|Forum topic by jonah||posted 12-10-2013 02:50 PM||2034 views||6 times favorited||28 replies|
12-10-2013 02:50 PM
People have linked various explanations of electric motors and horsepower numbers, but I still see people throwing motor horsepower ratings around as if they are accurate. See here for some basics:
What I wanted to address is some basic truths about motor operations at 120VAC. 95+% of 120VAC motors are designed to function on a 15amp household circuit. That circuit allows the motor to exceed 15amps for fractions of a second on startup (all do), but allows a maximum of 15amps under load. Some physics equations:
Voltage (V, measured in volts) = Current (I, measured in amps) x Resistance (R, measured in ohms)
So at 120VAC, 1 horsepower equals 6.21amps. So in physics fantasy world where motors are 100% efficient at converting electricity into power, a 1hp motor would draw 6.21amps. In reality, most motors are 50-70% efficient, so a motor developing 1hp of power would actually draw ~9-12amps. This is equally true of the kinds of induction and universal motors commonly used in tools. Also, keep in mind that many tools are designed for nominal operation at 110VAC, not 120VAC, and also that lots of power grids don’t even provide 120VAC. Stick a multi-meter into an outlet in your house. There’s at least a decent chance, depending on where you live, that your house has 110-115V coming out of those outlets.
Realize that there are some complicated vagaries of alternating current that make the equations much more complicated, but for our purposes we can ignore them. They don’t change the basic facts.
If your “3 1/4 horsepower” router actually developed that much power, it’d draw 27-36amps – which it clearly doesn’t. That router probably draws around 12amps (most manufacturers don’t even approach 15amps, let alone 20). Let’s say it actually is rated for 15amps. Your 15amp router is probably around 1.5hp.
What the manufacturers are doing, as has been mentioned before, are taking “peak hp” numbers from the milliseconds when the tool starts (and is drawing dozens and dozens of amps) and sticking them on the box. Shady? Sure, but it’s not illegal and considering how few people actually understand electricity, they can sell a lot more shop vacs by claiming they are 6hp than by actually telling the truth.
I hope this helps people make intelligent choices about what they’re buying.