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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. |