Woodworker's Journal 1992-16-5, страница 22
In The ShopAmps us. Horsepower: Demystifying Electric Motor Power Ratings 'v / ■ H11LIH LIT • -: ---- by Jim Barrett hen shopping for power tools, how can you compare the relative power output of different brands' it you can't li y out each tool, you'll have to rely on the manufacturer's specs. The two most common indicators are am/trttiKe lamps) and horsepower (HP) which are, or should In?, directly related lo each other. However, I've often noticed that two or more tools with the same horsepower rating have different amperage ratings. I've also sometimes lound that a tool with a lower amp or HP rating scented to have jtist as much power as one with a higher rating. So. which ligure do you go by—amps or HP? Or can you believe ihe figures at all? Assigning HP Ratings To establish I IP ratings for tools, most manufacturers attach the tool lo a dynamometer. This device measures changes in motor speed and torque as an artificial load is applied to the tool by the dynamometer's brake until the motor stalls (lock-rotor torque*. Speed and torque readings are taken at various points between the motor's no load speed at startup and motor stall. From these readings, a horsepower curve can be calculated, using ihe following basic formula: Speed (in rpm} x torque (in inch pounds) x .0000158 = HP. While the formula itself is simple enough, many variables enter the picture, which can affect test results. And, because toolmakers tesi their tools independently of one another, there's no common yardstick or established procedure for rating HP in the tool industry. Also, the resulting data is sometimes tweaked a bit lo rale a tool a bit higher—or lower—than il's actual power output, for various marketing reasons. One example of this is rounding out to the nearest HI5—who ever heard of a 11V16 HP router? Finally, when comparing horsepower, you have to make sure you're looking at horses of the same color. Tw o Types of Horsepower: Peak I IP (also called maximum developed HP) is the top HP a motor can deliver for brief periods under a heavy load, such as w hen plow ing through a dense knot in wood. \i peak HP. the motor may draw over three times it's rated safe amperage, and after about 20 seconds, the motor will start to overheat (if you don't pop a circuit breaker first). Continuous HP (rated IIP) is the amount of HP a motor can deliver during continuous operation without overheating: in other words, when the tool is operated for hours al a time at it 's rated safe amperage. Hnrsc|)«mtT and Motors All portable powci tools and many bcnchtop tools use universal (htiish lyjx'l motors, designed largely lor intetmilicnl it■■.< On ihe other hand, stationary power tools typically us, induction (brushless) motors, designed lor continuous op, i.uioii these larger, heavier motors run at a fixed spcol i. iih, i I 7.""* rpm or 3,450 rpm), and are able to chug along .u tin j> ed under a wide range of loads. Universal motors, on the oihei hand, can reach speeds as high as 25/MK) rpm. bui will low down considerably under load. As u rule, the ri.iniepl.iii on induction motors list continuous HP, although som ic'■■! ni.ik, v also include peak HP in their literature 1 hey usually don'i assign HP ratings to tools with universal motors Kouters and some bench-top stationary tools are exceptions, i> |ally giving peak HP ratings. If you're not sure which fiinm . hi ie looking at when comparing tools, check the amp laimgs rule of thumb is, a tool rated at 12 amps (at 12<i\ prodik.es about 21/- peak HP. or 1 continuous HP. Amperage The rated nmpeiagc ol a tool is the maximum amount of current it ean draw without overheating and burning up. Because the ratings are assigned by Underwriters Laboratories, which uses consistent lest methods, amps tend to be a more accurate indication ot actual motor power than HP. Even though amps are a measurement of electrical input, they reflect power output. Amps also tend to be a more precise measurement because they're Usually broken down in '/id increments (9.2 amps. 9.3 amps, etc. l. whereas HP is usually broken down into 'A increments i HP. 7+ HP. I HP. etc.). This is one reason why you'll sometimes see two different tools with the same HP rating, but wilh different amp ratings. It you're still not sure whether amps and/or HP are an accurate indication of power, there's one more indication: the si/e and weight of the motor itself. Generally, the bigger and heavier a motor is, the more power it has. (As unscientific as this may sound, several electrical engineers concur.) 22 The Woodworker's Journal |
