If you’re an electrical engineer, you can stop reading this article right now. This story isn’t for the gear heads — it’s for the rest of you woodworkers who use power tools every day but are occasionally stupefied by amps, volts, watts and horsepower. I’ll warn you, there’s just the tiniest bit of math to learn here. But if you can multiply and divide two numbers, you will open up a whole new world of understanding when it comes to the subject of motors.

The first thing to understand is that there are two kinds of motors that power almost all of the machinery in a home workshop: induction motors and universal motors. Each type has its strengths and weaknesses. The reason that you need to know the difference between the two is that some tools (table saws, planers and jointers, for example) can be powered by either type of motor. So you need to educate yourself so you’ll choose the right motor for the kind of work you do.

In general, induction motors power stationary machinery that must run for hours on end, such as big table saws, planers, band saws and jointers. Universal motors power mostly hand-held stuff: routers, jigsaws and sanders. However, this is changing. These days you’ll find more and more universal motors in benchtop table saws, small jointers, spindle sanders and portable planers.

I like to think of the two motors as the tortoise and the hare. Induction motors are the tortoise of the pair. They’re rugged, quiet, large, heavy, turn more slowly and can be stalled under heavy use. They are great for the long haul. Universal motors, on the other hand, have a shorter life span, they’re smaller, they make more noise, they operate at very high speeds, they offer the most horsepower per pound of any alternating current motor, and they are very difficult to stall. Universal motors provide large amounts of power in quick bursts with constant torque and at variable speeds.

It might help to think about how you use tools with universal motors. If you’ve got a chop saw, you need a burst of power for three or four seconds to make your cut. You need torque and you need it fast. Same goes for biscuit joiners and routers. Unless you are running parts for 100 doors on your router table, chances are that these tools are on for five minutes and then off for a while. Now think about how you use a jointer or a planer with a hefty induction motor. You might have 100 board feet of lumber to surface. Each board might have to go through that machine five times. Your machine might be running for hours on end.

So each type of motor has a type of job that it’s really good at. And it all has to do with the way that the motor is built. Here’s the inside story:

Induction Motors

The reason they are called “induction” motors is the way they convert electricity into a spinning rotor. To understand how induction motors work, let’s say you’ve got one of these puppies in your table saw and you’re about to turn it on. As you flip the switch, power flows into what’s called the “stator” and magnetizes it. The stator is a mass of copper windings that surround the rotor in the center, which is what spins the saw’s blade through a series of belts and pulleys. Inside the stator are two or four “poles” that become magnetically charged because of the electricity running through the wires. When the electricity changes direction or cycles, as it does 60 times a second in the United States (hence the term 60 cycles), each pole changes its magnetic strength, from a positive to a negative value or from a negative to a positive value.

The induced poles in the rotor are then attracted and repulsed by these ever-changing electromagnets in the surrounding stator. The motor isn’t running, but the rotor is excited. What this hulk of iron and copper now needs is a shot of power from another copper winding (called a “starting winding”) that is out of phase physically and electrically with the main winding. And that’s where the capacitor comes in. In most modern tools a capacitor (which is in series with the “starting winding”) helps with the starting torque. Then, when the motor reaches 85 percent of its speed, the capacitor and the starting winding drop out of the circuit and the motor runs on its main winding.

Whew. So, this is the long way to explain why these are called induction motors. As you can see, the rotor spins because it is “induced” by the electromagnets in the stator. Induction motors are large and heavy because the induction process takes a lot of iron and copper (a ? hp induction motor weighs about 25 pounds; a ? hp universal motor weighs 2? pounds). Induction motors are reliable because they’re simple, their parts are built for long life and they run at slow speeds (so they don’t generate as much motor-damaging heat). In fact, a well-built induction motor won’t heat up more than 40 degrees centigrade over room temperature. Induction motors are slow because the rotations per minute (rpms) are governed by how many poles are inside the stator and the number of times per second that your electricity cycles — which is standard at 60 cycles. So now you can understand why you wouldn’t want your router powered by an induction motor — you could barely lift it, and it probably would be too slow and not have enough torque.

Universal Motors

Universal motors get their name from the fact that many of them can operate on both alternating current (from an outlet) or direct current. The way that universal motors work is a little more complicated than their induction cousins, but there are similarities.

Instead of a rotor, universal motors have what’s called an armature that spins in the center. Instead of a stator, universal motors have what’s called a field, usually consisting of two coils surrounding the armature. Universal motors also have some parts that induction motors don’t. On one end of the armature is a part called the commutator. This part is round like the armature, but it is usually smaller in diameter and is made of small bars of copper. It’s through these bars that the armature winding is energized. Universal motors also have what are called “brushes.” Brushes are made from a carbon-graphite material and are usually held in place against the commutator by small springs. When you turn on a universal motor, current travels in what’s called a “series circuit.” One side of the electrical line goes through the field, then through the brushes, into the commutator, then the armature, and back to the other side of the line. Each of the bars in the commutator changes polarity as it contacts a brush, and this changes the polarity in the magnets in the armature. The magnetic forces in the armature react with the electromagnets in the field coils and the motor develops torque.

Universal motors make a lot of noise because they spin at a dizzying speed — sometimes seven times faster than an induction motor — and their fans suck a lot of air through the motor, which makes noise. Universal motors are less reliable for three reasons. The motor generates more heat, which can cause the components to break down. Second, the carbon brushes wear out. If they can be replaced then it’s a quick fix. If they can’t, you’ve 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 comparing 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 horsepower. 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 nameplate 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 commutator. 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

Sidebar: 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 110 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.

Quick Tip: Motor Care

The universal motors in most of your hand power tools will live longer if you follow this simple tip: blow clean air through the motor regularly.

Universal motors suck a lot of air through them because the motors turn at a high speed and they have large fans to keep the motor from overheating. Think about your shop. Pretty dusty isn’t it? That dust is being sucked through your router and is slamming into your armature like a meteor shower. This dust can also build up, cause the motor to run hotter and shorten the life span of the tool. If you regularly blow compressed air through the vents of the tool, you’ll dislodge the dust and keep your motor healthy.

In addition to sawdust, the carbon-graphite material from the motor’s brushes also builds up on the commutator. Blowing air through the tool also helps dislodge that stuff, too, and this also prolongs the life of your tool.

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.