Monday, May 6, 2013

V4.18 - Electric motors Part 4

We left off while talking about working a motor pretty hard...

But if you're going to push the motor for all it's worth, like with a contractor saw, where you work it well beyond its continuous power rating, you'll want to think about keeping the supply wiring short and heavy, or run it at the higher voltage. With twice the voltage and half the current, you get 1/4 the percentage of voltage drop through the same supply wiring, all other things being equal. With the torque curve sagging as the square of the voltage ratio, you get what amounts to 4 times greater torque curve stiffness compared to the lower voltage.

You can see that in the reduction in start-up time that folks often report, and in the apparent increase in power. It's not actually an increase in power; it's more like removing inhibitors to the motor operating along its design curve. It's a bit like running an engine at high altitude for a long time, where it's wimpy but you're used to it, then bringing it down to sea level and being amazed at how much power it has. It's not that it somehow gained power - it's just operating how it's designed to operate. If you have a continuous, light load on it, you wouldn't notice a difference, and it wouldn't matter, since it can drive the load without strain even at high altitudes. But push it as hard as it can go, and it won't go as hard at high altitudes, and may load up and start overheating.*** (*** My friend has to allow for the thin air with diesel hydraulic power units that he designs for use in Mexico City. Unless they're turbo-charged, where the high altitude torque rating at speed doesn't change much, which is one reason they use them. Same with some piston airplane engines.) Drop the altitude, and suddenly it seems to be a new engine. It's not, of course, but you're not choking it any more, much like supplying an induction motor with voltage within its design parameters by fattening up the conductors, or raising the voltage.

So the bottom line is: No, technically an induction motor will not be more efficient at the higher voltage, but the effects of voltage sag in the supply are minimized when operating at the higher of the two voltages. That may or may not be meaningful. Use short wiring of adequate size, and don't push it to nearly stall, and the voltage won't sag that much, nor will you experience any real gains. But it doesn't take much to drag down a big motor at low voltage, and it's a very slippery slope. Running at the higher voltage flattens that slope considerably.

If you stuck with this series, your knowledge of motors just got bigger.

Send your questions or comments to: and we’ll see what we can do to help you.

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