What makes a motor "Compressor Duty"?

[John McClanahan]

Recently I was comparing 3 HP single phase motors for a possible replacement for my table saw. My current motor is a 1.5 HP Baldor, rated for hazardous locations. By looking at it, it is obvious that it can get wet while running without causing any problems.

So I'm comparing and I can see where "open frame, drip proof" gets its rating. Then I run onto some "capacitor start, capacitor run". They use the start winding and a capacitor to give the motor more torque, but its still rated 3 HP?:confused:

Then I start finding "compressor rated" or "compressor duty" that look the same as the open frame, drip proof model.:cool:

And what about "Farm Duty"? I found plenty of them at TSC.:D

John

[Carroll Courtney]

John,that's a very good question so I'm going to guess and I really don't know for sure.But I would say slower rpms and since the motor has to start under a load due to air in the tank which will need the extra torque is the purpose of the two capacitors(start/run)
Farm duty is what I think is for dusty environments-Where tumble weeds roll:D

[David Kumm]

Farm duty motors are single phase, generally a little cheaper, and according to my motor guy, a better buy than regular single phase counterparts. The leeson ones are red as opposed to blue. Dave

[johnny means]

Compressor duty motors are more robustly built than non-compressor duty. The idea is that a conpressor motor is never running under a no load situation, from start-up to shut-off it is constantly under load. Also, they are often required to run for extended periods under these circumstances. It's my understanding that most motors that aren't compressor duty are junk that aren't meant for any real work.

[Gary Curtis]

Compressors have unusual demands for the motor. As Johnny said, the motor never idles or runs without a load. So heat becomes the enemy. Most compressors have a belt wheel with built-in fan blades to cool the compressor head. In summer months, it's not a bad idea to place a small portable fan aimed at the motor and the air pump.

My present compressor for the shop is a 4hp 220v model. I nearly burned out a 110 compressor painting cars. I like the idea of the 220, but even though it draws less amps, it is the wattage of the motor that determines how it much heat is developed. My saw has a 220v, 5hp Baldor, but I'm kind of gear head and like bigger stuff. A 10inch saw will run fine on 3hp, 110v. Dust is the culprit that kills saw motors. And most saws (not the European models) lack a dust shroud around the blade. So at a minimum, you want TEFC ratings. Totally, Enclosed, Fan, Cooled.

Do a Google search for Electric Motor ratings to get more info. It isn't complicated.

[Chris Mahmood]

"Compressor duty" is a marketing term so it can mean anything but probably it means the motor has a NEMA rating of C or D. It is designed with a higher starting torque, maybe a specialized pull-up torque curve, and a heavier start winding. Not something you'd need on a table saw but necessary for something like a compressor or pump that doesn't have some sort of clutch.

The start winding and capacitor don't really have anything to do power of the motor, they exist because a single phase stator doesn't produce a rotating magnetic field like a 3 phase one so a separate winding shifted 90deg provides a temporary second phase to get the rotor up to speed. A single phase motor designed for high starting torque would need this winding to be heavier to handle the additional heat caused by the starting load. Likewise for the duty cycle. If a motor is rated for continuous use it just means that once the entire motor has heated up to it's normal operating temperature the insulating components will remain intact indefinitely. Otherwise it was designed to a limited duty cycle (e.g., 60 minutes on, 10 minutes off). If normally you run your table saw for 60 minutes at a time then you probably want to spring for a continuous duty motor, otherwise it's just an extra expense. A limited duty cycle motor isn't necessarily inferior to a continuous one, they are just designed for different applications. For example, I think large crane motors operate with something like a 30-40% duty cycle and there's nothing cheap about those.

I've never figured out what "farm duty" meant. Maybe they use lower spec. bearings or something since the motor is expected to be used hard and not have a long life?

[Larry Edgerton]

Was not sure about all the ratings so when my brother an I built a compressor for our auto shop/cabinet shop[shared compressor] we used a oversize pump with the appropriate size compessor duty motor, and then we ran the pump at half speed by changing pulleys. Never had a lack of air and the motor is on about 15 years of hard use now. Had to replace the compressor head once.

I really should find out more about electric motors being as I have so many.

[Frank Drew]

Dust is the culprit that kills saw motors. And most saws (not the European models) lack a dust shroud around the blade. So at a minimum, you want TEFC ratings. Totally, Enclosed, Fan, Cooled.



Exactly; for a table saw, particularly, you want a dust-proof (TEFC) motor; I had a drip-proof motor for my jointer, and it was never a problem, but table saws produce much more dust and it's all around the motor.

[Mike Schuch]

I have an open frame drip proof 3hp motor on my PM65 table saw (A previous owner put it on as a replacement for a 3 phase motor). Every couple of years it will stop functioning and I have to take the plates off and give it a really good blow out with compressed air and then it will continue to chug along for another couple of years.

So my vote is also for a TEFC motor for a saw. Much better than using an open frame compressor duty motor in the dust box... uh, cabinet.

[Gary Curtis]

If the OP has a 1.5hp Baldor to be replaced, that is a high dollar item. Sell it on eBay. I see Baldor motors all the time on the Bay. I think you or someone here mentioned a Leeson as a replacement.

That's a highly regarded brand, as well. But Baldor is pure gold. Or call machinery dealers in a big city near you and see if you can swap out the motor. Those folks always have customers who want something other than what the OEM provided — a bigger motor, a smaller motor, a 3 phase instead of a 1 phase — so they have ready supply of used motors with no use on them at all.

[Chip Lindley]

Recently I was comparing 3 HP single phase motors for a possible replacement for my table saw. My current motor is a 1.5 HP Baldor, rated for hazardous locations. By looking at it, it is obvious that it can get wet while running without causing any problems.

So I'm comparing and I can see where "open frame, drip proof" gets its rating. Then I run onto some "capacitor start, capacitor run". They use the start winding and a capacitor to give the motor more torque, but its still rated 3 HP?:confused:

Then I start finding "compressor rated" or "compressor duty" that look the same as the open frame, drip proof model.:cool:

And what about "Farm Duty"? I found plenty of them at TSC.:D

John

John, look for a TEFC (totally enclosed fan cooled) 3450 rpm motor with a "cont" (continuous) time rating and you will be good to go. ODP (open drip proof) motors are not for the dusty conditions on a TS. You do not need a "compressor duty" motor with start and run capacitors because a TS does not start under a heavy load. But, it certainly WILL work for a TS. Most "farm duty" motors are TEFC, 1725 rpm--wrong speed for a TS. There are also "saw duty" motors which are usually 3-phase, but will most obviously work for a TS ;-)


...A 10inch saw will run fine on 3hp, 110v...

I am not quite sure what your line of thinking is here, unless you were "blessed" with a big 3hp motor built to run only on [actually] 120v. Such special motors DO exist. But, a 3hp motor on 120v will draw around 30 amps. A special, dedicated 40A circuit will have to be run with #8 wire to accommodate such an amp draw. Cut the amps in half (15A) by switching the motor to 240V and you can use #10 wire on a 20A circuit.

Actually, if the run is not overly long, the 240v 20A circuit will work fine on #12 wire, and be within most Codes, but I don't like to scrimp on any of my wiring.

[Curt Harms]

I am not quite sure what your line of thinking is here, unless you were "blessed" with a big 3hp motor built to run only on [actually] 120v. Such special motors DO exist. But, a 3hp motor on 120v will draw around 30 amps. A special, dedicated 40A circuit will have to be run with #8 wire to accommodate such an amp draw. Cut the amps in half (15A) by switching the motor to 240V and you can use #10 wire on a 20A circuit.

Actually, if the run is not overly long, the 240v 20A circuit will work fine on #12 wire, and be within most Codes, but I don't like to scrimp on any of my wiring.

Man, you need to upgrade your motor supplier. I had a Craftsman table saw years ago and it said 3 H.P. in big bold letters on the front. It only needed 13 amps @120 volts to produce those 3 h.p. if I recall correctly.

:D :D

To the O.P., is this saw a contractor saw with the motor hanging out the back? If so, be sure a larger heavier motor won't cause the motor mount to twist or distort when tilting the blade. If not, never mind.

[Bruce Wrenn]

Man, you need to upgrade your motor supplier. I had a Craftsman table saw years ago and it said 3 H.P. in big bold letters on the front. It only needed 13 amps @120 volts to produce those 3 h.p. if I recall correctly.

:D :D

To the O.P., is this saw a contractor saw with the motor hanging out the back? If so, be sure a larger heavier motor won't cause the motor mount to twist or distort when tilting the blade. If not, never mind.13 amps X 120 volts=1560 watts. 1560 watts divided by 746 watts=2.09 HP Now that is at 100% efficiency, which means no heat loss, friction loss, etc. Ain't no way, not any day.

[Chip Lindley]

Man, you need to upgrade your motor supplier. I had a Craftsman table saw years ago and it said 3 H.P. in big bold letters on the front. It only needed 13 amps @120 volts to produce those 3 h.p. if I recall correctly.

:D :D



Curt, if you honestly believe you had a 3 H.P. motor there, you also believe your Sears Shop Vac is 6 H.P. !! :D :D

[Larry Edgerton]

I have been thinking of taking the 1 1/2hp motor out of my contractors saw and installing the 6hp motor out of my Sears vacuum..............

[Mike Cruz]

I was wondering how long it would take for this thread to get silly...

[Mike Cruz]

Interesting about what compressor duty means, and the explanations for it. I would have thought that compressor duty meant that it wasn't built to run for long periods, but rather shorter (3-5 minute) bursts. I've noticed that my compressor motor gets a bit hot. I'd say, hotter than my non compressor duty motors. The explanation of immediate torque makes sense.

Funny, though. If compressor duty means that it is built stronger, why are compressor duty motors often less expensive?

[Curt Harms]

13 amps X 120 volts=1560 watts. 1560 watts divided by 746 watts=2.09 HP Now that is at 100% efficiency, which means no heat loss, friction loss, etc. Ain't no way, not any day.

What, you don't believe in magic? :D :D

[Curt Harms]

Curt, if you honestly believe you had a 3 H.P. motor there, you also believe your Sears Shop Vac is 6 H.P. !! :D :D

It's NOT??!! :eek:

:D :D

[Josiah Bartlett]

Compressor motors generally aren't even open, drip proof level of sealed.

They can be rated continuous, mine is. I run a 5 hp WEG compressor duty motor on my Quincy compressor. The compressor unit is rated continuous. NEMA rated motors have a "Service Factor" on them, anything >=1 means continuous rated at rated horsepower.

[Mike Henderson]

Here's (http://www.about-air-compressors.com/dutycycle.html)on explanation of "compressor duty". According to this web page, compressor duty motors need to have shut down periods between use, in order to cool down. If run continuously, they may get too hot, which will decrease the life of the motor.

Regarding "Service Factor": If you had a 1HP motor with a service factor of 1.2, you could load that motor to 1.2HP and the temperature of the motor would increase by 10 degrees Centigrade above it's rated temperature. Rated temperature is expressed as temperature rise above ambient, which is assumed to be 40 degrees C. So if the motor was rated for operating at 85 degrees Centigrade at full load (ambient plus 45*), it would operate at 95 degrees Centigrade when operating at it's service factor level. However, operating a motor at 10 degrees Centigrade above it's rated temperature will decrease the life by half.

Service factor is different than duty cycle and you still have to respect the duty cycle, no matter what the service factor.

Mike

[The only thing I could find on "farm duty" is that those motors have a higher starting torque.]

[Mike Cruz]

Mike, while the many other explanations seemed plausable, your post just confirmed what I had originally thought. Not sure why I had thought that, maybe because I had read it somewhere, but for some reason, compressor duty nagged in the back of my head that the motor needed "shut down" time. That said, the fact that compressors start up under load (unlike most other motors), might have something to do with it, too. But two things: Take a look at how our compressors run...on (for a few minutes directly under load) then off, then on, then off. Hence, compressor duty would suggest a motor that is frequently on and off (maybe by need for the life of the motor) and under constant torque. And secondly, MOST home shops could probably use compressor duty motors in all their machines. I mean, most of our machines are turned on, then off, then on, then off. Professional shops may run planers, jointer, and table saws for 30 or more minutes at a time, but I think most hobby shops don't.

[Joe Jensen]

Mike is right. Compressor duty is designed for more frequent starts but they don't run continuously. Compressors have unloader valves so the motor does not start under load. Farm duty motors are designed to start under load, say a grain auger full of grain. It starts and stops with grain in it. Tough for a motor.