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Is their a fairly easy way to tell the horsepower of an electric motor? I need to be accurate to with about 1/8th of an HP. I picked up a cache of old electric shop tool motors (think drill presses and table saws) but none of them have data plates. Most of these motors are pre-WWII as they are repulsion-induction types and don't use capacitors. A few do use caps though. Motors of that age tend to be very large for their horsepower output, so I can't just go by the physical size of the motor. They also tend to be less efficient then modern motors so I can't just go by their ampere draw either. Some appear to be from at least the early 20's and others might be as late as the 50's, so their efficiency is all over the map. I doubt any of these motors pull more then 1.5 HP but the smaller ones could be as little as 1/4th HP. Most are about 1700 RPM but there are a few 3400 RPM ones also.
Attack weight x to it and see how long it takes to lift the weight given distance y. Then you can calculate its hp.
Faggot
>>986800
http://lmgtfy.com/?q=how+to+determine+horse+power+of+an+electric+motor
>>986813
I already hit up google or I wouldn't be here. Just to humor you I followed your link. I went to the first 10 links in that, they were all useless. They were either all simple 'volts + amps = horsepower' calculators (not accurate enough) or required I know the efficiency of the motor. If I had the dataplate with the efficiency spec on it I would already have the horsepower. I skipped links talking about cars of course.
>>986800
Get a few grinding wheels (or whatever) of different rotational inertias. Fire them up on-camera and count frames to compare how long it takes to reach idle speed. Some math should give you a pretty accurate delivered power. However, note that sustained power delivery is RPM-dependant. For power at a particular RPM, you'd need to do a bit more work.
>>986845
I don't think that will work. Some of these motors are repulsion-induction types. As they have no starting capacitor they take longer to spin up under load than more modern designs. Then there are just normal variance based on manufacturer, age and such. Some just take longer than others to come up to speed. Thanks for the idea though.
>>986800
>HP
>not kW
>>986820
You are a fucking moron.
You come to a sight make unrealistic requests, and rage at the best help you are going to get.
Let me spell it out...
If you want a motor accurate to 1/8th horse power don't use something that you don't know what it is. You are going to waste more time trying to determine the horsepower and you will never find out to the level of accuracy you want.
Several of those links tell you how to determine the horse power.
The only other option would be to take it somewhere where it can be tested. But I bet that it would cost less to just buy the precise motor you want. What you want is magic or clairvoyance.
I sense you want this info for something more than the actual use of the motors.
Take your useless crap and go elsewhere.
>>986800
It sounds like you're asking the wrong question.
Why do you need exactly 1/8HP? There are other ways to control speed, starting time, and torque than just purchasing a different motor. Tell us the application and we can help you find a solution that fits it without beating around the bush all day.
>>986848
>As they have no starting capacitor they take longer to spin up under load than more modern designs.
That's the idea. Spinning up takes an input of energy over time, i.e. power. The time it takes to spin up depends on the power of the motor. Therefore, with a camera, some measurement, and a bit of math, you can determine the power used to spin up a wheel. Analyzing multiple wheels can let you account for the rotational inertia of the motor itself.
>>986852
>Why do you need exactly 1/8HP?
I don't need it exactly but right around there. It would be nice to be able to tell a 1/4 HP motor from a 1/3 HP motor from a 1/2 HP motor, which is about 1/8 HP difference between each. I don't want to put an underpowered motor on a tool because it limits the capabilities of the tool and shortens the life of the motor. I don't want to put too powerful a motor in there because motors are expensive and vintage ones are hard to find, so it would be a waste.
Up from there it gets easier. Almost all motors larger than 1/2 HP are either 3/4 HP, 1 HP or 1 1/2 HP. I doubt any of these are more powerful than that.
>just purchasing a different motor
I restore vintage tools and sell them on the side. Its a rewarding hobby. Most of the people I sell to want vintage motors in them. If I went out and bought a bunch of brand new motors it would wreck my sales because my prices would just about double. No joke. Single phase motors are not cheap. I have almost a dozen tools that need motors and this batch would go a long way to getting them operational again. I was able to pick them up cheap from a scrapper. First thing the turd did was peel off the dataplates because they were brass but he hadn't gotten to the windings yet so I was able to get them running again pretty easily (mostly shot bearings or wiring and a few bad capacitors).
Used motors are available of course but they get really expensive if they are company branded (check eBay for Rockwell/Delta, Walker-Turner or even Craftsman motors, its insane). Generic motors (GE, Westinghouse, Sunlight, Marathon, A. O. Smith, Dayton, etc) are cheaper but still not cheap. They just don't come up for sale that often where I live either. I've had to buy pretty wrecked and useless tools before just to get the motors in them.
>Tell us the application and we can help you find a solution that fits it without beating around the bush all day.
I said in my first post this was for shop tools.
>>986866
I barely passed high school math and I don't own a good video camera. Just a cheap cell phone and an old Canon PowerShot ELPH. I was hoping for a more mechanical method, if thats possible. I wouldn't even know where to start once i had the video captured.
>>986885
OP, you're not gonna get numbers accurate to the 1/8th. Not by a long shot.
AvE on youtube has several videos in which he explains and does the math on horsepower on an electric motor. You need to know the power draw and RPM. I know his blend tec blender disection video has that sort of info. Try there.
>>986891
I know both the RPM and the power draw.
>>986906
Long shot, but any Hobart motors?
>>986924
Unknown. Most of them have no markings left on them. One has the old GE logo stamped on the end cap of the motor.
>>986800
your best bet is going to be amperage draw. No-load is gonna be lower then load.
so if you're reading the motor and with no load, you're getting about 8-9 amp draw, with load its going to be around 15 amp. or about 1.5HP.
15amp@120volt = 1800watts of power. 1800watts is 2.4 Horsepower. rounded. now that's at MAX load, if you hold a motor at this load, it will most likely over heat and cause any thermal overload switch to cut on. and your motor is most likely going to be rated at 1 or 1.5 horse power.
so just measure the voltage basically! the rest is some pretty simple math.
>>986800
This is not easy, but you could make a dynomometer you could use on all the motors.
They work like this picture where the blue is a fishing scale. Then you have the foot-pounds and you do the rpm calculation.
The hard part would be making the clutch in green. For small motors that won't tear your hand off, you could use a length of belt on the pulley and squeeze down on it with your fingers until the motor starts to stall.
Directly attach your motor to a small alternator and put a multimeter on the alternator.
745 watts is one horsepower.
I actually had to do this as a science project in school (fond the hp of a motor) and that's what I came up with. Was never sure if I was right because the teacher didn't know the answer either.
>>986812
If it works, you may have lucked out. If you overload the motor, it was rated for less.
I think OP wants a non-destructive method.
>>986800
>attach a horse to the motor and have the horse walk in the opposite direction that the motor turns.
>Start the motor, and make the horse walk at the exact same time to make it fair
>if the horse can keep walking the motor is less than 1 horse power
>if the horse can walk away still, keep attaching more horses until they can walk away
>the second they horses cant walk away, you'll know what the horse power is of the motor
>>986800
You could set up a dynamometer. A variable mechanical load such as a water pump with a restricting valve. Add to that some method of measuring the motor torque. Mount the motor on a swinging arm and attach a spring scale. Also, get an accurate tachometer (non contact optical will do).
Run the motor unloaded and start applying mechanical load (closing the pump valve). Measure the RPM and increase the load until it drops to its rating*. An unloaded motor will run near 1800 (or 3600) RPM, but 'slip' at 1700 (or 3400) RPM at full load. Measure torque (and current draw). Calculate HP from torque and RPM.
*Of course this assumes you now the motor's full load RPM.
If you don't know the motor's rated RPM, there are methods involving measuring winding resistance (very accurately), running the motor at various load levels and then measuring the winding resistance again. These measurements will allow you to calculate average winding temperature rise. Some motor design manuals can be consulted to get winding hot spot temps for average temps. Plot all this on a graph and keep the hot spot temp under the maximum value for the winding insulation type. That's the motor's rating.
All of this is too much work for cheap motors. If you need a motor within 1/8 HP, buy one. If you just want to learn motors and measurement techniques, go for it. But pre-WWII motor insulation is probably pretty weak, so it would be best to derate these oldies quite a ways from their original design.
>>986945
That wouldn't be accurate enough because I don't know how efficient the motors are. I have an old 1/2 HP motor and it pulls 8 amps. Its pushing 100 years old but its only slightly older than some of the ones in my cache. I have a much newer but still vintage Rockwell (1960's) that pulls 6 amps for the same HP. I also have an old 1/3 HP motor from the 50's that pulls 6.2 amps. Efficiency has a big effect and you can have a 30% swing on the horsepower of a motor for the same amp draw. So you see my problem.
>>986948
Hey now, thats an idea. I have enough spare motors that I could run some motors with known ratings through their paces, get some readings and then run the new guys through and see what they generate. I will look into that.
>>986964
>If you need a motor within 1/8 HP, buy one.
I doubt any of these motors are 1/8 HP. The weakest is probably 1/4 HP but a few of them could be a full horse if I am lucky. A typical 1/2 HP 1750 motor new, that isn't made in China, is close to $100 new. I generally sell a restored drill press with a half HP motor for about $175-$200. Theres no way I could shift it at $300.
>it would be best to derate these oldies
Marketing has already done it for me. Modern motors are rated based on peak horsepower, old ones like this were rated under operating load. This is why little shit shop vacs or garbage table saws can have a '3 horsepower' motor yet be usable on a 110v/15a single phase line. Meanwhile Rockwell/Delta used to sell their 10 inch Unisaw table saw with a 1 horsepower motor back in the day and it did the job.
