I know there's a lot of ambiguity about horsepower in electric motors, and maybe I shouldn't even go there. So I'll get right to the application.
I should be done next weekend with my KMG clone, a 2X72" grinder. I'm told that I'll want 2-3hp running it, I believe it.
I may have found a freebee motor off of an ancient-looking machine. I haven't run it, but looks in good shape. The specs are:
1160 rpm
110/220V & 9.2/8.6 amp .5 HP
it is something a little over a foot in diameter. At least by hobbyist woodworking standards, I consider it quite a large motor. It is labeled 1/2 hp, but is bigger than my 2hp bandsaw at home, and far bigger than my "2hp" hand-held router.
Is this just a matter of semantics? And this motor might be powerful enough to run my grinder just fine?
Or, someone at a used machinery dealership once told me that such motors ARE in fact merely as powerful as their faceplate says, no stronger, but that because of their massive construction they'll just never burn out whereas those hand-held routers eventually do?
The specs seem inconsistent. It's drawing a lot of amps for a 1/2 HP motor, and should draw about half the amps at 220 that it does at 110. WRT the other question about motor ratings, some small tool and vacuum motors are deceptively rated at peak power output.
If it says half horse then that is what you've got. Many high speed motors such as routers and shop vacs claim 5 and 6 developed hp. but this is not true horse power as they soon loose rpms with any load. It takes nearly
Oh, sorry; the 8.6 is a typo. I believe it was half of the 9.2.
Why the enormous size then? Does it just have a larger casing, or are even 1750/3500 rpm motors of more realistic size also going to lose rpms under load compared to this half horse that looks like a 5 horse in size?
Even assuming that the 9.2/8.6 amps is really 9.2/4.6 amps, the specs seem to be out of whack. That works out to 1012 watts at full current draw. 1 hp is 746watts plus add some for various losses so that motor should put out well more than 1/2 hp or else it will be getting very HOT!
Old motors were always much larger than newer ones. On top of the design issues, this one is an 1160 rpm motor which means it is a 6 pole motor compared to the usual 4 pole (1750 rpm) or 2 pole (3450 rpm)
This motor will have a big torque curve as well a huge reserve capacity when you try to bog it down. Downside is that you will likely want to use a belt drive to > Hi,
So do you think, even being 1/2 HP, that it would work for this grinder? I was planning on a belt drive anyway, so that's no big deal at all. Are 2hp only needed because of the load the grinder puts on it anyway, so with the reserve capacity I could get away with it?
From direct experience: I have a VERY old 20" bandsaw with a motor about that size. It is rated at 800 RPM. and the lower wheel is driven directly off the motor shaft. It takes about 30 seconds to come up to speed, and I can slow it down easily by forcing the wood into the blade.
There are a couple of reasons your motor is so large: 1.) It is old 2.) It is low RPM.
Routers use brush (AC/DC) motors, and turn at very high RPM. Power is the product of RPM and torque. Your motor turns slow, and has lots of torque. Routers turn fast and have low torque.
You can try, but I am afraid that if you use a belt/pulleys to get the RPM up, you will be able to stall it when you grind.
I'm not a motor expert by any means, but I can relate my recent experience in replacing the huge 1140RPM motor off my '33 South Bend (the original and rated at 1/2 HP) with a modern motor/VFD rated at 1HP that is less than half the size and weight of the original.
The new motor is clearly more powerful. The original motor weighed 100 pounds easy, but the new motor, while smaller and not nearly as cool looking is way stronger. I posted about this subject while I was going through the process of deciding whether I should replace or rebuild, and the responses I got might be interesting reading for you. Try a Google search.
A large part of the size difference is because of the allowable temperature rise. Old motors used insulating materials that would not stand very high temperatures. They had to be larger to dissipate the power losses without getting too hot. Newer motors use high temperature rated insulations and can safely run hotter, so can be smaller. The old motors tend to have more losses which also requires them to be bigger.
Because of the larger thermal mass, an old motor would take longer to overheat when overloaded but motors are basically rated for their maximum continuous power within allowable temperature rise. I do not think that an old 1/2HP motor should be expected to be satisfactory for an application needing 2HP to be available. Don Young
It perhaps isn't quite so much of a minefield as you think. Most decent industrial quality motors are honestly and unambiguously rated. Many motors on consumer goods, cheap power tools, cheap compressors etc. are not. Two of the most common ways to boost a motor's rating for marketing purposes are to (i) quote the input power rating instead of the output power, or (ii) quote an intermittent power rating, which the motor cannot sustain continuously. There are also no doubt less scientific methods which verge on sheer lies.
As people have mentioned, there are a few reasons why your 1/2 hp motor is so large. Firstly it's old. Old motors run cooler and operate at a lower magnetic flux density than modern motors. In order for them to run cooler they require a larger surface area to conduct their lost power to the atmosphere, which requires the motor to be physically larger. They also require a larger diameter rotor in order to generate the same torque in a field of lower magnetic flux density. Lastly, it's a slow running, six pole motor. Old or new, a slow running motor will be larger than a fast running motor of the same horsepower rating because it needs to produce more torque.
Old motors do have the advantage of greater thermal capacity, which means that they may be capable of withstanding short term overloads better. I have a 3/4 hp motor (probably dates from the 1950s and weighs about 40-50 lbs) which has seen quite a lot of abuse during experiments. I remember noticing that after being stalled repeatedly over the course of about 10 minutes it was barely warm. Old motors are also inherently nicer in their construction and appearance - you don't get problems with fins breaking off, cracked fan cowlings, etc. - but there's some luck in the draw. Some ancient, scrounged motors run perfectly and seem almost indestructable, but others will need a rewind.
There's no harm in trying your motor out to see if it will run your machine. Set it up to give the correct grinding speed and see if it provides adequate torque. If it does, run the machine hard for a few minutes and see how hot the motor gets. Touching it with your hand is a pretty poor way of telling if the motor is too hot, but if you quickly disconnect it from the electricity supply you can measure the change in winding resistance and from that calculate the temperature rise. If it's of interest I can scan a page from one of my books which explains the procedure.
Give it a go (I'm a great enthusiast for old motors).
Pulling those currents, it's around half a horse. If it's old enough, it's probably got woven cotton or maybe silk insulation, those are very bulky compared with the insulation coatings we have today, they needed a lot of space for the same number of turns of wire. The magnetic materials were less efficient than what we have today and the allowable running temperatures were lower, too. As others have said, it's a low-speed motor, more poles so it's bigger. Might be something other than a capacitor-start split-phase unit, too, there were some other, less-common, starting technologies in the past. I don't think you're going to be happy replacing a 2 HP unit with it. If it's free, give it a whirl, though.
Router units are high-speed series AC-DC motors with brushes, they're MADE to wear out. Regular single phase motors will last as long as the starter switch contacts, bearings and insulation hold out, decades for most.
Thanks for the answers! They helped a lot. I'll keep on looking, then, for a 2-horse, but might as well pick this one up. I'll always have a use for another decent motor; many things can run off of
1/2 hp.
One more question out of curiosity: If 3500 rpm motors can be smaller than 1750 rpm motors, isn't it cheaper to exclusively manufacture 3500 rpm motors and then always use a belt to slow it down to 1750 if that's what you wanted? Often I want a belt even if there is no rpm change; it is more forgiving than direct drive to set up, and if I overload it the belt can slip, which is safer for the motor's health.
So long, of course, as the bearings aren't too likely to wear out much faster on the faster motor. Naturally I wouldn't find a 2hp router off ebay and then gear it down to run my bandsaw, even if it actually does have the horse power I think I'd just wear it out too quickly :-)
Each shaft/sheave combination absorbs some energy, and adds cost. It isn't always true that faster motors are noisier, though. I have a 3450 rpm 3-phase motor which has sleeve bearings which runs quietly.
I've seen it done on a few cheap machine tools, but I don't think it gives the advantage you might expect. Both 2-pole (3500 rpm in the US) and 4-pole (1750 rpm) motors are pretty cheap. Using a faster motor may well require an additional belt drive stage, which adds to the cost and reduces efficiency. Faster motors are also noisier and wear out more quickly, so they have significant disadvantages in a quality machine tool. Where it is more common is when an output speed of a few hundred rpm is required. Rather than buy (or build) a bulky and expensive 8-,
12- or 16-pole motor, a 4-pole motor will usually be used and the speed reduced using a belt drive or gear train.
SOME of the big motors have repulsion start, which gives more STARTING power than the smaller cap start motor, which gives more starting power than a split phase or shaded pole. There are applications where a repulsion start 1/2 horse will start a bigger load than a 1 or 2 hp capstart, and the bigger motor(1 or 2 HP) is only used because the start load is too big for today's common cap starts.
We had a 2HP repulsion start Leyland motor on a bale elevator - took a
5HP cap-start to start with the same load. Could likely have RUN the elevator on 1HP if you never started it under load. We used it for bale threshing, so it OFTEN started with a full load.
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