Tell me about electrical motors

If you have a low speed compressor, I would recommend going with a 1750 RPM motor, if high speed then one can go with a 2 pole motor. I really question the rating and durability of these inexpensive motors for hard loads like a compressor, Chinese 5Hp in a 143T frame size vs. the Baldor/WEG equivalent 182/184T frame size for the 3450 RPM and 1750 RPM motors. Also note the lower temperature insulation rating. WEG is commonly used for compressors and a bit of a middle road price wise.

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If I was looking for a true 5hp to drive a compressor at full rated output, I wouldn't buy a generic Asian motor, they often "push" the ratings. I'd get a real industrial brand, that's proven reliable and backs up their claims. Also go with 1750 rpm, running the compressor at lower speed will help it last longer and it will be much quieter.
I've had good luck buying motors from theses guys:

Electric Motors | Motor Controls | VFD | AC Drive

dealerselectric.com dealerselectric.com
 
If you are looking for a single phase motor, look for " Farm Duty ". These are just marketing but generally decent build motors for farms because only single phase is available. They tend to be priced a little less and the build is a little better. Dave
 
Eddyde said:
If I was looking for a true 5hp to drive a compressor at full rated output, I wouldn't buy a generic Asian motor, they often "push" the ratings. I'd get a real industrial brand, that's proven reliable and backs up their claims. Also go with 1750 rpm, running the compressor at lower speed will help it last longer and it will be much quieter.
I've had good luck buying motors from theses guys:

Electric Motors | Motor Controls | VFD | AC Drive

dealerselectric.com dealerselectric.com
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I understand the thought but I am not certain I agree. If you run at half speed but it takes twice as long to charge your system your really not saving money, electrically, nor wear and tear. All rotating equipment has a design life cycle, X number of revolutions if you will, before you should have any issues and most will out live their design life cycle; but the cycle is relevant in this discussion, and the design life cycle is based upon the Horse power, torque and RPM the rotating equipment is designed to function at. If the compressor is designed to operate with a 5HP motor at 3450 RPM you are not doing yourself any favors by using a 5HP motor at 1750, in fact you are likely shortening the life of the compressor.
 
Beckerkumm said:
If you are looking for a single phase motor, look for " Farm Duty ". These are just marketing but generally decent build motors for farms because only single phase is available. They tend to be priced a little less and the build is a little better. Dave
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If you are putting the motor on a compressor, you want a motor designed for that application due to the pressure in the cylinder which must be overcome at start up. Farm duty motors are good motors but this application has its own rating for a good reason.
 
While compressor duty motors are designed to meet the needs of compressors and a good choice, I'd not be afraid of a farm duty motor if i got a deal on it. They are designed as high starting torque motors for farm machinery, including compressors. My comparison was more to a normal duty TEFC motor than a compressor duty. I'm a three phase guy so take my advice on single phase with verification.

Running a piston compressor at higher than recommended speed is a quick way to kill it. Most new pumps and valves are as cheaply made as can be sold at a price point and the splash lube systems are not excellent. For longevity there is no substitute for a slow running oversized pump, and a 1800 rpm motor of similar HP has twice the torque of a 3600 rpm as well. Dave
 
Beckerkumm said:
While compressor duty motors are designed to meet the needs of compressors and a good choice, I'd not be afraid of a farm duty motor if i got a deal on it. They are designed as high starting torque motors for farm machinery, including compressors. My comparison was more to a normal duty TEFC motor than a compressor duty. I'm a three phase guy so take my advice on single phase with verification.

Running a piston compressor at higher than recommended speed is a quick way to kill it. Most new pumps and valves are as cheaply made as can be sold at a price point and the splash lube systems are not excellent. For longevity there is no substitute for a slow running oversized pump, and a 1800 rpm motor of similar HP has twice the torque of a 3600 rpm as well. Dave
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I understand and appreciate your point on the farm duty, I have owned a few of them, I would still buy the motor with the correct rating. I am a mechanical guy so i default to the motor and compressor manufacturers recommendations.

I suggest, this was my point in my earlier post, looking at the compressor head tag and matching the motor spec to the head, if it is a 1750RPM head that is what I would buy. I have known guys who stick a 1750 on a 3600 RPM head thinking it will increase the life of the head, It doesn't because the head has to run longer to build up pressure which increases the cycles and in most shop conditions it will be running when it should be idle.

I tend to not over think these things, I look at the forums as a place to have discussions and share knowledge and opinions, and I respect everyones knowledge and opinions, at the end of the day I am not trying to talk anyone into anything, just sharing in hopes I might add some value to their thoughts as I hope everyone can add to mine. :)
 
verbotenwhisky said:
I understand the thought but I am not certain I agree. If you run at half speed but it takes twice as long to charge your system your really not saving money, electrically, nor wear and tear. All rotating equipment has a design life cycle, X number of revolutions if you will, before you should have any issues and most will out live their design life cycle; but the cycle is relevant in this discussion, and the design life cycle is based upon the Horse power, torque and RPM the rotating equipment is designed to function at. If the compressor is designed to operate with a 5HP motor at 3450 RPM you are not doing yourself any favors by using a 5HP motor at 1750, in fact you are likely shortening the life of the compressor.
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Most quality, commercial/industralr grade compressor pumps run at around 600-800 rpm with a 1750 rpm motor. that's why their flywheel/pulley is much larger that the motor pulley. The pumps are designed with larger pistons, to compress more air per stroke using torque rather than speed. Most "big box store" compressors, are 3400 rpm they use smaller, cheaper to manufacture pumps, run at higher speed to achieve their rated output. However, operating the pump high speed, will shorten is life, Heat is the big killer.
It all comes down to what your budget is and what your usage is. Pay less than a grand for a import "rated" at 5 hp or pay 3 grand for a real 5hp compressor that will outlast the cheap one tenfold. But if you are only using it occasionally for light duty work, the cheap one may be the best way to go.
 
Eddyde said:
Most quality, commercial/industralr grade compressor pumps run at around 600-800 rpm with a 1750 rpm motor. that's why their flywheel/pulley is much larger that the motor pulley. The pumps are designed with larger pistons, to compress more air per stroke using torque rather than speed. Most "big box store" compressors, are 3400 rpm they use smaller, cheaper to manufacture pumps, run at higher speed to achieve their rated output. However, operating the pump high speed, will shorten is life, Heat is the big killer.
It all comes down to what your budget is and what your usage is. Pay less than a grand for a import "rated" at 5 hp or pay 3 grand for a real 5hp compressor that will outlast the cheap one tenfold. But if you are only using it occasionally for light duty work, the cheap one may be the best way to go.
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Eddyde,

If the pump comes with the pulley as most do and has a tag which says to use a 3600RPM motor then that is what it is designed for. They are designed to run for x length of time then shut down; so, if you use a 1750RPM motor, you are not doing yourself any favors, heat will be an issue either way, the longer it runs the more heat will build up slowly. PSI is pressure per square inch meaning for every square inch of piston surface area you are dealing with that much force. If the compressor drops to 80 PSI before kicking on and your piston has 2.5 square inches to before it ever turns over it first has to overcome 200 pounds of force and that force will build as the pressure builds until it shuts off. The big heavy pulley is also a flywheel which once moving provides inertia through rotating mass to help over come that force, rotating that mass at a slower than design RPM actually creates additional load and heat. As I said in a previous post, I am not against either the 1750 nor the 3600 RPM motor, I am advocating to use the motor specification the compressor head is designed for.
 
I think, from my experience everyone is sorta right at the same time. First understand this is a 'from scratch build', I got a deal on a new tank and then started sourcing parts to build it into a compressor. I bought a compressor pump, actually a Harbor Freight. That pump is rated at around 900 rpm. The motor, whichever one I end up with, will have a correctly sized pulley added to it so that the compressor does in fact turn at or just below 900rpm. If the motor is a 1750rpm it will get a larger pulley than a motor turning 3450. I would like a lower rpm motor because it does have more torque, and like a low rpm compressor it 'should' last longer than a higher rpm unit. Since I'm building from scratch, I made sure the unloader will remove pressure from the head and about 18" of piping each time the compressor stops. When the pump starts up there should be no pressure to overcome.
 
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