Popular Woodworking 2000-11 № 118, страница 9

Assembled armatures and commutators at Makita's plant in Georgia.We were impressed with the rigorous tests Makita performed on its universal motors. Note the small bars on the commutators.

paring the electric lines in your house to a water hose. Voltage is like water pressure. The more voltage you have, the more force with which the electricity moves through your wires. Amperage is like the amount of water in a hose. You can have the faucet on low or high. Wattage is harder to explain. It is, in electric terms, the amount of energy that a device consumes. You can calculate wattage by multiplying the amperage of a tool (usually found on the information plate on the motor) by the voltage (which for home shop people in the United States is 120 volts or 240 volts). Why would you want to calculate wattage? Because 746 watts equals one horsepower.

So with that formula you can attempt to calculate the actual horsepower (as opposed to the advertised horsepower). This is one of the most important aspects of this whole article. Remember it. Here's an example of how you can estimate how much horsepower a tool has compared to how much horsepower a tool says it has on the box: Does a 9-amp router live up to the 2 horsepower rating on its box? Let's see: 9 amps multiplied by 120 volts equals 1,080 watts of power. To get horsepower, we divide 1,080 watts by 746. The answer is 1.44 horsepower. Hmmm. You can probably guess that either this router will develop 2 hp right as it's ready to crash and burn, or that the manufacturer used that other horsepower equation, which uses rpms and torque, to calculate horsepow

er. And as pointed out earlier, universal motors in routers have very high rpms, which can skew that equation. (My apologies to the gear heads here because I left out some of the other complicated factors in calculating power, such as the power factor and line losses).

So if horsepower is a bogus measure, what does that leave us with? Amps. Amps tell you how much power a tool consumes, and that's the simplest way to compare similar motors, especially universal motors. Unfortunately, a lot of manufacturers tell us that the amperage on the name-plate is not always the amperage you get. Three different 7-amp motors can all draw a different amount of current.

Even worse, amperage doesn't tell you how much of that energy is wasted. Here we're talking about the elusive "motor efficiency." Motor efficiency is not something advertised on many universal motors, but you can sometimes find it on the nameplate of induction motors. It is a percentage, usually between 50 percent and 80 percent, that explains how much of the amperage going into the motor is converted into work coming out. When you shop for an induction motor, look for a motor with the highest efficiency, highest amps and best horsepower for the job.

If you can't tell a motor's efficiency, there are other ways to judge it in the store. One expert told us to peer through the vent fans in a tool with a universal motor to see if you can see the bars on the com

mutator. The smaller the bars, the better the motor. Smaller bars mean there are more coils in the armature winding, and that makes a smoother-running motor. If you can't see the commutator bars, there's still one final way to choose a motor: buy a trusted brand name.

A couple years ago our editor toured several manufacturing plants in Taiwan. At one facility, his tour guide pointed to a pile of rusting commutators sitting outside. Those, the guide explained, would be cleaned up, repaired and put into motors for off-brand tools. Installing used parts isn't something that happens just in Taiwanese off-brands. And don't assume this is a typical practice of Taiwanese manufacturers because it isn't. Manufacturers of cheap motors anywhere can lower the cost of a tool by reducing the amount of iron and copper in a motor. This will lower the life span of the motor because all that metal acts as a heat sink to dissipate heat generated by the motor. They also can skimp on the brushes.

So do the math when you shop for motors. But even that can be misleading. One 14-amp chop saw can be $100 more than a similar-looking 14-amp chop saw. What's the difference? Probably the motor. Should that deter you from buying the cheap saw? No. If the tool won't get heavy use, a less expensive tool will allow you to spend that money somewhere else. But it should make you think twice about what you're buying and what to expect in the long run. PW

INDUCTION VS. UNIVERSAL?

you make the call

In the old days, table saws,planers and jointers had induction motors. Small tools had universal motors.Alas,that line has blurred in the last decade. Some manufacturers, such as Ryobi,DeWalt and others, put universal motors in their table saws.The universal motors are much smaller and are much less likely to stall in a cut, but they are much noisier and their life expectancy is shorter. Universal motors have also become the mainstay in portable planers — a tool that would have been a lot harder to design with a huge induction motor driving it.

If you think you can run a table saw or planer for an extended period of time and it's powered by a universal motor, you'll be replacing the motor a lot sooner than you think. How can you determine if your tool has an induction or universal motor? Turn on the tool.A really noisy motor indicates it's probably a universal motor. If you're still not sure, look at the motor. Many universal motors have coin-opened hatches so you can easily change the brushes.