Re: (ET) motor current

[David C Robie <mycroftxx1 juno com>]

A good thought. If your meter is accurate enough and you are well
connected. That's how the 'load meter' on the tractor works.  The little
$5.00 or so digitals are not real accurate on low ohms.   You could
instead, take a MA reading across the same wire with the motor running. 
but MAKE SURE it's the same wire, or the meter will go away.

Here's another way to test these motors against each other for efficiency
without a meter:

Remove the deck's power plug andupend the deck.  Spin a blade by hand
just give it a flip..  You'll notice that the other 2 blades spin. 
Tha'ts because the blade you spun acts like a generator, feeds the other
two motors.

So  - if you spin the center motor, the other two shoud both do fairly
equal movement.
   
      - if you spin the 'good' end one the other ones should do likewise.

      - but if you spin the 'bad' on and don't get the same effect,
something's wrong with it.
        It's efficiency as a generator will be a lot less.  you will get
less (if any) motion from  
        the others 

      -  If there's no discernable mechanical resistance difference,  you
likely got a cracked
          field magnet.  Possibly a cracked thn repaired one.

Try this first before metering.   Incidentally, this won't work on a deck
that's been laying around for a while, too much mechanical resistance. 
Only good for decks that have recently been in service.

Dave
Weymouth MA




On Thu, 27 Jul 2006 16:22:57 -0400 (EDT) Michael S Briggs
<msbriggs alberti unh edu> writes:
> 
> The leads themselves won't have 36V dropped across them - by the 
> lead 
> voltage, I'm intending to measure the voltage drop from the PTO 
> outlet to 
> where the wire lead connects to the motor, and use the voltage 
> across that 
> wire, and its resistance, to calculate the current through the 
> motor.
> 
> Mike
> 
> -- 
> 
> --------------------------------------------------------------
> Michael S. Briggs
> UNH Physics Department
> (603) 862-2828
> ---------------------------------------------------------------
> 
> On Thu, 27 Jul 2006, Elie, Larry (L.D.) wrote:
> 
> > The motor is rated 3/4 hp at 36V.  Even on a full charge of 39+V, 
> the
> > droop due to leads will be about 36V.  That's the max power under 
> load,
> > not the free spinning power rating.
> >
> > 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: Wednesday, July 26, 2006 6:02 PM
> > To: Neil Dennis
> > Cc: Elec-trak cosmos phy tufts edu
> > Subject: Re: (ET) motor current
> >
> >
> >
> > On Wed, 26 Jul 2006, Neil Dennis wrote:
> >
> >> First, if you have an accurate voltmeter / ohmmeter, measure the
> > voltage
> >> drop along the power lead, check the resistance - that should 
> give you
> > a
> >> bllpark reading for the motor current.
> >
> > D'oh! Good idea, I should have thought of that - essentially use 
> the
> > lead
> > for the motor as a shunt. *smack* (sound of me smacking myself in 
> the
> > forehead)
> >
> >> Do that before you try the 10
> >> amp meter.  I'd guess that the motor draws more than 10 amps but 
> maybe
> >> not.  Wonder, aren't they rated about 1 1/2 hp, that's about 1100
> > watts
> >> so go from there.
> >
> > Damn, that's 28 Amps, assuming 39 volts. They presumably don't 
> always
> > pull
> > that much power, only when initialy starting up, and as needed to
> > maintain
> > speed while cutting. Still, that initial startup would likely just 
> blow
> > the ammeter fuse. I'll use the lead as a shunt.
> >
> >> Have you taken the motor apart, maybe the field magnet is 
> cracked,
> > that
> >> could cause the problem.
> >
> > Nope, haven't taken it apart. I figured I'd first see if I can 
> find some
> >
> > other problem causing it before disassembling the motor.
> >     If that is the problem - do the large diameter and small
> > diameter
> > motors use the same field magnets? (I have a spare small motor, 
> but the
> > three I'm using are large ones) If not, perhaps some place has
> > replacement
> > magnets?
> >
> > Thanks
> > Mike
> >
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> >
> >
> >
> 
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