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(ET) Rewinding 120 VAC hedge trimmers for 36 VDC



Larry, John, and others,
    Here is the text of an earlier posting concerning rewinding a 120 VAC chain saw with universal motor for 36 VDC use.  The "tool" is different, but the concept remains the same, and in changing voltage (and current) there is more than a simple rewinding involved.
 
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.