuse protects not only the wiring, but
the device as well, but these cases are few and far between. I don't know of
any in a car of the era we're interested in. Power windows might be an
example in a modern car, but there the makers use torque switches (I think -
anyone know for sure?) to shut off the motors when the window travel is
obstructed. Having to replace a fuse everytime one of the kids hung out the
window as it was going up would be a pain (and you certainly don't want the
motors powerful enough to cut the kid in two!).
My purpose in posting what I did was not to correct you, but to get people
thinking about how to use fuses when installing aftermarket devices. If you
are protecting the device with a fuse, you will put the fuse as close to the
device as you can. If you are protecting the wiring, you will put the fuse as
close to the power source as you can, which is as far away from the device as
you can get. Installing a radio with the fuse located right at the radio is
almost as bad as not having a fuse at all. The power feed to the radio is
completely unprotected. It's far, far more likely that you will have a
problem with the wire insulation chaffing and exposing the bare wire to the
car body than it is that you will have a problem inside the radio. Especially
if the person doing the installation is not experienced in wiring techniques,
as most of us are not.
Regards,
Dan Masters,
Alcoa, TN
'71 TR6---------3000mile/year driver, fully restored
'71 TR6---------undergoing full restoration and Ford 5.0 V8 insertion - see:
http://members.aol.com/danmas/
'74 MGBGT---3000mile/year driver, original condition - slated for a V8 soon
'68 MGBGT---organ donor for the '74
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