Big shock from new router
I’ve known for a long time that peak horsepower ratings tell you very little about a motor. But the router I bought this past weekend brought home that point in a striking way. The box it came in touted the router as a 3 1/2 horsepower monster. For some reason, I don’t really know why, I decided to go back and reread a short piece by Barry Wixey in Fine Woodworking #175 about universal motors. Wixey explains that there is a difference between peak horsepower and continuous horsepower. Peak horsepower is a sharp spike that the motor can’t maintain, but continuous horsepower is the power it works at under normal conditions. There is a big difference between the two.
Here’s how to calculate continuous horsepower. Multiply the motor’s amp rating by the number of volts being feed into the motor to get the watts output. For my router that would be 15×120=1800. Divide the result by 746 (the number of watts in one horsepower). 1800/746 = 2.41. That’s right, my 3 1/2 horsepower router is capable of putting out 2.41 horsepower on a continuous basis.
But wait, there’s more. That 2.41 is possible only if the motor is 100% efficient, not loosing any mechanical power to heat or other wastes. Well, a typical universal motor is between 65% and 75% efficient. So, my 3 1/2 horsepower router is really outputting between 1.56 and 1.80 horsepower continuously.
That might upset you, and I was a little peeved. But then I remembered, every router I’ve used has been more than capable of doing what I asked of it. So I’m not too worried about the somewhat misleading horsepower claims. The router will do the job.
By the way, the best way to get a sense of a motor’s power is to look at its amp rating. Ignore peak horsepower claims entirely. For more on horsepower, read Martin Seifert’s article in Fine Woodworking #135.