Thursday, December 18, 2014

V5.51 - Electric motors Part 3

We left off while talking about the voltage sagging on its way to the motor…

As long as the applied load doesn't exceed the torque associated for the rated hp at the rated speed, a continuous rated motor will work fine for continuous operation within the voltage range at the motor leads. The supply wiring can normally deliver the current without suffering excessive voltage sag at the motor leads, but if the run is very long, a bump up in size or two will take care of that. Where the problems start is with voltage that sags too far, causing the current to rise too high, like a refrigerator or air conditioner during a serious brownout (the compressor torque load is fixed), and when manually loading a motor, like with common woodworking machines.

Since you don't know how hard you're working the tool, folks tend to push them pretty hard, easily way past the continuous output rating of the motor. Induction motors can normally output anywhere from 150% to 300% of rated power, with three-phase motors generally able to handle larger overloads, but that's just a generalization. As the load goes beyond rated load, the current also goes beyond rated current, and with the rise in current, the voltage will droop. When the voltage droops, the torque curve becomes more depressed, so the rotor runs slower, causing greater current to flow, and with greater current flow comes greater voltage droop, depressing the torque curve further, slowing the rotor, . . . and so on until equilibrium is reached.

If you don't believe me, do some heavy ripping and get a feel for how it performs, then put your saw on a long extension cord and try again. Add another long extension cord, and so on. With enough cord, you'd have trouble ripping a Popsicle stick. Startups will also get longer as cord is added.** (** Startups are just extreme overloads to the motor - from zero rotor speed up to near rated speed results in 'starting current', usually 5-8 times rated current, which can be calculated from the letter in the "Code" or "kVA Code" box on the nameplate, the rated hp, and the voltage it's configured for.)  Remember that the torque curve sags as the square of the voltage ratio, so small reductions in voltage have a disproportionate effect on current and max torque. To prevent excessive current problems, you need to make sure the voltage drop over the supply wiring isn't excessive. For 'normal' loads, like fans and pumps and whatever, that's not a problem. Just size the conductors to keep the voltage within range. 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.

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

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