Can someone tell me the formula for electrical horsepower? In other words, how do you determine the horsepower for a table saw. I understand that P = IE. Where I is the current drawn and E is the applyed voltage. Where do I go from there?
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Replies
746 Watts is the equivalent of 1HP, but there's peak HP and other things to consider. Are you looking for output power? If you're looking at some kind of power tool and specs for it, I don't know of anyone who states output HP.
So 2238 watts = 3 hp
P = IE
2238 = I x 120
I = 2238 / 120
I = about 19 amps
Therefore a 3 horse tablesaw has to be a 220V circuit???
Am I correct???
25amp 110 circuit or 15amp 220 circuit.
The reason I am trying to understand the horsepower formula is as follows.
I can have a 3.25 horse Porter - Cable router with a 120 V plug on it but I can't have a 3 horse tablesaw with a 120V plug on it.
What am I missing here?
Because the router manufacturer is giving you the peak HP the motor will develop at stall, which doesn't do you a whole heck of a lot of good as you probably would have backed off long before when it first started to bog down. Sears used to do the same thing w/ some of it's little benchtop saws... I think I heard of some of those little buggers labeled as something like 5 horse or something ridiculous. You see it on some brands of shop vacuum... I've seen those labeled at something like 6.75HP, running on a 15A 110V circuit... Basic math is volts (110) times amps (15A) equals watts (1650w), divided by 746 watts per horsepower gives us... 2-1/4 HP is about it. There's a reason why all the little electric space heaters that run on 110/15A plug ins are listed as... 1500A, or about 5100 BTU. There's only so much power to suck out of a regular power cord! There may be some trickery w/ speed control, gearing, whatever on some stationary machines that may cause one to use that power more efficiently, but otherwise... ain't marketing great?Monte
For induction motors (like on a cabinet or contractor saw), motor output power is hp=(V x A x pf x eff)/746 where V is voltage (use nameplate voltage, usually 115 or 230V), A is current (stated on nameplate), pf is power factor (a measure of how much of the current is usable), eff is efficiency, a measure of how much input power is turned to output power (the rest is turned to heat), and 746 is a constant (746 volt-amps, or 746 watts, equals one hp if pf and eff both equal 1).
The problem with trying to make such a conversion is that you won't know what pf and eff are for a given motor unless you have the actual spec sheet. They also vary depending on the type of motor (motors with a run capacitor have very high pf, and somewhat higher eff than those that don't), and very small motors have very low efficiency (I've seen them less than 50%), while very large ones can be in the high 90's.
3 hp cabinet saws have motors that draw 12A (Unisaw) up to around 17A (some imports) at 240V. The Unisaw motors (WEG and Marathon) use a run capacitor, while the Baldor on a PM, and the imports, do not. The 3 hp rating, by the way, is a continuous output power rating - thermal equilibrium is attained after some period of time while outputting 3 hp, without overheating the insulation system. They will output much more power at their peak, typically 2 or more times continuous rated power, but then they burn up. The higher the overload, the shorter the life.
Routers, on the other hand, use universal motors, which don't have a continous rating. They are whatever someone says they are. They will, I'm sure, output 3 hp or so, but for a very short period of time, at which point they burn up. So for routers and such, the best you can do is to look at the current rating, knowing they won't deliver anywhere near 3 hp for any useful period of time, regardless of what the marketing people say.
Be seeing you...
Edited 9/2/2005 2:44 pm ET by TKanzler
I've got a 3/4HP capacitor start motor on my bandsaw. Sometimes I plug it into a Variac when I need slow speed cutting. The Variac's I have are rated to 25 amps.
When I first turn on the Variac with the band saw switch on, I have to crank the voltage up for the starter windings to get the motor started. The current gets up to about 15 amps before the centrifugal switch kicks out. After that, the current drops down to around 3 or 4 amps and I can slowly increase the voltage to get the desired speed. Many times, this voltage is only 30 or 40 volts. At full voltage and no load, the current is about 8 amps. The only problems when running at such low voltage is that the torque is not as much, and if I cut to fast I run the risk of slowing the motor down enough that the centrifugal switch kicks back in. Not something I do a lot - just mostly when wanting to cut metal. I occasionally touch the case of the motor to make sure it's not getting to hot.
I use two Variac's in my electrolysis process going through a full wave bridge. They both have AC volt and AC amp meters.
PlaneWood by Mike_in_Katy (maker of fine sawdust!)PlaneWood
when I need slow speed cutting. ???Gee.. I just don't push the wood in so hard!
Question , How many Countries still us Horse Power (HP)
compared to Watts?
Hilmar.
I got a fairly new motor at a garage sale for $5.00, but it doesn't have any labels. If I plug it in and get an aperage reading, and multiply that by the voltage, I can get the wattage (watts = Amps x volts)? Then multiply my wattage x voltage and divide by 746, and it will give the horse power?
Excuse my calculations, I am just trying to figure out the horsepower of this motor. Feel free to make corrections, you won't hurt my feelings!
Thanks!
I'm afraid it isn't that simple. In fact, it's virtually impossible to determine the original hp rating of a motor without the nameplate, and of course, if you had the nameplate, you would know the hp rating.
If the motor was rated for continuous duty, then while outputting it's rated hp, the temperature of the windings climbs and eventually reaches thermal equilibrium - it doesn't get any hotter. The terminal temperature will be lower than the temperature rating of the insulation (allowing for hot spots deep inside the windings), but the temperature rating is unknown (it's on the nameplate).
Induction motors are also capable of outputting much more power than they're rated for, but of course, when overloaded they draw more current, generate more heat, and the windings run hotter. If you had a motor lab, you could instrument the windings and monitor temperature, or calculate temperature changes from resistance changes, as you slowly increase the load (making the motor output more and more hp), but you'd have to make assumptions about what the temp rating of the insulation is, and what the max ambient temperature it was designed to run in (40C is pretty standard). You'd also need to know how much power it's outputting, hence the motor lab (with a dyno).
As far as the no-load current reading goes, that's not useful in any way. The motor will draw more current as a load is applied, and if you know the no-load current and the full-load current, you can estimate actual motor output while it's operating between those two points, but you can't extrapolate anything from the no-load value alone, and you wouldn't know the full-load current without the nameplate. You can't even compare it to another motor, unless you have one that's the same make and model. Motors from different manufacturers, and even within a single manufacturer's product line, can have vastly different no-load current values (for the same hp rating), due to design differences. For that matter, the full-load current values can be all over the place (for the same rated hp) - 3 hp WEG and Marathon Unisaw motors are rated 12.0 and 12.4A @230V, while the Baldor is 15.5A, and some Chinese motors are 18A (Grizzly G1023S). They all put out a real 3 hp continuously. The WEG and Marathon use a run capacitor, and the Baldor and Grizzly do not (run capacitors reduce current draw for reasons I won't go into here).
Here's a link to a spec sheet for the 3 hp Baldor Unisaw motor. You can see (at the bottom) how the current goes from 5.9A at no load, up to 22.1A at 150% of rated load (4.5 hp). You can also see that it will output up to 250% of rated torque, which with the slowdown in the motor speed at that torque, is probably about 2 times continuous rated output (6 hp). http://www.baldor.com/products/perfdata.asp?1=1&catalog=WWL3606&product=AC+Motors&family=General+Purpose%7Cvw%5FACMotors%5FGeneralPurpose
About the only thing I could suggest is to put it on a tool, and see if it does what you want it to. That won't tell you much about it other than whether or not it'll work for your application. Just remember that at the point where it's thinking about stalling, you're working it much, much harder than it was designed to work. I wouldn't use it on a fan, since it would be too easy to grossly overload (and overheat) it under continous running conditions, and burn it up before you know it.
HTH
Edited to add: If you take the no-load current (5.9A) of the Baldor linked above, multiply by voltage (230V), and divide by 746, you get 1.8 hp when it's actually outputting zero hp. That's because most of the current is just sort of bouncing back and forth between the utility and the motor, with a little bit making heat. You only pay for the little bit that's making heat, but the current you would read on an ammeter is much higher. That's tied to what I didn't want to get into above about the run capacitors. If you want to know more, send me an email.
Be seeing you...
Edited 9/3/2005 12:05 pm ET by TKanzler
Thanks for the followup! I knew it would probably wouldn't work like I thought it might. The only way to find out was to ask.
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