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Old motors: Why so different than today's

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markba633csi

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#1
I've heard folks say stuff like "that old 2 HP motor would be like 20 HP today"
What does that mean exactly? Are current motor ratings exaggerated that much? Or is it certain manufacturers only?
Mark S.
 

RJSakowski

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#2
There are lots of different ways to look at horsepower. An honest way would be to look at the maximum sustained load. in terms of work /time. Some manufactures use the input power of volts x amps = watts and 746 watts/hp to calculate their horsepower rating. This neglects losses due to electrical resistance and mechanical friction. and gives an inflated value for the useful output. Even worse is when they use the stall current instead of running current as that is in no way a sustainable value. The final straw is when they talk about peak horsepower. In no way relevent to the actual operation.

I used to have a Hoover vacuum cleaner that had a rating of 12 hp. It would be lucky to actually be generating a 1/2 hp. Marketing!
 

Ulma Doctor

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#5
i think a lot of the old motors were rated without the efficiency factored in.
for example the same motor factored 2 different ways,
v=120
I=8
Hp=746
without the efficiency factored in this motor would have 1.286 theoretical hp

let's say the motor was 82% efficient and we then multiplied V x I x EFF then divided by 746
we'd get a theoretical hp of 1.056
the difference is .23 hp on a 1 hp motor dependent on the calculation you based the motor on
 

RJSakowski

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#6
An exerpt from Ritzenbatt Vacuum (http://www.ristenbatt.com/xcart/Power-of-the-Vacuum-Cleaner-Suction-Motor.html)
Peak Horse Power
is an exaggerated rating of the output of a vacuum cleaner suction motor. Most motors used in power tools are rated in actual Horse Power in which one horse power equals 746 watts. Horsepower can also be derived from the formula in which one horse power equals 550 foot-pounds of work per second. The rating for Peak Horse Power is found by taking the suction motor without its fans and adding as muchload as possible without burning it out and then measuring the horse power using the formula which is based on torque per second. This results in a rating that is many times higher than the actual horse power of the motor. Keep in mind also that this motor rating does not take into account the number of fans which the motor is driving. An example of this is that a 4.0 PHP motor with two fans produces about 33% more suction than a 4.0 PHP motor with only one fan.
 

Hukshawn

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#7
Wasn't there some kind of a law suit recently where someone was suing compressor manufacturers for falsly advertising hp ratings?
 

darkzero

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#8
Wasn't there some kind of a law suit recently where someone was suing compressor manufacturers for falsly advertising hp ratings?
Yeah it was a while ago though, well the one I remember. Sometime in the early 2000s. There was a class action suit against many of the major air compressor manufacturers. I remember Sears, Devilbiss (they made some of the Craftsman models back then), & Campbell Hausfield were a few. I think they tried to add cfm ratings & tank sizes as well.

Not sure what ever happened though. After that the air compressor ratings changed & got conservative but they're still a lie. I remember my old 20 gal Craftsman/Devilbiss 110v air compressor said 5HP on it. Haha, that would mean that little air compressor is as powerfull as both my lathe & mill together! My current 220v 60 gal compressor says 3.5HP on it. My 7x12 bandsaw has a motor bigger than my lathe, mill, or air compressor & it only says 1HP on it
 

Groundhog

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#9
Awhile back we built a new drive station for a smaller ski lift. When the new 125HP motor arrived I tried to send it back. No way was it a 125 HP motor. They had to fax drawings with dimensions before I finally accepted it. (the trucker was p*ssed - I wouldn't let him take it off his truck until I got it straightened out - cost me a case of beer and a crow sandwich.)
The old 110HP (AC wound rotor, slip ring) motor was about the size of a 55 gallon drum. The new one was the size of a round kitchen waste can. It was so small I was sure it was a 12.5 HP motor and the paperwork just got fouled up.
We had to add a big flywheel to the output shaft because the lift would stop too suddenly without the mass of the old rotor keeping it turning.
 

RJSakowski

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#10
A quick check is to multiply amps x volts = watts. Then divide by 746 to get hp. The output mechanical hp will always be less than this number.

One catch is this should be running amps, not peak amps. A clamp-on ammeter will verify this.
 

Silverbullet

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#11
They did the same thing with small gas engines. Fifty years ago all mowers had about 3 1/2 HP ratings then about fifteen years ago they took off crazy saying there engines were now 6.75 HP. Big law suit and now there ratings have dropped HP and replaced with cubic ins. Same deal people are gullible ,,Barnum said,, sucker born every minute. Yupp
 

Superburban

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#12
A quick check is to multiply amps x volts = watts. Then divide by 746 to get hp. The output mechanical hp will always be less than this number.

One catch is this should be running amps, not peak amps. A clamp-on ammeter will verify this.
Wouldn't the motor need to be under a load? and then, are you not just calculating the load on the motor?
 

RJSakowski

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#13
Wouldn't the motor need to be under a load? and then, are you not just calculating the load on the motor?
Yes, it should have a typical load. What you are mesuring is the input power which is always more than the output power. Output power = input power x efficiency. As I recall, typical efficiencies are in the 80% - 90% range.
 

Superburban

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#14
Ok, I'm following. Your just calculating the load on the motor, not what the motor is rated for right? Just like a car, if you can't find the specs, you need expensive equipment to find the max output.
 

RJSakowski

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#15
Clamp on ac ammeters needn't be expensive. HF has three for under $20.
 

tq60

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#16
Clamp on Ammeter indicates current at the time.

However a 7.5 hp motor on a compressor may not approach 7.5 hp in current until it reaches full tank pressure.

There does not seem to be any standards for simple comparisons so one needs to read and understand the data sheets and hope they are correct.

Pressure washers use "cleaning unit" where they multiply volume of output X pressure to come up with some sort of common data point that one can use to compare units.

Same could be used for say compressors where amps and operating pressure and volume could be used.
 

Mr Mike

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#17
I used to have a Hoover vacuum cleaner that had a rating of 12 hp. It would be lucky to actually be generating a 1/2 hp. Marketing!
Please correct me if wrong, I believe it may say 12 Amps on the vacuum, I've seen that stated many times.

A 1750 rpm one Hp Motor will have more torque then a 3450 rpm one Hp Motor, and several times more torque then a 6000 Rpm one Hp Motor. A vacuum requires high rpm not torque. all 3 motors use the same amount of energy - 746 watts X 1 hour = 1 HP.

A high rpm motor can be considerably smaller and use the same amount of energy as the larger higher torque motor, the trade off is speed for torque.

A 1750 rpm motor would look funny on your vacuum, and would turn the brush well enough to rip the carpet out, But couldn't create the vacuum needed with the tinny fan to suck out any dirt due to the rpm of the motor. The small fan has to reach high speeds to create good suction using a high rpm motor.

Pull your vacuum agitator & belt off and look how fast your little motor is going, takes allot of energy to do that.

There is no gimmick here, No magic going on - just volts times amps = watts.
 

Allan

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#18
Lawn mowers. Yeah, what a joke. My brother bought one years ago that said in big letters something like, "7 HP at 5000 RPM". Then a small sticker said, "Never run above 3000 RPM".
 

RJSakowski

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#19
Please correct me if wrong, I believe it may say 12 Amps on the vacuum, I've seen that stated many times.

A 1750 rpm one Hp Motor will have more torque then a 3450 rpm one Hp Motor, and several times more torque then a 6000 Rpm one Hp Motor. A vacuum requires high rpm not torque. all 3 motors use the same amount of energy - 746 watts X 1 hour = 1 HP.

A high rpm motor can be considerably smaller and use the same amount of energy as the larger higher torque motor, the trade off is speed for torque.

A 1750 rpm motor would look funny on your vacuum, and would turn the brush well enough to rip the carpet out, But couldn't create the vacuum needed with the tinny fan to suck out any dirt due to the rpm of the motor. The small fan has to reach high speeds to create good suction using a high rpm motor.

Pull your vacuum agitator & belt off and look how fast your little motor is going, takes allot of energy to do that.

There is no gimmick here, No magic going on - just volts times amps = watts.
Unfortunately, that claim was on the packaging which is long since gone. The rating plate on the vacuum says 10.4 amps/120 volt, which would work out to 1.67hp input (that may include the power to the brush motor and light as well)

However, I did buy a new Shop Vac this year with a label in plain sight that proclaims 4.5 peak hp and the rating plate says 9 amps/120 volt. which works out to 1.44 hp input. At 75% efficiency, this would be about a 1 hp motor. I have to say, though, it is the most powerful vacuum that I have ever owned.

As the excerpt I quoted in post #6 explains, vacuum cleaner claims of peak hp are based on a totally unrealistic method of measurement done strictly for marketing purposes and no real indication as to how well the vacuum cleaner will perform.

The point being, though, that you can't trust advertising claims when looking at expected performance. A modern motor may look like an economical replacement for your tired old motor but you may be disappointed with the outcome. If you calculate the horsepower by the rated input power (volts x amps), you may have a better understanding of what to expect. Factor in for efficiency and if the labling says anything like peak, figure a sizable factor for that as well.
 

Mr Mike

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#20
Unfortunately, that claim was on the packaging which is long since gone. The rating plate on the vacuum says 10.4 amps/120 volt, which would work out to 1.67hp input (that may include the power to the brush motor and light as well)

However, I did buy a new Shop Vac this year with a label in plain sight that proclaims 4.5 peak hp and the rating plate says 9 amps/120 volt. which works out to 1.44 hp input. At 75% efficiency, this would be about a 1 hp motor. I have to say, though, it is the most powerful vacuum that I have ever owned.

As the excerpt I quoted in post #6 explains, vacuum cleaner claims of peak hp are based on a totally unrealistic method of measurement done strictly for marketing purposes and no real indication as to how well the vacuum cleaner will perform.

The point being, though, that you can't trust advertising claims when looking at expected performance. A modern motor may look like an economical replacement for your tired old motor but you may be disappointed with the outcome. If you calculate the horsepower by the rated input power (volts x amps), you may have a better understanding of what to expect. Factor in for efficiency and if the labling says anything like peak, figure a sizable factor for that as well.
You'll get no argument from me on the advertising being somewhat misleading, And efficiency.. well If you lived here I'm sure we could have a great conversation about all this.
 
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