RE: mower deck motors, bearings, was: Re: (ET) Weeder / Cultivato r

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

Yes, I DO have one I use on other applications.  If this is too high 
for anyone here (real wiring is involved...) ignore it.  Else, this 
is an 'elegant' fix.  Heavy electric warning!

Typical magnet wire insulation today is good to 180 to 190 Deg. C.  When 
the ET was made, it was probably 160 or 170 Deg. C.  Exceed that temp for 
any length of time and the motor will 'burn-up' which really means the 
insulation between the wires has flowed, dropping the effective resistance 
of the coil and pushing current and temp even higher until there you get 
serious smoke.  If the temp never exceeds the insulation temp, the motor 
will never 'burn'.  How to be sure?  Read on.

Since we can MEASURE the room temperature of the coil accurately (a bridge 
is good for this, but a good multimeter that has nulling capability may 
work, 
and best is measure current and voltage accurately at low values and 
compute) 
we can actually SET the max temperature.  Copper resistance changes with 
temperature.  A RTD (resistance temperature device) works by this method.  
One can calculate what the elevated temperature resistance WILL BE for a 
coil 
of a given room temperature resistance.  The Formula is:

(Change in Resistance to a given temperature)/(Resistance at room 
temperature)=
Alpha * (Change in temperature to given temperature)

As an equation its:

Delta R(T)
___________ =  Alpha * Delta T

R(20 deg. C)

Alpha (resistivity) for copper is 4.07x10-3/(deg. C), and given 
temperature is 
the temperature we target.

For example, if a coil has a 2 ohm room temperature resistance, it's 
resistance 
at 170 Deg. C (a increase from room temperature of 150 Deg. C) will be:

Delta R (T) = Alpha * Delta T / R(room temperature)

Delta R (150 C) = {4.07E-3 /(deg. C)}* 150 C / 2 ohms =2.03 x 10-2.

Since the total R is R(RT) +Delta R, Total R is 2.00203 ohms.

Not a big change you say?  Actually, it's quite substantial.  If one now 
wires a current limit to prevent the thing from ever exceeding a current 
that 
will produce 2.00203 ohms, the thing will NEVER exceed 170 Deg. C, 
guaranteed.

How do you do that?  Well, what I do at work (not on the ET... yet...) is 
essentially build an constant voltage/constant current supply with 
crossover 
set to never exceed the current I would choose for the 2.00203 ohms, at 
ANY 
voltage, and then set the voltage as high as I choose.  Why set current 
and 
float voltage high you ask?  Well, many motors and solenoids are NOT wired 
for continuous duty; they NEED more current than I would supply if I just 
followed the above design rule.  By setting the voltage high, the thing 
can suck plenty of current until the resistance starts to increase 
substantially.  

I know this was a bit deep for many out there, but it is how I wire more 
complicated 
magnetics and keep them safe.  BTW, the above is faster than any fuse and 
will 
match a Zener diode if you do it right.

Oh yeah; you do have to worry about lead resistance too.


Larry Elie

 


-----Original Message-----
From: Don Cohen [mailto:bond007d netsync net]
Sent: Thursday, June 14, 2001 4:44 PM
To: MR23 (Christopher M. Meier, Mpls, MN); Pestka, Dennis J; 'Elec-Trak
Email Discussion List'
Subject: Re: mower deck motors, bearings, was: Re: (ET) Weeder /
Cultivator


For what it is worth, I just ordered some new cultivator parts from Bill 
and
had a chat with him about bearings and gear box lube. He says that there 
are
no bearings in the cultivator - they are bushings. He also says that they
should last a long time before becoming a problem. As to the gear box lube,
he says there is no specification for the type of lube. Bill says that many
try to use the same type of gear oil that is in the transmission, but the
gear box was not designed for that type of lube. When I asked him what type
of lube to use, he recommended Lubriplate - a white soft grease, similar to
many medical creams in consistency. I actually knew what it was because I
have some (use it in my electronics business). It is just thick enough to
stick to things, but soft enough to move around an relube parts as needed.
Incidentally, I had to order a new field and armature because mine burned 
up
after about 15 minutes use last week. I think a rock got stuck in the tines
(had happened several times) and since my switch was sticky, power was not
immediately shut off when I let go of the trigger. This must have caused
wires to over heat and start melting (which they did). Motor still actually
works, but starts heating up very quickly (must have a shorted winding?).
This time I am going to put in a circuit breaker because there is none in
the cultivator. It uses the 50A ET breaker - way too large. I guess I will
start with fuses to find the right size and then get a DC breaker. 10A
should be a good starting point. Actually, I guess I should get a dual
device that is both thermal and current activated - kind of like the 
breaker
on the mower motors. Anyone have any thoughts or ideas here?

Don Cohen

----- Original Message -----
From: MR23 (Christopher M. Meier, Mpls, MN) <mr23 mn rr com>
To: Pestka, Dennis J <Dennis Pestka mkg com>; 'Elec-Trak Email Discussion
List' <elec-trak cosmos phy tufts edu>
Sent: Thursday, June 14, 2001 1:31 AM
Subject: mower deck motors, bearings, was: Re: (ET) Weeder / Cultivator


> I spoke with Bill Gunn last week, during our conversation (about mower
> deck motors) he mentioned that the units are designed to have their
> bearings replaced every 10 years.  This 'mentality' may explain why
> the cultivator bearings may not be able to be externally lubed.
>
> Has anyone also had bent mower deck motor shafts?  Have you
> lived with them, or had the motor units rebuilt or replaced?  Since
> my last discussion with Bill about bearings, I've checked my motors
> and found they all 3 have bent shafts.  I need to get on the horn to
> him again, and find out how much he wants for rebuilt motors, and
> decide if I'm just going to re-bearing the one motor that has a stiff
> and noisy bearing, or if I'll trade one or all 3 in for rebuilt units...
>
> -Chris
>
> ----- Original Message -----
> From: "Pestka, Dennis J" <Dennis Pestka mkg com>
> To: "'Elec-Trak Email Discussion List'" <elec-trak cosmos phy tufts edu>
> Sent: Wednesday, June 13, 2001 12:52 PM
> Subject: (ET) Weeder / Cultivator
>
>
> > Looking for some advice on the Weeder / Cultivator attachment.
> > My runs but it seems fairly slow.
> > Talked with a electric motor shop and he was surprised it didn't have a
> > place to oil the top sleeve bearing.
> > He also suggested checking the gear box for dried up oil/grease.
> >
> > Does anyone have any experience fooling with these.
> > Is there a maintenance manual available ? maybe Bill Gunn.
> > Does anyone know what type of oil/gear lube that goes in the gear case.
> >
> > Any help would be appreciated.
> > Dennis
>