Monday, April 15, 2013

V4.15 - Electric motors for woodworking

This week, and for maybe a few more, we are going to talk about electric motors that are used on woodworking machinery. This came about because one of the forums that I frequent had a good discussion going on. A good friend of mine from up in New York is an electrical engineer and he had some fine comments. I’ll change some terminology so that it’s easier to understand. Many woodworking machines are equipped with motors that can be wired for a low and a high voltage. These are called ‘dual-voltage’ motors. Most of the time those voltages are 120volt and 240volt, so that is what we will be discussing. The initial question was: “Will my motor somehow be more efficient if wired for 220v? - but if not, then why would anyone bother to run it 220v, when there are always 120v sockets around?"

Dual-voltage single-phase motors use two main (run) windings that are connected in series for 240V operation, or parallel for 120V use. Connecting two equal impedance windings (impedance means- resistance. In other words, the windings have some resistance) in series results in half the supply voltage being felt across each of the two windings, and the current through one winding is exactly the same as the current through the other winding. Half of 240V is 120V, so each winding 'sees' 120V and some amount of current (depending on the % of rated load, but for consistency, we'll just stick with rated load and rated current at rated voltage).

When one reconnects them for 120V, the two windings are in parallel, so the supply voltage is split off to two windings together, and they both 'see' 120V. But since they're in parallel, and the impedance of each individual winding is still the same, the current in each winding is still the same, but there is twice as much current through the supply wiring. Half the supply voltage equals twice the supply current, as compared to the 240V series-connected arrangement.

So assuming the supply voltage is exactly that of the motor's nameplate voltage for both cases (115V or 230V for NEMA compliant motors), the current at full load will be exactly what the nameplate shows* (* There is a tolerance on this, of course, but that just muddies the water even more.) and for that matter, the efficiency, power factor, and torque curve will be exactly the same. The motor won't know the difference. In theory. Where the difference shows up in practice is that no circuit has zero impedance, so with any current flow, the voltage at the motor will not be the same as the voltage at the source. The higher the current, the greater the voltage difference between any two points on the circuit, or more importantly, at the motor leads.

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