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  • Another electric motor question

    I have a question regarding electric motor ratings. I recognize that manufacturers don't all spec their motors the same, so here is what I typically do to get a rough idea of HP rating:

    conversion: 1 HP(electric) = 746 Watts

    Name Plate: Voltage = 230V (RMS)
    Full Load Current Draw = xx.x AMPS

    I multiply 230V by the full load current draw to get the power, then divide by 746 to get my approximate HP rating. I must say, it does seem to give "ballpark" estimates. However, comparing this HP rating only occasionally gets me "real close" to the HP rating on the motor and almost NEVER close enough to be considered equal, regardless of whether it's a single phase or 3 phase motor. "Regular" motors typically over-estimate the HP rating by my standard (my calculated rating is typically between 0.75 and up to 2-3 HP lower than nameplate rating when avoiding the crap "peak HP" rating so prevalent, particularly with Sears), whereas air compressor motors are typically underestimated (my calculated HP rating is higher than the nameplate HP rating)

    Does anyone know how an honest manufacturer tests and specifies these figures, or do you think it the difference between the power input (which is what my calculation computes) and power output?

    btw, the 5HP motor on the rotary phase converter I built draws ~62amps peak when I start it up (probably less than 0.2 seconds) -- that would make it about a 19HP motor according to "Sears Standards". It draws only 2 amps when up to speed and under no load.

  • #2
    Effeciency and servcie factor come into play when rating an electric motor.

    Most motors sold in the US are tagged with thier full load amps at peak effeciency(usually between 85 and 90%).Most motors have additional capacity beyond this effeciency,but as the motor approches the upper limit(locked rotor amps the point which the motor either stalls or drops the start whinding back in)the amperage increases dramatically until failure(meltdown).
    This also affects service factor of the motor.The closer the motor is pushed to the upper limit the greater the heat it generates.Since heat is the enemy of insulating varnish,the greater the heat,the shorter the life span of the varnish.

    Most motors in the comsumer market will be rated with either a 1.00 or 1.25 service factor.If mememory serves a 1.00 service factor indicates 100,000 hours at rated capacity,1.25 would be 125,000 hours and so on.But this seldom is ever achieved,most never see 1,000 hours due to other factors.

    Industrial motors are often pushed beyond the rated efficency depending on the desired service life and application.
    I see 1.00,and 1.25 on things like conveyors,1.5 and 1.75 on machinetools and presses ,2.00 and 3.00 on things like elevators and bridges.Basically the more demanding the application the greater the service factor.

    You mentioned the amerage spike when starting your phase converter,this is common and pressent in all AC inductions motors single or three phase.
    The starting current can be as much as 10 times the nameplate current depending on the starting load.Thats why you see special fuses on motor loads.

    Compressor motors on consumer units are cheap motors,they have poor cooling and often run a higher load than rated just before set pressure is achieved.
    I have a small Chinese motor infront of me,its off a pump.There is no amp rating on the tag,just KW(.370)and voltage (110v) and they give the hp as 1/2hp.
    Is this to be belived?
    Well 746/2 would equal 373 watts,close.
    So 373/110v would be 3.39 amps,also close.
    So,do any of these figures match performance?The answer is I have no idea because the mfg can put any BS they want on a tag.I can hook the motor up,turn it on and load the motor unitl the amp meter reads 3.39 amps,but is it developing 1/2hp?I have no way of knowing without a brake test or a dynomometer.

    Usually if a motor is built to NEMA standards the tags are pretty good.If its a spec motor for somebody like Sears or Campbell Hausfeld all bets are off.
    I just need one more tool,just one!


    • #3
      Calculating an induction motor's HP from nameplate electrical data by means of a simple formula plain doesn't work for ordinary people. You could possibly use it for brushed DC motors but you have to crank in a few fiddle factors to account for efficiency, the size of the motor, and in a used PM motor the condition of the field magnets.

      There's a non-linear relationship between calculated watts and actual developed HP that could be reduced to a single formula but it would be a lengthy thing to contend with. Even then it would be a guess.

      It's best to compare the mystery motor's nameplate data with a motor catalog. Find a few motors in the catalog that are electrically equivalent and do some study.

      Single phase induction motors 1/2 HP and under may draw 1300 Watts (calculated) per HP. Single phase motors 2 HP and under may draw 1100 watts per HP. 5 HP and under 950 watts and so on until you reach an irreducible minimum of about 850 watts per HP.

      Here are two complicating factors. The rough figures I quote do not take into account service factor. They apply if the service factor is 1.00. If the motor is designated 1.15 service factor the motor's rated full load Amps will be that much larger; the motor being overbult by that amount to meet conditions of increasing severity.

      Three phase power. You have to multiply the three phase amps by 1.73 to get the single phase equivalent for your Watts conversion.

      746 Watts per HP is an accurate conversion of electrical units to mechanical units but only in a theoretically perfect motor. No such motor exists even in the laboratory. I'm fond of saying that EE undergrads have a high attrition rate because of second year AC theory. They often go on to successful careers in the fast food industry.

      Some hype obsessed marketing people think it's OK to inflate the HP ratings of consumer grade compressors and shop vac to ridiculous figures. It's common to see "6.5" connected with "HP" on the sides of Sears' and Rigid's larger shop vacs when the electrical data clearly shows 115 Volts at 12 Amps. A more realistic figure for a series wound motor having these electrical ratings in this application is about 3/4 HP. A real 6.5 HP motor running a dust collector would require an 8" suction and will have enough power to suck the shoe right off your foot and spew it into the cyclone in a few milliseconds. DAMHIKT

      I suggest all of you who cherish the 746 Watts per HP conversion as applicable to consumer level induction motors firmly put the concept out of your minds and in the future use catalog data to make your mystery motor determinations.

      [This message has been edited by Forrest Addy (edited 01-13-2005).]


      • #4
        "so on until you reach an irreducible minimum of about 85 watts per HP."

        Hey, cool, perpetual motion. I'm sure you meant 850. I just received the Sears tool catalog. It has two pages of compressors. Not one has an accurate or even possible hp rating advertised. One is even advertised as 7hp on a standard 120vac household outlet. I am seriously considering complaining to the Canadian Competiton Commissioner as such false advertising is unfair competition.
        Free software for calculating bolt circles and similar: Click Here


        • #5
          I'm always amused and sometimes amazed by the information offered on this forum by self-professed experts. Unfortunately, the respondents to this question of motor horsepower apparently don't know service factor from power factor or watts from volt-amps. The general message may be correct, (a 5 HP Sears motor will not continuously produce 5 HP) but the supporting "facts" are iffy.

          Horsepower and watts are synonymous; 1 hp is exactly 746 watts. The confusion is between applied and produced power (actually measured as torque and mathematically converted to power). There are always losses from resistive heating and viscous and coulomb frictions that lower produced power. Starting current and running current are vastly different but they produce starting and running horsepower.

          The simgle (sic) formula referenced for calculating horsepower from "watts" is indeed simple; 1 hp = 746 watts. Determining produced horsepower from input watts is complex requiring consideration of physical dimensions and such variables as velocity, hysteresis and eddy current losses, power factor, and input power.

          As far as electrical engineering students are concerned, AC theory can be a tough concept. But I'll wager that by their second semester, they can spell, write a coherent sentence, understand verb tense and even handle formulae with fractional exponents.

          The internet and forums such as this can be a great asset, but when people won't bother to proof read their work and show no regard for spelling and grammar, the dissemination of information is cheapened and a disservice is done to all exposed to it. This problem is akin to the garbage produced by Hollywood. People view the immorality, the excessive drinking, hear the swearing, and think that's how normal people act and society degenerates. Let's keep the information flowing but strive for accuracy. And, if you consider yourself an authority, set a good example by at least spell checking your work.


          • #6
            Get up on the wrong side of the bed Phil?

            I'm well aware of the difference between service and power factor. I'm also aware of my defects in spelling and proofreading etc. I usally come back later and correct my errors because I quite literally can't see them when I first make them. We all have blind spots.

            I didn't engage in a lengthy and excrutiatingly correct discussion of AC theory, definitions, and applicability to everyday problems for good reason. My goal was to illustrate to the unwary a hazard associated when electrical to mechanical unit conversions are applied to everyday problems.

            If someone wants to know the time you don't tell him how to build a clock.

            [This message has been edited by Forrest Addy (edited 01-12-2005).]


            • #7

              I think your comments about Forrest are completely out of line.

              I have found over the years that his information and experience are exceptional. When Forrest talks, I listen.

              You have established no credibility on this forum and I take your comments accordingly.



              • #8
                The metric folks, (bless them...with a brick) DO rate motors in watts.

                BUT of course, that is watts output at the motor shaft(I hope).

                And of course, 746W = 1 HP.

                But, there is of course an efficiency issue converting input electrical watts to output torque-rpm watts.

                Without bothering to calculate all the various factors, your hand tells you that, the motor gets hot.

                Probably as good a rule as any is that a motor will pass thru say 50% or so of its input watts ("real" watts) as output shaft power.

                So under that rule, a 1/3 HP motor would produce about 250W output power, and draw maybe 420w to 500W. "Theoretically" that is 3.5A to 4A . You likely won't ever see that....

                Motors have a crummy power factor, so the actual line current and "indicated volt-amps" will be more than that. Unloaded, the motor probably draws at least a couple amps, but its at a lagging phase angle, and amounts to lower power than the "indicated volt-amperes" would suggest.

                The overall power factor gets better at more heavily loaded conditions, but full load amps may be as much as 5.5 or 6 amps.

                You can roughly estimate the motor power by measuring the no-load amps.
                Subtract that figure from the full-load amps, and take the "real power" to be 50% or 60% of the result of a power calculation based on that current.

                You'll be wrong, of course, but probably considerably less wrong than if you do a straight conversion of amps and volts (which would be "volt-amperes", or VA) to power.

                [This message has been edited by J Tiers (edited 01-12-2005).]

                Keep eye on ball.
                Hashim Khan


                • #9
                  While not an "expert" I have changed,rebuilt and rewound my fair share of motors.

                  It still goes back to the old saying"if it sounds to good to be true,it probably is"

                  Sure that Sears motor will produce 7hp,right before it burns up

                  Does anybody else understand the term locked rotor amps?

                  [This message has been edited by wierdscience (edited 01-12-2005).]
                  I just need one more tool,just one!


                  • #10
                    Oh,and after 12 hours of ACTUAL work,spell checking your own work ain't that "simgle"
                    I just need one more tool,just one!


                    • #11
                      At "locked rotor" condition, the motor produces NO POWER WHATSOEVER, but it sure absorbs some.....


                      Keep eye on ball.
                      Hashim Khan


                      • #12
                        Sears is famous for rating their motorized stuff in terms of peak power drawn from the line, though they make it a little obskewer about what is actually meant. When you see that big SIX POINT FIVE HORSE sticker on the side of the vacuum cleaner, that's how much power it will draw from cold, and not yet turning. For that fraction of a moment after switch on, it will indeed draw that much power through your speeding power meter, calculated using the highest peak voltage that a normal 110vac outlet would have. That would be when the generating plants are all pumping full tilt boogie, then everyone decides to shut off their dryers, ranges, and electric hot water heaters all at once. That voltage might jump to 130 vac or so.
                        They like to say 'produce', or 'develop', but they never give a spec like 'shaft horsepower', which is what people generally like to take the power rating to mean. Not so, not even close. Now maybe for a vacuum cleaner that would be ok, since the guy who buys one (with that big horsepower figure in his head) can say 'that must really suck'. Then when he gets it home and finds it's not much better than the old one, he can still say 'that really sucks'. As far as an air compressor, maybe it's the same, although different words have to be used. If it doesn't perform as powerfully as the horsepower rating would seem to suggest, you could still say 'that really blows'. For all other motorized items, just treat them as hand warmers which do double duty by having a little power left over for the intended function as well. Think of this as the 'power to spare!' feature.
                        Sears is not alone in this deception of the public. It's just that they're fun to pick on. I'm sure we'll soon be seeing 'THE most powerful 1500 watt heater made', and 'the fastest 3600 rpm motor ever produced'.
                        Anyway, if you're looking to get a motor that will get the job done without overheating, get one with 'powerful horses'. I'm not sure of the difference is watts, or where this rating system came from, but it stands to reason that if there's powerful horses, then there's also less powerful ones.
                        As far as 'locked rotor' goes, that just plum sounds powerful. I gotta get me one a doze!
                        -------- --------
                        I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-


                        • #13
                          Looking at the Sears ad it says in the ad "120 volt household outlet". Ok, 7hp X 746 divided by 120 equals 43 amps. From a 15 amp circuit. Don't think so. Not even locked rotor/starting/whatever amps. The wires in the line, the power cord and motor are too thin (too much resistance) for that to happen.
                          Free software for calculating bolt circles and similar: Click Here


                          • #14
                            I guess webbch's thread has been pretty well hijacked and I apoligize for my part in it - but continuing along the tangent:

                            I understand there was self serving legislation some years ago that had the effect of allowing among other thing misrepresentaion of HP in motor driven equipment. From that we have motor HP ratings based on laboratory trickology rather than actual mecchanical output accessible by the user.

                            So rather than a motor's HP being based on steady state absorbtion dyno readings we have the present mess where the HP is an instantaneous peak value impossible for the user to take advantage of. An executinve from Rigid explained it to me in these rough terms. The motor is instrumented and clamped in a test fixture, spun to some max reading where the RPM is recorded, then the motor is abruptly braked and the resultant peak torque noted. A torque/RPM calculation is made and that's the HP used by the hype meisters of the tool world.

                            It's significant to note that the motor's peak torque is strongly influenced by the rotating mass of the armature and attached rotating parts. In effect this "peak HP" is about as useful to the user as is a distant lightning strike is to your power bill.

                            I understand Sears, Rigid, Campbell Hausfield et al lost a class action lawsuit about fraudulent HP ratings in compressors and had to make reparations to purshasers in the form of $50 discounts. Can anyone flesh out the details?


                            • #15
                              There was a class action suit a while back, that offered a discount certificate to those offended buyers who responded. I think it was Campbell Hausfield compressors, and rebadged C/H sold by others, and they had to be used on air handling equipment. Why someone would want to purchase another over rated compressor from a seller who had already duped them is another question.

                              Like all class action suits, the offended party was sort of mollified, the lawyers enriched and the cost ultimately passed on to the consumer.

                              PhilB has been an infrequent contributor, but his response in this topic is interesting.

                              My god, the typos, missing apostrophe and other glaring errors indicate an unconscionable lack of proofreading on his part. I do not wish to affend(sic) him, just point out his mistakes so he can do better in the future.

                              Most of the contributors here are passing on their knowledge and attempting to be as factual and accurate as possible. Some of this knowledge has come from reliable sources, experience and education. Some has come from other, less reliable sources. It is up to the reader to sort out the good from that which he might question. None of it is presented as the be-all and end-all, take it or leave it.

                              On the whole, in my mind they are a more reliable source than a paid professional like, say Dan Rather.

                              Add to that, most of us are mechanical types, many typing with a fistfull of hotdogs, and the occasional typo or grammatical error may occur. That's life I guess.

                              [This message has been edited by JCHannum (edited 01-13-2005).]
                              Jim H.