fractional HP motor tester needed

[patmichael@q.com]

I need a tester to measure the HP output and amps drawn by fractional horsepower motors. These motors are for sewing machine collectors who do not want to put an after-market motor on their machines - they want the original rebuilt. I need a device that will allow me to see if the motor is producing the horsepower that is is supposed to, rotating at the right speed, and not drawing too many amps after I put new bearings and brushes in it (or before whenever I can). Sometimes motors just run sluggish - I need to be able to figure out why. I think sometimes the armature or stator are just defective - not enought to stop working but maybe enough to deliver reduced power. If I can figure out what it is I might be able to pull one from another motor to fix it.

I'm sure there must be an off-the-shelf device someplace but I am at a loss as to where to look or even what it would be called. The shafts are always 1/4" in diameter and the motor always 110v AC at around 1/10 to 1/20 HP so that should simplify things. I saw one on eBay several years ago and bid on it but lost. That one allowed for increasing the load on the shaft so it wasn't just spinning freely which would have been perfect, and maybe the only way to measure HP. Can anyone help me by at least pointing me in the right direction? A used device would be best as a new one would probably cost an arm and a leg.

Pat

 

[BrownOut]

You can use a DC generator, attached to the output shaft of the motor, and high wattage resistors to put the load on the motor, then measure the shaft's rotational speed. You can get a rough idea of the output power by using P=V/R^2, where V is the output voltage of the generator, and R is the load resistance. That number will be in watts, then use a conversion factor ( it's something like 750W/HP ) to calculate HP, or whatever.

 

[Reloadron]

The simple approach BrownOut suggest is likely the least expensive. Really depends on how accurate you want to get. To load the motor you could get a brake unit either small magnetic brake where the load can be adjusted or a small disk brake assembly. The torque could be measured by a rotary torque transducer and a good rotary torque transducer will have a provision for a tachometer speed pickup for rotational speed. Also, for working on small motors a really slick tool for checking and testing armatures is called a "Growler". Google "Armature Testing Growler" for some interesting results. You may be able to find a used on on Ebay for the cheap. For working on small motors the Growler could be your new best friend.

Ron

 

[BrownOut]

Hi Ron,

I used a growler during one summer I spent working in my father's friend's starter rebuild shop. Odd piece of gear that things is.

 

[Reloadron]

Hi BrownOut, yeah, they are a slick little device. I wish the hell the many times I had a chance to snag one from scrap I did. Seeing what they charge for a new shiny one today.

Ron

 

[patmichael@q.com]

BrownOut,
Thanks but that is way too technical for me. I am math-challenged so need something that does that part of the work for me. The armature growler suggested by Ron sounds like the place to start.

Pat

 

[patmichael@q.com]

Ron,
Thanks for the suggestions. I have some homework to do Googling armature growler, magnetic brake, and rotary torque transducer. A few hours on Google should give me what I need to know. Nothing on eBay at the moment but will keep checking. Actually, it looks like something new might not cost as much as I was afraid of. It looks like the technology is actually pretty simple (except to me). The challenge is finding something that will give me all the info I need in one unit or a couple things I can put together.

 

[KeepItSimpleStupid]

I did a tension controlller once using a magnetic clutch. The only thing I didn't do was make the current able to go slightly negative.

Voltmeters are easy.
Tachometers are easy.
A voltage source can be a VARIAC.
Electronic fuses are probably more difficult.
A light bulb in series can also act as your first unloaded test.
The magnetic brake loads the motor. You can indirectly measure torque by knowing the current through the brake.

Now all of that is a manual piece.

Just something to think about.

 

[shortbus=]

Sometimes motors just run sluggish - I need to be able to figure out why. Pat

A lot of old motors, especially sewing machine and desk fans, were lubricated with 3-in-1 oil or something similar. If it is used regularly no problem, if the motor sits idle for any real length of time (think an antique) there is a problem. 3-in-1 oil and similar brands, oxidize over time and actually get sticky. Before taking a motor like this apart try putting a little acetone or denatured alcohol in the bearings and letting it sit for awhile. If the motor shaft turns easier then throughly clean and re-lube the bearings and see how it works.

 

[duffy]

^ I think you mean WD-40. That stuff turns to something like rubber cement after a couple of years, I've seen it.

 

[shortbus=]

@duffy - no, 3-in-1 is what I was talking about. Didn't have WD-40 back in the day. The 3-in-1 gets gummy too.

 

[Reloadron]

History 101:

Created in 1894, 3-IN-ONE Oil has seen its can of oil "morph" throughout its 106-year history—from a glass bottle in the late 19th century, to the familiar red and white tin can around World War II, to the recent introduction of a high-tech plastic Telescoping Spout.

WD-40 Company, manufacturer of 3-IN-ONE Oil, is calling on consumers to ransack their homes for the oldest can or bottle of 3-IN-ONE Oil they can find. The winner of the contest—that is, the person whose bottle is deemed the oldest—will receive a $1,000 cash reward and a Dremel prize package that includes a Dremel Professional Tool, Right Angle Attachment, Auto/Cycle Repair Kit and Home Improvement Kit. The second place winner will receive a Dremel MultiPro Kit and Right Angle Attachment, while third place will receive a Dremel MultiPro Kit.

Every household had 3 in 1 oil, it was like a rule.

Ron