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

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?

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.

will you determine what the amps the two motors are using

Pretty simple explanation ..........http://i207.photobucket.com/albums/b...largehorse.jpg

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

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.

"...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.

Did you follow standard electric motor break in procedures?


















(just kidding)

D

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

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

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.

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....

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

I agree with Jim. It's fuzzy math.

"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!