RE: (ET) Traction motor current

[Larry Elie <lelie ford com>]

It doesn't shock me.  But I don't have the whole answer.

Gas engines are a bit wimpy.  A "20 HP" gas garden tractor is really 20 HP 
under ideal conditions on a test stand, at one specific RPM, at one 
temperature, 
one altitude or barometric pressure.  Real life is often 75-80% of that at 
full throttle; 
partly because full throttle is just that-- it doesn't at all imply max 
HP.  For example; 
full throttle standing still just puts the engine at max RPM-- for a 20 HP 
garden tractor, 
full throttle standing still is probably 1 HP, if that.

Slug a gas engine by asking for too much torque, and the RPM drops off.  
For a 
3600 RPM/20 HP engine at 1800 RPM and you do NOT get 10 HP; the torque 
curve isn't that flat.  You might get 5 HP.  Slow an electric motor and 
the 
torque does just fine.  That's why a GE pulls so good.  Only a steam 
tractor 
would beat it (steam puts out max torque at 0 RPM).  Diesels do a bit 
better than 
gas for low end torque, but not in the class of steam or electric.  BTW, 
for low torque, high 
RPM use, a gas engine is quite good.  That's why we have gas cars, 
motorcycles, etc.  

Then there is drivetrain loss.  Many garden tractors are hydrostatic, the 
output of the 
shafts is 75% of the input power, the rest lost to heat.  Belt drives are 
95%, and a 
belt often drives either the transmission or the hydraulic pump.  GE's 
also use a 
belt(s) so that part is even.

I did a test of circuit breakers for an unrelated FEA some 20 years ago 
for my 
previous employer.  I'm sure you can look it up, but most trip in 
milli-seconds 
at TWICE their rating, and after tens of seconds at 15-20% above their 
rating.  
At say, 5% over their rating, they trip after HOURS, depending on the 
ambient.

Now, let's talk about that tractor pull.  Say the E20 put out 65 amps for 
10 seconds at 
an honest 36 volts (My tractor is usually 38 V except right after charge 
when I get 39V+).
I get a tad over 3 HP.  But that isn't what happens in a tractor pull.  I 
could use a 1/2 HP 
electric winch and pull SEVERAL tons at a low speed.  All that is required 
is pull.  You 
get that from two things; traction (and GE's are heavy!)  and torque.  I 
have a 20 HP gas 
Oliver Cleat-track (a bulldozer) that weighs 10,000 lbs and (at 1 mph...) 
can pull my dad's 
Massey Harris 44 (a 50 HP gas tractor weighing 8,000 lbs with duels) 
backwards 
quite handily.  It can't work a field as well because the work depends on 
the HP.  
Traction and gearing are important.  Assume the competing 20 HP gas 
tractor is a hydrostatic.  The engine is running at a high RPM, but NOT 
the max because 
the designer matched the pump load well to the engine.  The engine may be 
putting out 
15 HP.  The Hydro drops it 11.25 at the axle.  If everything I said was 
true, it should win 
a pull with an E20.  Neither is pulling the load far, so something like 
inertia may make a 
difference.  I don't think HP is the best measure of what goes on.  All I 
can think is that 
the hydrostatic has waste-gates (spring-loaded poppets) on the pressure 
lines to prevent 
it from an overload from damaging the pump, and it really couldn't put 
more than a few HP 
to the wheels.  That's conjecture.  I really don't know why an E20 can 
beat a 20HP gas.

Any better ideas?

Larry Elie


----------
From:   Rhett T. George[SMTP:rtg ee duke edu]
Sent:   Tuesday, October 26, 1999 11:13 AM
To:     elec-trak cosmos5 phy tufts edu
Subject:        (ET) Traction motor current

 - GreETings -

Occasionally someone raises the question of maximum traction motor 
current.  Circuit breaker CB 1 is in the armature circuit.  If these
are marked as circuit breaker are usually marked, that is, the current
rating is stamped on the end of the reset button, then the E 20 has
a 50 A breaker and the E 15 a 40 A breaker.  Now 50 A times 36 V yields
1800 W or 2.4 HP.

Does this seem reasonable in terms of the performance of the E 20 or 
other tractors in the line-up?

                                        Rhett George