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RE: (ET) AC motor theory, dynamic braking, and regeneration
- Subject: RE: (ET) AC motor theory, dynamic braking, and regeneration
- From: Larry Elie <lelie ford com>
- Date: Mon, 11 Jan 1999 12:40:06 -0500
- Sender: owner-elec-trak cosmos5 phy tufts edu
Steven Naugler wrote (in part);...
; Here is why I don't think that it is big enough. 1.5 kW x 0.85 %
efficiency x 1.341 hp/kW = 1.71 hp. Multiply by ;3 to get gasoline
horsepower and you get 5.13 hp. When 22 inch push mowers have 3.5 hp,
you'd at best get ;enough power for a 32 inch deck. And that's assuming a
100% duty cycle at 100% power for that inverter. Also, 5-;7.5 hp 120 VAC
single phase AC motors are somewhat hard to find.
; The GE mower deck motors look small because they get their power at
high rpm. Consider that if a 22 in deck ;with a gas engine turns 3600 rpm
to get proper blade tip speed, a GE deck motor with the 14 inch blades
would have ;to turn 5660 rpm to get the same tip speed. I really ought to
measure the blade speed with a strobe some time.
; Dan, I too think that your best bet is either a 24 VDC motor or a
bigger inverter. If there were such things, a 24 ;VDC to 240 VAC 3 phase
would give you the best chance of finding a properly sized motor.
; Hope this all helps someone. Dan, I'm not sure it helps you a lot.
; Steve Naugler
I agree, but more so. The GE deck was originally designed with 4 (yes, 4)
small motors instead of 3. The outside
portion of the blade does MOST of the cutting (it is going the fastest
folks). Under load, an electric motor puts
out more than it's rated torque, but it isn't going to cut much. Case in
point; I have spent much time on a
Bollens 20 hp, 56" cut tractor, a Sears 48" cut, and the GE 42". Now of
course, gas engines are mis-rated
(as are some electric motors), and the Bollens probably cannot get even 10
hp to it's blades. In heavy grass
(8" or higher), the gas tractors slug... they don't have a flat torque
curve, but they keep cutting... if the motor
speed drops more than 50% or so under these loads the engine would stall.
The GE blades slow so much
that they aren't cutting at all. In all 3 cases, if you lower the ground
speed, the thing works better.
The problem is that the blades just can't turn very fast under a large
load. BTW, there is even a difference
between side and rear discharge decks under these loads, because slow
turning blades don't push the grass
out of the chute well either. In either case, you can hear the blades
practically stop-- remember, they are
carrying more and more current as they are trying to turn, they just
aren't doing any effective work and running
the batteries down. I suspect this is the biggest reason that the GE
didn't do great in the market. Although
I don't own one, I suspect the single stage snow thrower may be turning
slow in wet snow as well... but the
motor is putting out enough torque to fool the user into thinking that it
isn't over loaded.