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.
Chris Schwarz is a Senior Editor for Popular Woodworking.