Re: (ET) 36V Chain Saw

["Steve & Sandy Naugler" <snaugler earthlink net>]

Dennis and John, and other GE chain saw enthusiasts,
    While rewinding a motor for a different voltage without commutators and
brushes is conceptually straightforward.  In this case of 120 VAC RMS to 36
VDC the number of windings would go down to 0.3 (36/120) of the original
value.  In order to handle the higher current, the cross sectional area of
each conductor would have to go up by a factor of 3.33 (120/36).
    Rewinding a motor with commutators and brushes is harder.  The number 
of
commutator segments and their spacing is selected in order to keep the
voltage between any adjacent commutator segments small, so this works in
your favor when rewinding a motor for lower voltage.  However, as the
voltage goes down, the current must go up proportionately, so the 
commutator
segments and the brushes must become larger to handle the higher currents.
And you still have to change the number of and sizing of conductors to
handle the changed voltage and current.
    I agree that in the absense of GE's 36 V chain saw, an inverter used
with a 120 VAC chain saw is the best option.  However, one must look very
carefully at inrush capacity of the inverter.  My MacCullough electric 120
VAC chain saw draws about 1.5 kw running, but would need an inverter with
about 5 kw of inrush capability.  Some electronic inverters are sold 
without
any significant inrush capability, especially the cheaper units.  A cheap
design might need a 5 kw rating to start a 1.5 kw saw.  More expensive
electronic inverters can handle an inrush of 2 to three times their
nameplate ratings for several seconds.  This type would allow a motor to
start properly.  If you go with an electronic inverter, either test it with
your chosen load (chain saw) or be conservative with your kw rating.
    A better option here is a rotary inverter.  GE offered a rotary
inverter, essentially a small motor generator, with a higher nameplate than
their electronic inverter.  What they traded was frequency stability for
inrush capability.  With one of these a 2 kw unit would suffice for a 1.5 
kw
chain saw, and its fun to listen to it change speed when you apply and
remove a load.
    Lastly, these inverters should be and usually are rated in kva, not kw.
With non resistive AC loads current and voltage are not in phase.  Here kw
(kilowatts) = kva (kilo volt amperes) x power factor.
    Power factor is defined as the cosine of the phase angle.  We could 
talk
for hours on power factor, but here's whats important.  Under full load a
small motor could have a power factor between 50 and 90 %, depending on
design and efficiency.  The only place where power factor is 100 % is a 
pure
resistive load, which a motor decidedly is not.  A motor is an inductive
load.
    What this means is that your inverter kva rating must always be larger
than the kw rating of your motor.  The way to size your inverter is to
multiply the motor's full load current rating by the motors voltage.  A 115
VAC motor with a 12.5 full load ampere rating has a kva of 1.44 (115 x 12.5
/ 1000)(divide by 1000 to convert va to kva), but would only draw 1.08 kw 
if
the power factor was 75% (1.44 x 0.75).  And don't forget this is all input
power.  Shaft power is always lower still due to the motors conversion
efficiency, which for small, cheap motors tops out at 85 %.
    John, I think you're on the right track so long as you are conservative
with your inverter selection.  My solution has been long extension cords 
and
a garage outlet.

Steve N.

----- Original Message -----
From: <JohnKelly oaktech com>
To: Pestka, Dennis J <Dennis Pestka mkg com>
Cc: <elec-trak cosmos5 phy tufts edu>
Sent: Friday, October 22, 1999 5:30 PM
Subject: Re: (ET) 36V Chain Saw


>
> Dennis,
>
> If your chainsaw has a universal type motor , that is if it can be run on
> 110V DC as well as AC,  a good motor repair shop should be able to re 
> wind
> the motor to run on 36 VDC but this would be an expensive proposition.  A
> much better idea would be to add one of the newer 36 Volt  DC to 110 Volt
> AC inverters to your tractor which will allow you to run all types of
> electric equipment.  A one to two kilowatt inverter should do the trick.
>
>                          John Kelly
>
>