Canadian Plastics

Power to the people

Here's a question: What is "power"?

October 1, 2010   By Jim Anderton, Technical Editor



Here’s a question: What is “power”?

Conventional parameters for shop floor electric circuits use terminology like “voltage”, “current” (sometimes called “amperage”) and “load”. At its simplest, think of these as similar to the flow of water in a garden hose. The pressure available at the faucet is voltage, the rate of flow in litres per minute out the nozzle is current, and the restriction to flow from the nozzle or kinks in the hose is the load.

In our applications, of course, the load is really the conversion of electrical energy into useful work like melting and injecting resin. And in keeping with the garden hose analogy, it’s flow that really counts here, so current in amps is the parameter you’re really interested in for plant distribution. Fuses and breakers limit excess current, current determines the gauge and allowable length of wiring runs, and voltage is the push that drives current. If the current is low enough, there’s no danger. Ever see that demo where the kid’s hair stands on end while he/she touches that big silver ball? There might be 50,000 volts on the surface of that ball, but minimal current. Touch a household 110V AC hot wire, on the other hand, and you can be killed by as little as 60-thousandths of an amp of current.

WHAT’S A WATT?

So where does “power” fit in? We measure the amount of energy used by a simple load like a band heater by multiplying the voltage times the current in amps to give power measured in watts. How many watts used in an hour is a watt-hour, or kilowatt-hour for bigger loads, which shows up on your electricity bill. For the band heater example, however, look at the math again: for a fixed wattage of heater, doubling the voltage halves the current for the same wattage, which means smaller, lighter wiring runs. The key point is that the band heater’s output is measured in watts, which means it doesn’t care whether you use high voltage and low current or the other way around. Naturally there are limits, and the heater will have recommended ratings, but for purely resistive loads like band heaters you can easily guesstimate the bill for melting that resin

But what about the other loads, like electric motors? AC motors are very efficient and reliable, but they impose both simple resistive loads and a more complex form of load with ugly names like “inductive reactance”. Motors are usually rated in horsepower (1 HP equals 746 watts), which is a quaint historical measure that’s really only useful in comparing motors. You often see ratings in “VA” on AC motors, which is a measure of apparent power, as opposed to reactive power, et cetera. Unless you have a power quality or serious load balancing issue, you don’t need to go there.

PERILS OF HIGH VOLTAGE

From a shop floor perspective, there’s a safety issue, of course. You can survive high voltage exposure with low currents, but what controls the current? Mainly the resistance of your body, which isn’t enough, unfortunately, to save you in a high voltage strike. And high voltages can spark, with resultant hazards.

So why not just use low voltage and high current to get the same wattage? That’s what Thomas Edison wanted, and if he’d has his way 100 years ago, today there would be generting stations every few kilometres to minimize the resistive losses in the low voltage, direct current lines. As it turned out, Edison lost the battle with George Westinghouse, who wanted high voltage AC distribution.

Good thing, too, because if we lived in a low voltage world, we’d probably be melting resin with steam boilers — and I, for one, am not shovelling coal!


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