Carleton MacDonald wrote: >I recently bought a Shop Vac 6000. It works quite well but I have a question >about the specification "6.0 peak horsepower". > >The vacuum indicates that it uses 11.0 amperes. At a nominal 120 volts this >would mean 11x120 = 1320 watts. > >The "horsepower", a non-SI unit, is defined as being exactly 746 watts (an SI >unit). A watt is a watt is a watt, of course, so it would seem to me that >1320/746 or approximately 1.77 hp is more accurate. (I have no idea what a >"peak" horsepower is. The watt is the standard unit of power; the horsepower >is an outmoded unit used only in the USA.) This assumes no losses within the >motor itself, of course; actual "hp" of output would be lower due to this. > >A 6.0 hp motor would be huge, and would likely require a dedicated 240 volt, >30 ampere circuit. Further, due to the inefficiencies of single-phase motors >in large sizes, it would probably be polyphase. > >Any explanation would be welcome, and interesting. Your math is correct. The trick is the word "peak." Electric motors will produce quite a bit more than their continuos power rating, but only for a short time. This is one advantage electric motors have over internal combustion engines for powering portable tools. For example, if a tool such as a saw needs 500 watts for general cutting but 1.5 kW for the occasional knot, a 500 W electric motor will work because it can briefly deliver the required 1.5 kW. However, if the saw were powered by a gasoline engine, the engine would have to be rated for 1.5 kW. If it's only rated for 500 W, the gasoline engine will stall when the saw hits a knot. A gasoline engine won't put out more than its rating, even for a short time. Of course, the 500 W-rated electric motor will draw a lot more current when it's putting out 1.5 kW, but most electric power circuits will supply the extra current for a short time, even if it exceeds the circuit breaker rating. I have no idea how long a "peak" is with a shop vac. It could be a few seconds, or it could be a few minutes. As a practical matter, the "peak" might be limited by the circuit breaker feeding the circuit. My bench saw at home usually trips the garage circuit breaker before it trips its internal breaker, if I try to rip too much wood too fast. I recently had to translate someone's reference to horsepower into watts for an article in our trade association's journal. I found six different forms of horsepower with slightly different conversion factors in the NIST Guide for the Use of the International System of Units. I particularly liked the boiler horsepower, one of which is equal to 9 809.5 watts.
