Woodworker's Journal 1985-9-5, страница 22

In The Shop

Amps us. Horsepower:

Demystifying Electric Motor Power Ratings

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by Jim Barrett

hen shopping for power tools, how can you compare the relative power output of different brands' it you can't li \ out each tool, you'll have to rely on the manufacturer's specs. The two most common indicators arc aniftrtttgr lamps) and horsepower {HP) w hich are, or should In?, directly related lo each oiher. However, I've often noticed thai two or more tools with the same horsepower rating have different amperage ratings. I've also stmiciimcs found thai a tool with a lower amp or HP rating seemed to have just as much power as one with a higher rating. So. which ligtire do you go by—amps or HP? Or can you believe the figures at all?

Assigning HP Ratings

To establish HP ratings for tools, most manufacturers attach the tool lo a dynamometer. This device measures changes in motor speed and torque as an artificial toad 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 poinis between the motor's no loud speed at startup and motor stall. From these readings, a horsepower curve can be calculated, using the 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 resuhs. 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 iriarkeling 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 HP (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. ihe motor may draw over three times it's rated safe amperage, and after about 20 seconds, ihe 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.

Hnrsi'iMmtT and Motors

All portable powci tools and many bcnchtop tools use universal (biush type) motors, designed largely lor intermittent u < On the other hand, stationary power tools typically tist induction (btushless) motors, designed lor continuous ■ r i.mon Nurse larger, heavier motors run at a fixed speed i. ill*. i I rpm or 3,450 rpm), and are able to chug along ai lln ■■(>■ ed under a wide range of loads. Universal molors. on ili> oihei hand, can reach speeds as high as 25.000 rpm. bui will low dow n considerably under load.

As u rule, the iianicpl.iie- on induction motors lisi continuous HP, although som in I in.it. v also include peak HP in their literature 1 hey usually don't assign HP ratings lo tools with universal motors Koutcrs and some bench-top stationary tools are exceptions, typn dly giving peak HP ratings. If you're not sure which fieun . hi ie looking at when comparing tools, check the amp taimgs a rule of thumb is, a tool rated ai 12 amps (at 120\ produce-, about 2¹/- peak HP. or 1 continuous HP.

Amperage

The rated iimpeugc ol a tool is the maximum amount of current it tail draw without overheating and burning up. Because the ratings are assigned by Underwriters Laboratories, which uses consistent lest methods, amps tend lo 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 '/io increments (9.2 amps. 9.3 amps, i it . i. 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 (he same HP rating, bui with different amp ratings.

II you're still not sure whether amps and/or HP are an accurate indication of power, there's one more indication: the size 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