Motor horsepower an Apples/Oranges Question

[Jeffrey Makiel]

Manufacturer's stated horsepower ratings have a lot in common with Wall Street and credit card companies these past few of years.

Caveat emptor ("let the buyer beware")

-Jeff

[Larry Edgerton]

So you guys are saying I really don't have a 5.5 Hp vacuum cleaner?

[george wilson]

The rule is 3 amps per H.P. at 220 volts.

[Peter Quinn]

The rule is 3 amps per H.P. at 220 volts.

I wonder why then my old DeWalt RAS, with a 2HP motor and a motor plate rating of 17A@220 requires a 20A circuit? And why my new Grizzly 1HP feeder draws 4.7A@220? Or why my 3HP PM66 TS draws 22A and requires a 30A circuit per manufacturers recommendations? Seems like electricity might be a good place to require industry standards?

I can tell you that I doubt my 6HP shop vac motor could power my TS for very long, and that the motor on my TS weights more than a half dozen shop vacs combined. Some of that could be the cast iron mount, but no doubt some of it is all those pesky windings and magnets?

Start throwing in efficiency ratings, duty ratings, and motor class designations and a simpleton like myself starts to get a headache just thinking about these things.

[Tom Veatch]

The rule is 3 amps per H.P. at 220 volts.

George, I believe it would be considerably higher than that. At a power factor of 1 (almost as hard to achieve with an electric motor as 100% efficiency) 220v/3a is 660 watts. 1 HP is 746 watts.

Throw in reasonable values for efficiency and power factor and you're closer to a consumption of about 1200 watts consumed per HP produced.

I believe a better rule of thumb is 1 HP produced is about 240v/5a or 120v/10a consumed. Since the overall efficiency can vary significantly from one motor to the next, more exact values for a particular motor would be available from the motor's dataplate.

[Frank Hagan]

Doesn't NEMA publish standards for labeling of motors? I can't seem to reach NEMA.ORG this morning, but if the motor has a NEMA nameplate there are only a few "tricks" the manufacturers can play. One is with the service factor on the label; a 1 HP rating with a 1.5 SF (service factor) rating is the same motor as a 1.5 HP rating with a 1.0 SF.

I worked for a pump manufacturer, and our motors were from AOSmith, GE and Franklin. We had what we called "full rate" (service factors above 1.0) and "up rate" motors (service factors at 1.0). It wasn't as bad a marketing game as they play now, and it did serve a purpose. Homeowners would invariably choose the higher rated horsepower for their pool pump even if it wasn't needed.

What was interesting was that 1.5, 2 and 3 HP pumps were being used for pool filtration when a 1/2 or 3/4 would have been just as good. We sold two speed pumps for energy savings ... rotor speed would drop from the 3450 rpm standard in that industry to 1725, flow would be reduced to half, and energy use would be about 1/4 the total. The same effect could have been acheived with a 1/2 HP pump for hundreds less, but the homeowner buying the pool would often choose a "package" including the equipment, and the pool dealers trying to sell the right HP pump would lose out every time.

The reason we have 6.5 HP shp vacs is because none of us would buy a 1/3 HP shop vac that was next to a competing brand that was a 6.5 HP one.

[george wilson]

Actually,that might be for 3 phase. I can't recall now. That's what a machinery dealer told me. His old industrial machines are mostly 3 phase. Also,start up amps is a lot higher.

My 2 h.p. Bridgeport type milling machine,which is single phase,220 volts says it's 15 amps at 220 V. I just went out and checked it. It would be 31 amps at 110 V.,according to the motor plate. Maybe someone else can talk about 3 phase. I had a Weiler lathe,whose 3 h.p. motor was more the size of a 1 h.p.,but my 1 h.p. Hardinge HLVH lathe has an enormous 1 h.p. motor that must be over 12" in dia. X 18" long. Can't get down there to measure it.It is huge. The Weiler is no cheap lathe.Probably $50,000 new,as is the Hardinge HLVH. Why the huge difference in motor sizes? The Hardinge must have a very large overload capacity.

I do not know why a Dewalt 2 h.p. motor would need 17 amps,when my milling machine is 15 amps at the same power. My milling machine motor is a large motor for its power,probably easily weighs 2X the weight of a Dewalt RAS motor.

[Mike Henderson]

Doesn't NEMA publish standards for labeling of motors? I can't seem to reach NEMA.ORG this morning, but if the motor has a NEMA nameplate there are only a few "tricks" the manufacturers can play. One is with the service factor on the label; a 1 HP rating with a 1.5 SF (service factor) rating is the same motor as a 1.5 HP rating with a 1.0 SF.

I don't think that's correct. As I remember the definition of "Service factor" it's the amount of power the motor can produce as a multiplier of the rated HP (so for a 1HP motor with a 1.5 Service factor that would be 1.5HP), with a rise in operating temperature of 10*C, but the life is cut in half when operating at that level. [A SF of 1.5 would be quite high. SF is usually much smaller, probably 1.2 or less because of the operating temperature rise limit of 10*C when operating at the SF HP.]

So if you have a 1HP motor with a 1.5 Service Factor, you can get 1.5HP out of it, but the motor will fail much quicker.

A 1.5HP motor with a 1 SF would have a full life.

Mike

[See this discussion of Service Factor.]

[Rod Sheridan]

Manufacturer's stated horsepower ratings have a lot in common with Wall Street and credit card companies these past few of years.

Caveat emptor ("let the buyer beware")

-Jeff

Jef, I disagree, any reputable motor (Leeson/Baldor/Westinghouse/Kato etc) will have accurate information on the motor nameplate.

I have however seen some motors from China labled as 7 amperes, 110 Volts, 1 HorsePower...An absolute miracle! Unity power factor, 100% efficiency........I should have purchased a boat load of those and retired wealthy beyond my wildest dreams.

Regards, Rod.