Electric Motors

[theoldwizard1]

I am just interested in your basic single phase, induction motor. What is typically found on stationary power tools like drill presses, table saws and compressors.

• How do manufacturers "get away with" rating motors at ridiculously high HP My shop vac is rated at 7.5 HP but it is clearly much smaller than my table saw motor.
• Do statements like "Compressor Duty" or "Farm Duty" actually mean anything, or are they just marketing terms. How about SPL (special?)

Beside Baldor/ABB any other recommended brands ?

 

[WillyBoy]

If I recall correctly, Sears started this decades ago with their motors. Essentially, lock the shaft on the motor and measure the amps it's drawing just before the motor lets out the magic smoke. Then convert the numbers to "DEVELOPS" ** amount of horsepower based on the amp draw. No laws on the books seem to be violated, just misleading numbers.
Possibly your shop vac manufacturer is using the method.

As far as "compressor duty" I've heard that it refers to motors that are intended to run intermittently, not continuously.
I'm sure that there are folks ready to post that know more about it than me.

 

[djbmw]

I am just interested in your basic single phase, induction motor. What is typically found on stationary power tools like drill presses, table saws and compressors.

• How do manufacturers "get away with" rating motors at ridiculously high HP My shop vac is rated at 7.5 HP but it is clearly much smaller than my table saw motor.
• Do statements like "Compressor Duty" or "Farm Duty" actually mean anything, or are they just marketing terms. How about SPL (special?)

Beside Baldor/ABB any other recommended brands ?

The same way that audio amplifier manufacturers get away with nutty wattage ratings. 10,000 watts at 2 ohms in stereo for $200!

We used to joke that ratings lile these were only valid "when struck by lightning".

I find motors to generally be closer to spec than audio amplifiers though...

As for duty cycles, the contruction of the motor can tell you more about its intended use than some labels can. But, as Im sure you know, continuous use is intended to be ran without a worry in the world about cycling on/off. For others that are duty cycled, a lot has to do with heat and start capacitor ratings (and number of cycles they can see in a given period before failing).

 

[micromind]

Basically, there's running HP and peak HP. Running is what the motor can produce continuously, peak is the maximum it can produce anywhere in its power curve. It can produce peak HP for a short time then it'll overheat and burn up.

If the HP is listed as SPL, then it is a chintzy motor that doesn't list the actual HP because the marketing department told them no to.

Farm duty means it has bearing grease that can withstand very cold temperatures plus it had shaft seals that are better at keeping moisture out.

Compressor duty usually means that they are lying about its HP, plus it's built as cheaply as possible.

Elevator duty means it can start more often than normal without damage.

Crusher duty means it has higher starting torque than normal.

Fan and blower duty means it has lower starting torque than normal.

2 basic types of motors, NEMA (National Electrical Manufacturers Association) and IEC (international Electromechanical Commission). Nema motors are built like tanks, tough and reliable and can take some serious abuse. IEC are built to the lowest possible standards and barely tolerate normal use, much less any kind of abuse. NEMA =inches, IEC = metric.

A good part of NEMA motors will have a service factor of more than 1.0 Basically, if a 10HP motor has a service factor of 1.15, it is actually an 11.5HP motor sold as 10HP. Most IEC motors don't have service factors because it's always 1.0

Baldor is a good brand, I like Leeson best, those are what I normally spec.

 

[sz0k30]

Hey Micromind, great writeup! Thanks.

 

[mm08822]

There are numerous threads regarding this marketing deception and class action lawsuits circa 2004.

Personally, I won't buy any motor unless it has a SF= 1.15. I always check FLA's against nameplate HP.

One of the claims of compressor duty motors was their supposed starting torque when the motor started under pressure.
With most (all?) compressors having some form of unloader valve, I call BS because I have never seen a compressor start at cut-in pressure with a failed unloader.

Never buy a compressor based on tank size and hp. Use SCFM values @ X psi. This is the working deliverable. The hp required is dependant on the scfm. Tank size is secondary at best IMO.

 

[Bert_]

Mostly just products marketed directly to consumers are lying about the rating.

I deal with baldor a lot. They are good but not perfect. Weg makes a good motor for a little less money.

 

[Firebrick43]

You forgot inverter duty! But great job with the explanation.

 

[mm08822]

You forgot inverter duty! But great job with the explanation.

OP was focused on single phase motors.

 

[fitter30]

Imported motors use new math for hp rating or they use ponies not horses. Marketing bottom line. Average consumer/ homeowner looks at hp rating for a comparison.

 

[fitter30]

That

There are numerous threads regarding this marketing deception and class action lawsuits circa 2004.

Personally, I won't buy any motor unless it has a SF= 1.15. I always check FLA's against nameplate HP.

One of the claims of compressor duty motors was their supposed starting torque when the motor started under pressure.
With most (all?) compressors having some form of unloader valve, I call BS because I have never seen a compressor start at cut-in pressure with a failed unloader.

Never buy a compressor based on tank size and hp. Use SCFM values @ X psi. This is the working deliverable. The hp required is dependant on the scfm. Tank size is secondary at best IMO.

That's why they have a cast Iron flywheel to help with rotation at compression.

 

[mm08822]

That

That's why they have a cast Iron flywheel to help with rotation at compression.

When running sure, but from standstill, it is just more inertia the motor has to overcome to get moving and then up to speed.

 

[mm08822]

Imported motors use new math for hp rating or they use ponies not horses. Marketing bottom line. Average consumer/ homeowner looks at hp rating for a comparison.

Having Metric units on the nameplate instead of imperial units is a poor excuse to not know what you're buying.

They use kw ratings instead of HP. 1HP =~ .75 kw. There are multiple duty types as well. All nameplate info. Selecting the correct duty type is equally as important as hp/kw.