Popular Woodworking 2000-11 № 118, страница 8

DIFFERENT KINDS OF POWER FOR YOUR HOME SHOP

You probably know that most of your house is wired for 110-120 volts.And you might know that certain appliances,such as your electric range,dryer and big air conditioners, are wired for 220-240 volts.And perhaps you've heard about three-phase power.What's the difference between these and which should you be using in your shop?

110-120 VOLTS • This is the standard current that most of your hand power tools run off of.And except for special circuits that power 240 appliances, this is the voltage to all the outlets in your house. Remember that voltage entering a

house can vary. So some people get I 10 volts, some people get 120.Tools and appliances can handle a 12-volt variation, so don't worry.

220-240 VOLTS • This heavy-duty circuit uses two hot lines from the main panel that act as returns for one another.These heavy-duty circuits are good for a variety of reasons. First off, machines on these circuits use only half the amperage as they would on 120-volt circuits, so you are less likely to trip a breaker or blow a fuse on a well-wired 240 circuit. Plus, 240 circuits are much less prone to voltage drops than 120 circuits.This means you can have a table saw that's more than 20 feet from your service box. Operating a motor at low voltage causes the torque to drop and the motor to heat up (shortening the life of the motor). Many induction motors can easily be switched over for 240 power. In the box on the motor where the electric cord goes in there will be a diagram to show you how to reconnect the different leads. If you can afford the wiring change, do it. However, one myth about 240 power is that it is cheaper. Don't believe the myth.You buy power by the watt.

THREE-PHASE POWER • What's three-phase power? Well, the power coming into your house is single-phase power.This means that there's one electric pulse changing direction 60 times a second.Three-phase power has three of those pulses changing direction at slightly different times.The fluctuations are timed so that when one phase is at its lowest power, another phase is at its highest.The result is a very steady stream of energy. Three-phase power is typically used in factories, not homes.You need a special motor to run three-phase power, but three-phase motors are less expensive, extremely reliable and more efficient than single-phase motors.Three-phase power is not available to most residences.But you can purchase a "phase converter." Some manufacturers don't recommend static phase converters but say that rotary phase converters are OK. Bottom line, for the home shop, it's cheaper to buy a single-phase motor for a saw than it is to convert your juice to three-phase power.

Here's the box on the motor for our Jet table saw.You simply move a couple leads and your saw is then ready to take 240 — as soon as you change the plug, that is.

got trouble. And third, the big fan that cools the motor brings in a lot of junk such as sawdust and foreign objects. This junk can damage the windings and insulation.

Learn to Shop

Now that you know the differences between induction and universal motors, you need to know how to compare motors when tool shopping. First consider how you will use the tool and whether it should be powered by an induction or universal motor.

If you need your table saw to be portable or you're only turning it on for short times, a universal motor will do. But if you expect to sometimes run your saw for longer periods of time, get an induction motor.

Things become more complicated when you start comparing one motor to another. Motors are measured differently by different manufacturers. Should you use horsepower? Amperage? Wattage? Motor efficiency? All of the above? The answer is that all these factors are related and all

play a part in judging whether a motor has got a lot of guts or is just a loafer on the job.

First off, let's clear the air about horsepower, which is the way you measure induction motors and some universal motors. It's almost a meaningless number, unfortunately. That's because there are several ways to measure horsepower, and this makes comparing two 1-hp motors almost impossible.

Some manufacturers measure horsepower with the motor under no load. Some measure horsepower as the saw almost reaches the point where it is about to stall — called the point of "breakdown torque." Some lock the motor in a dead stall, turn on the power and see how many amps the motor pulls from the outlet and calculate the horsepower from that. This is one way to measure "developed horsepower."

Developed horsepower is probably the least accurate measure of the motor's day-to-day abilities. When you lock the motor in a dead stall and turn it on, the motor will pull a lot more amps than normal because it's trying desperately to pull itself out of this stall.

Instead, try to find a "continuous-duty" horsepower rating, which is found on most high-quality induction motors. If the motor's nameplate doesn't state its horsepower rating is for continuous duty, ask the sales person. If they don't know, have them find out, or call the manufacturer yourself.

Why is this so complicated? Keep in mind that there are a couple different formulas to calculate horsepower. One way is to multiply the rpms of the motor by the amount of torque (which is in foot-pounds). Divide that number by 5,250 and you have a horsepower rating. Keep in mind that a universal motor's really high rpms skew this equation. The other horsepower formula involves the electricity going into the motor.

For this calculation you need to know how amperage, voltage and wattage are related (this is that math that I promised you). Almost every basic electricity textbook explains these different terms by com-