Re: (ET) motor current

["Elie, Larry \(L.D.\)" <lelie ford com>]

Than you have not zeroed them properly.  Shunts also create error as the
resistance of the shunt changes with temperature; i.e. as you use them.
In the case of the clip-on, the error is about 1/2 of 1%.  In the case
of a shunt, the error depends on how long you leave it connected.  Sure,
a large value shunt won't warm as much, but then the error is greater.
There is no free lunch. 

Larry Elie


-----Original Message-----
From: Michael S Briggs [mailto:msbriggs alberti unh edu] 
Sent: Friday, July 28, 2006 8:44 AM
To: Elie, Larry (L.D.)
Cc: steves; elec-trak cosmos phy tufts edu
Subject: RE: (ET) motor current


On Thu, 27 Jul 2006, Elie, Larry (L.D.) wrote:

> Don't you have a Hall clip-on meter?  They are fine for this
> application.

Nope. I've never found them to be very accurate, so I don't use them.
        I realized last night that the approach of using the leads as a 
shunt to measure voltage across (to calculate current) won't work, since

the resistance along the leads is essentially zero (tested to confirm) -

it's on the order of milliohms, and the leads from the multimeter
probably 
add more resistance than the 14 Ga motor lead wiring itself.
        I'll just use my multimeter in ammeter mode, and short across it

when the blades first start up, so the startup current doesn't fry the 
fuse.

Mike

>
> Larry Elie
>
>
> -----Original Message-----
> From: elec-trak-bounces cosmos phy tufts edu
> [mailto:elec-trak-bounces cosmos phy tufts edu] On Behalf Of Michael S
> Briggs
> Sent: Thursday, July 27, 2006 4:32 PM
> To: steves
> Cc: elec-trak cosmos phy tufts edu
> Subject: Re: (ET) motor current
>
>
>
> On Thu, 27 Jul 2006, steves wrote:
>
>> I thought the motors were more like 1 HP. I have measured the current
>> with a DVM (if I recall correctly) and it can get over 10A (I think).
>> Problem is if you blow the DVM fuse - the replacement fuses are
>> expensive. I'm pretty sure 'idle' current is < 10A so you could
> measure
>> each motor running but not cutting and see if the suspect motor pulls
>> excess current.
>
> I have gobs of fuses for DVMs here at work (students are constantly
> blowing them in our labs), but I think I'll still just try measuring
the
>
> voltage drop along the leads to the motors instead.
>
> Mike
>
>
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