From the archives
Motors in the Woodshop
Differences Between Universal and Induction
My buddy Roger, who spends most of his time feeding his retirement check to a herd of horses in Wisconsin, talks about guys having something called a HP rating. I asked him about it once and he got this Tim the Toolman Taylor gleam in his eye as he began to explain. What you do, he said, is add up the horsepower of every vehicle, appliance, tool, electric toothbrush, tractor, snowmobile and anything else with any kind of motor that currently sits on your property. Whether or not they run seems to be irrelevant. And whomever has the highest HP total is the winner. I always figured old Rog had me beat pretty handily because he has a huge barn with several old vehicles in it, including an aluminum bread van which he once converted into a camper. It hasn't housed rolls (bread or bed) in several years, because every gasket in the big block blew on a trip to the Black Hills a long time ago. But apparently it still counts toward his HP rating.
So, when a reader asked me a few weeks ago if I'd try to explain the difference between induction and universal motors, I began to think that maybe I was mistaken about Roger having a bigger HP rating. I have a pretty decent woodshop and everywhere I looked, there was another motor. True, they weren't Chevy big blocks, but there were so many of them that I began to wonder if they might add up to a halfway decent challenge. It truly is amazing how many motors we own. The computer I'm writing on has one for the hard drive, another for the internal CD player, the Zip drive, the scanner, the CD writer and, of course, the printer. But these are small fry compared to the ones in the woodshop, where they're in everything from the 1919 Wallace mortising machine to the laser-guided miter saw - almost a century of motors, and all still running like clockwork.
Induction motors have a series of large metal conductors which spin inside the windings we're all familiar with - rolls of wire wound around and around, covering the motor just inside the housing. How they work is actually quite simple. We plug in an AC cord to send alternating current (normal household current) through the motor. It's called that because it literally alternates numerous times every second between being positively and negatively charged. When the plate on a motor says it's 60 cycles, that's what it's talking about: the current alternates 60 times every second from being positive to being negative.
As the charge changes from positive to negative, those big metal conductors are alternately attracted and repelled (remember how two magnets behave?), and this keeps them spinning inside the windings. The charge is in the windings and this creates a magnetic field: the shaft keeps turning so the conductors are lined up with the magnetic field. As long as power flows through the windings, the magnetic field is maintained and the conductors constantly chase it. One of the easiest ways to tell whether you have an induction or universal motor is this: induction motors don't have any brushes to change.
Universal motors are noisy because they're equipped with small, removeable chunks of metal called "brushes", which need to be replaced every now and then. That's because the brushes rub constantly against something called a commutator, which is a thick disk attached to the shaft with grooves running in the same direction as the shaft. The shaft runs inside one set of windings, which in turn spins inside a second outer, stationary set of windings. Those brushes transfer electricity to the shaft, and thus to the inside windings.
When electricity is introduced to both sets of windings (the moveable ones inside the stationary ones and the stationary ones, too), the shaft will turn less than one single revolution. That's all that is required to line up the two magnetic fields caused by the electric current coursing through the windings. So, manufacturers of motors have devised a way to keep introducing the current in the outside winding so its just behind or ahead of the field in the inside one. The result is that the inside windings keep chasing alignment, so they keep spinning.
Comparing the Two
Universal motors are a whole lot smaller, too, which makes them ideal for portable power tools. The bigger the tool, the bigger the motor. Well, actually, that's not always true. Universal motors are measured in terms of their peak horsepower, which is the speed it spins at under no load whatsoever. Of couse, if it actually operated at that speed for more than the blink of an eye, it would fry. Everything about its operation (even the cooling fan) reduces the speed, and thus the power delivered. So, a 1/2 HP universal motor is rated for its 'potential', and not the amount of work it actually does. On the other hand, a 1/2 HP induction motor can deliver that much power constantly under load without frying. The system used to measure induction motor power is therefore called "usable HP". That's why a benchtop table saw with a universal (direct drive) motor and a 1/2 HP rating is so much less powerful than a cabinet or contractor's saw with the same rating based on an induction motor.
To compare these apples and oranges, you could look at how much power they draw, rather than how much they deliver. It's not an exact indication, but it's pretty close. Just check the labels and see how many Amperes (amps) each draws. If the voltage is the same (they're both 120 and one isn't 12, 18 or even 230 volts), and the phasing is the same (single rather than 3-phase), you're going to be in the ballpark.
Kinda makes a feller feel inadequate, don't it?