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  • table ratings

    been looking at table saws have notice hp ratings are a little confusing some have higher lower ratings at the same amp draw and rpms are differant too whats best

  • #2
    im not an expert but some company's list max output of the motor (usually when they are starting ) sears is a good example. and other companys list the constant duty output of the motor. I think the latter is a more honest measure, after all what good is a 2hp motor if as soon as it developes the hp it blows a breaker or overload switch bill

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    • #3
      BVWW has hit the nail on the head. There are two ratings that you will see for tools with induction motors (the type you see on a cast iron table saw), one is a rated horsepower, the other max developed. The rated horsepower is a relatively standard rating from motor manufacturers and is the most reliable for comparison. Max developed hp is a rating that pushes the usefulness of the motor. A general rule of thumb for induction motors is the actual rated Hp is 1/2 the Max developed.

      Jake

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      • #4
        So Jake in the case of the TS2424, it is rated at 1-1/2 true usable HP... It could develop 3 HP at startup or with no load... Do I understand this right? Not that it matters that much, just trying to figure it all out…
        Regards,<br /><br />Big Johnson<br /><br />Pictures: <a href=\"http://www.woodworkersweb.com/modules.php?set_albumName=albuv85&op=modload&name= gallery&file=index&include=view_album.php\" target=\"_blank\">http://www.woodworkersweb.com/modules.php?set_albumName=albuv85&op=modload&name= gallery&file=index&include=view_album.php</a>

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        • #5
          You are correct. The rated Hp for the TS3612 is 1 1/2 hp meaning under normal use that is about what the saw motor is developing. This is the same as 3 hp max developed.

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          • #6
            Not to nit-pick, but (watch out for the "but"): "max. developed" hp is usually not achieved when the motor is running free with no load, but rather when it is loaded to the point of stalling. It is a meaningless number for practical purposes.

            For comparative purposes, given so many different ways hp is "measured" or calculated by different people, use power consumption in amps. In a perfect world, one horsepower is 748 watts, which is about 7.5 amps at 110V. In the real world, some of the power consumed by an electric motor is wasted in the form of heat, and some of the power consumed is needed to overcome the internal resistance of bearings and the like. However, if you assume that two motors are more-or-less equally efficient, then comparative power consumption will give you a good idea of comparative power. If you stick to comparative conclusions, translating them into absolute horsepower is unnecessary.

            A footnote: comparing universal motors and induction motors directly will give erroneous results. From an electrical perspective, universal motors are generally less efficient than induction motors, and therefore will consume more amps to give the same effective power. As a practical matter, however, a tool that may be expected to run for some time, like a table saw, should not have a universal motor, period. Unfortunately, some table saws out there, generally at the lower end, do have universal motors.

            How does one tell whether a given machine has a universal motor or an induction motor, if the specs don't reveal that? If you can get a close look at the machine, see if the motor has those humps on the side; these are starting capacitors, which only induction motors have. If the motor is easily convertible from 110V to 220V, it is most likely an induction motor. If you can peak inside the case and see a commutator and brushes (or if the manual talks about accessing or changing the brushes), it is a universal motor. Finally, if the motor's speed is continuously variable (like a drill), it is a universal motor.

            Sorry; end of lecture.

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            • #7
              Oops; I meant to say "peek inside."

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              • #8
                ""max. developed" hp is usually not achieved when the motor is running free with no load, but rather when it is loaded to the point of stalling."

                Would you be willing to go for a compromise position? A friend used to be an Engineer at a tool company name of Porter-Cable. They make routers and stuff.

                He told me the basic regime vfor at least the universal motor developed horsepower is to run it to full speed, full power, and stop it as fast as possible. It's the train wreck school of power measurement, he said it is unusual for the motor to do much but smoke afterward.

                If I may add a couple to the list of distinguishing marks:
                - Weight. Even a pretty light induction motor is pretty heavy.
                - Size. Induction motors are generally big in comparison with universal motors.
                - Sound. Universal motors have a mind-wrenching whine to them, caused by the brushes I suspect.
                - Speed. If a motor does 20 thousand RPM, it's univeral. An induction motor that does more the 3800 is a rare beast indeed.

                Dave

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                • #9
                  So far all of what you guys have pointed out is correct.

                  A couple points to make:

                  No horsepower is measured with the motor running free since the motor is making no horsepower at that time (yes it is making a little to keep itself spinning). Since horsepower is a measure of work being done, the motor will not develop horsepower until it is loaded

                  Concerning universal motors, we do not rate the horsepower for those units for the reasons listed above. Generally a better comparison of universal motors is amp draw. Granted there will be some differences in efficiencies, but it’s a good baseline.

                  Finally a 15-amp motor is the largest you will see on a 120v tool. This because, generally the largest circuit you'll see in a house is 15 amps.

                  Jake

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                  • #10
                    "we do not rate the horsepower for those units for the reasons listed above."

                    Good for you guys, Jake! (psst, go tell the vac. guys this though, OK? I can't even find the amp draw sticker on my Ridgid vacuum).

                    "Finally a 15-amp motor is the largest you will see on a 120v tool."

                    You mean from Ridgid, right? Jet routinely uses motors with 18 FLA at 110v. Unless things have changed recently, they also don't have UL stickers on them.

                    dave

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                    • #11
                      To clarify: You will not see a 120v tool with a UL listing bigger than 15 amp

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                      • #12
                        Dave: I'm not sure you and I (or your engineer friend and I) are so far apart.

                        The power of any motor is ultimately the amount of load it will carry while maintaining its rotational speed. By definition, if the load is such that the motor cannot maintain its rotational speed, it isn't carrying the load. If you measure the amount of load it takes to stop the motor, you are measuring the sum of the motor's power and the power contained in the momentum of its flywheel (or whatever part is performing a flywheel function). Which is why "max. developed" as a rating for motors is a bit of a scam.

                        As for your other distinguishing characteristics: right as rain (and I should have included them in my list).

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                        • #13
                          Ja, absolutely.

                          Dave

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