Motor Madness (or "When is a horsepower not a horsepower?")

[David DeCristoforo]

I mentioned recently that my bandsaw motor had been pronounced DOA by the repair shop. I took them at their word and ordered a new motor from an industrial supply house. It took a week for the new motor to arrive. It took me two more days to get around to installing it. I immediately noticed a difference with this new motor. With the old motor, my 14" bandsaw w/ 6" riser block would slice through an 11.5" thick chunk of hard green wood with no issues. But with the new motor, it was starting to bog down going through a seven inch thick piece.


Here's the facts:


A) I was cutting the same wood from the same tree both times.


B) Both motors carry identical specs: 1.5HP, 1725 RPM, 5/8" shaft, 2.5" pulley.


C) Both motors were configured to run on 110v and both were plugged to the same outlet with the same piece of wire.


And yet, the new motor seemed to be much less powerful.

OK, now here's the good part. This morning, I get a call from the motor shop and the guy cheerfully tells me that my motor is fixed and ready for pickup! This was the same voice that told me over a week ago that my motor was "shot". He explained that when I called, he thought I was another guy who was the one who actually owned the shot motor. Mine just needed to have a bit of tweaking done and the fix was only going to cost forty bucks. Since I just happened to be right around the corner, I popped over and picked up the motor.


Now, I thought, I can see if I'm imagining things or not. I brought the old (and now un-shot) motor home, pulled the new motor out and popped the old one back in. Sure enough, it sliced through the same piece of wood with no difficulty. So now, I'm buzzing. I always thought that a horsepower was a horsepower. 110… 220… volts… amps… however you figure it, the motor's output should be the same. But here are two 1.5 horsepower motors that are obviously not equal in power output.


The possibilities are:


A) The original motor is underrated.


B) The new motor is overrated.


C) Horsepower is calculated using some kind of "fuzzy" math (much like economic indicators) and the motor's power rating can differ depending on who's doing the math.


D) I'm going crazy.


No tricks here. Both of these motors are almost identical. Same size, same frame type, same approximate weight, same specs on the plates and, as I said, they were both configured the same way.


Anyone?

[Kyle Iwamoto]

I do know that there is continuous rated horsepower, and peak horsepower. That is how they get a 5 hp 110v air compressor. My 5 hp table saw motor is bigger than the motor and compressor. Compressor is 5hp peak and table saw continuous.
For your case though, if the motors are identical looking, there may be something mis-wired in the new motor. My bandsaw can pull the blade through a full capacity blank too. Or, it could be wired for 220. That would cut your rated power in half.

[ray hampton]

will you determine what the amps the two motors are using

[Curt Fuller]

Pretty simple explanation ..........http://i207.photobucket.com/albums/bb319/clfuller1_2007/smallhorselargehorse.jpg

[Nate Davey]

Maybe it's a Torque issue, like diesel vs gas engines

[Jim Heikes]

I would say that fuzzy math is the key. Most manufacturers state HP, amps, or watts (HP x amps = watts) BUT they don't reveal the actual efficiency of the motor. Most motors probably are in the 60% efficiency range BUT it varies significantly from motor to motor.

[David DeCristoforo]

"...there may be something mis-wired in the new motor..."

FWIW... s'not Both motors are wired to run on 110v. The new motor runs fine... just not as strong.

[J.D.Redwine]

Did you follow standard electric motor break in procedures?


















(just kidding)

[David E Keller]

D

Although, I must admit that I like Curt's explanation better.

[Dirk Lewis]

old motor had a much higher efficiency rating, thus more usable power?

[Ed Morgano]

Brand name of old and new motors? If the new motor isn't a Leeson or a Baldor motor, it's made in China and that could account for the problem. Are they both Capacitor Start - Capacitor Run motors? (do they have one or two capacitors) A bad capacitor could cause a problem if there are two capacitors on the motor. If it only has one, that is a start capacitor and wouldn't make the motor "run' weak.

[David DeCristoforo]

If the...motor isn't a Leeson or a Baldor...it's made in China..."

Both motors are made in China. Both have two caps. Just to see, I swapped the caps (they are identical) and it made no diff. The thing is, the new motor seems to work fine. It just is not as strong under load. I'm thinking it might be just the ticket for powering a shop built buffing station....

[Bob Hamilton]

A motor that is wired for 220 V will seem to run fine on 120 V but will not have very much power and will get hot. I found that out when I was putting a new motor on my band saw. I had assumed that if it was wired for 220 it would not run at all on 120 and I could not understand why it was so underpowered. When I discovered the problem and switched the wiring it made a world of difference.

Bob

[Rich Greinert]

I agree with Jim. It's fuzzy math.

[David DeCristoforo]

"A motor that is wired for 220 V will seem to run fine on 120 V but will not have very much power..."

Good try. But both of these motors are wired correctly for 110v. The new one has plenty of power... just not as much as the old one. It does not run hot either. Still a mystery!

[Roger Chandler]

David,

the motor on my 14" bandsaw [Tiawanese} went out, I replaced it with a baldor with identical spec............no comparrison......the baldor just has so much more torque and will cut through a piece of wood that the other motor that came with the saw would stall with............American made, baby...........gotta love it!

[Josh Bowman]

David, it comes down to Volts and amps in reality. Craftsman is famous for "peak HP" ratings. Here you have a little motor that produces 5 hp! Well it does, but not in the way we use it. The 5 hp is "break HP" or to say it differently, it's the max amps drawn as the shaft is slowed to a stop (sometime before it burns out). So that creates a unit of measure that is not very useful. But Amp draw or better yet wattage can be used for comparison. These units should give you the average draw of energy as the motor is used. You may also have a defective motor. I'd take it back and swap for a different one and see if it gets better. Or better yet, take it back and re install the old one and never look back.

[David DeCristoforo]

"...take it back and re install the old one..."

Done. The old motor is back in the machine and the new one is going to gather some dust whilst I ponder it's fate... I wonder about the "peak" ratings thing though. I understand how this would apply if I were comparing say a 2HP router to a 2HP table saw motor. But these two motors are pretty much identical so I did not think that would apply. FWIW, the voltage and amperage specs on both are the same.

[phil harold]

Pretty simple explanation ..........http://i207.photobucket.com/albums/bb319/clfuller1_2007/smallhorselargehorse.jpg
Not as simple as that

A Shetland pony can pull twice its weight, while a draft horse can only pull half its weight...

[Craig Matheny]

Made in China could explain a lot lack of quality control for one. There are only so many parts to a motor that can be modified. You checked the caps and swapped them motor to motor so they should be good, my understanding is the power and the torque is created by the magnets and the windings if the wingdings are done with cheaper material or the unit got wound on Friday afternoon then that can be the problem i simple twist in the winding can break the wire. I believe the best advise was return the unit and get a new one see if it iany better if it is not then they are not built internally the same and by looking inside you can not tell.

Good Luck

[Paul Williams]

My bet is on thinner copper wire in the windings of the new motor. Less copper equals more resistance so the same amp reading for two motors with vastly different efficiencies. You could stick a meter across the windings of the two motors and see if the resistance of the mew motor is higher than that of the old.

[Jeff Bratt]

There is a lot of garbage around when it comes to HP ratings - the numbers on routers and shop vacs are all lies. (My webpage (http://home.roadrunner.com/%7Ejeffnann/WoodWorking/Shop/HP/Horsepower.html) documenting all these shenanigans.) However, the ratings on induction motors are generally pretty reliable. And you said the motor nameplate ratings for voltage and amperage were identical. It could be that the old one was just rated conservatively.

The math is actually not fuzzy at all: HP = torque (lb-ft) x speed (RPM) / 5252. I guess it's possible that the motors were different designs (internally), which can affect the torque curves. But that would be unusual for general purpose motors that are used in WW machines. Is there any more info about the different motors from the nameplates? Do you know the manufacturers of the two motors?