Re: (ET) one blade not cutting well

["Konstanty, Walter \(GE Indust, ConsInd\)" <Walter Konstanty ge com>]

This thread is getting so long I can't remember the key points anymore...
- Blades are turning all same way?
- Switch motors around may isolate the motor
- Switch blades around may isolate a heavy/bad blade
- Armatures really don't "get tired"....current flows to maintain terminal 
volts/speed 
    and if shorted, they blow up  (go to ground, melt, etc)...if the 
permanent magnets get weakened, speed increases
but torque decreases.  Magnets get weak from removing the rotor too many 
times or
external demagnetization.
- Measuring motor currents lets you know load between motors...measuring 
speed also
would be "neat".
- Rear or side discharge may affect the right-most motor more if all grass 
thrown that way.
- Friction or bad bearings show up quickly.

  Maybe we all need PLC upgrades to monitor motor temperatures, load, 
speed, amount of
grass cut/snow plowed...that would be real neat.

....Walt

-----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: Wednesday, July 26, 2006 3:35 PM
To: Klein Robert W NPRI
Cc: Elec-trak cosmos phy tufts edu
Subject: Re: (ET) one blade not cutting well



On Wed, 26 Jul 2006, Klein Robert W NPRI wrote:

> That's what your doing with an ammeter which has an internal shunt to 
> the meter.

Using a voltmeter to measure the voltage across a shunt isn't quite the 
same as what I'm saying (although of course most common ammeters do work 
that way). The main difference being that with the voltmeter and shunt 
approach I'd need to go out and buy some low resistance shunt to put in 
series with the motor, whereas I already have multimeters that can measure 
currents up to 10A. Me being cheap, I'd prefer an approach that doesn't 
involve buying anything new if possible. :)
        Whether installing a shunt or just using an ammeter, the same 
thing is going on electrically (since most ammeters just measre V across a 
shunt), but from the pocketbook perspective, they're different. :-)
        But, the ammeter approach wouldn't work if the current through the 
motors is typically >10A. It will just blow the fuse in the meters, which 
I have plenty of, but it would mean I couldn't use that approach, and 
would have to go buy a shunt.
        I think you mentioned previously that it sounds like a bad 
armature - I don't see how the armature could be bad really. The armature 
is just a winding of coils for current to flow through, for the field to 
make a torque on to spin the motor. Isn't the only way the armature could 
be bad when there is some break in the coil circuit, such that current 
couldn't flow through the armature at all? (which would mean the motor 
wouldn't spin)
        Or perhaps some of the wires in the coil could short if the 
coating is worn off?

Thanks,
Mike



>
> -----Original Message-----
> From: Michael S Briggs [mailto:msbriggs alberti unh edu]
> Sent: Wednesday, July 26, 2006 1:04 PM
> To: Klein Robert W NPRI
> Cc: 'steves'; Elec-trak cosmos phy tufts edu
> Subject: Re: (ET) one blade not cutting well
>
>
>
> Why not just put an ammeter in series? How much current do these mower
> deck motors normally pull? As long as it's <10A, I have some nice
> multimeters that can measure currents up that high. I can just pop the
> lid, disconnect the leads, and put the multimeter (in ammeter mode) in
> series with the motor.
>       Or is the current >10A normally?
>
> Thanks,
> Mike
>
>

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