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Tool Talk Discussion Forum

5 hp motor

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MF294-4

12-09-2006 19:53:33




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I see some compressor motors are 1725 rpm and some are 3450 and 3600. The higher rpm motors are cheaper. Is there a disadvantage to the higher rpm. This is 5hp 1ph motors.




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Dick2

12-10-2006 05:53:57




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 Re: 5 hp motor in reply to MF294-4, 12-09-2006 19:53:33  
The slower RPM motor will have a higher starting torque than the high speed motor. If the compressor is in an area where it gets cold (oil gets thicker), you should use the slower RPM motor. 1725 RPM motors have 4 coils and 3450 RPM motors have two coils, which also adds to the starting torque of the 1725 RPM motor. I did electric motor repair many years ago; high speed motors are generally built cheaper. I never liked them and never found many practical applications for them.

BTW, 3600 RPM is theoretical; friction losses reduce RPM to 3450.

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buickanddeere

12-10-2006 13:39:20




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 Re: 5 hp motor in reply to Dick2, 12-10-2006 05:53:57  
Old and Gerald J are correct.



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Gerald J.

12-10-2006 08:35:13




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 Re: 5 hp motor in reply to Dick2, 12-10-2006 05:53:57  
It isn't friction that make an induction motor run slower than synchronous speed. Its a requirement of the motor to induce a current at low frequency into the rotor. That's called slip and its the characteristic of the motor.

A 2 pole motor has the same torque for its speed as a four pole, but the slower speed of the four pole will double the torque to get the same horsepower. Life works that way all the time. When run through a speed reduction for a compressor, either motor will do the same torque at the 900 rpm compressor shaft, but the 2 pole being simpler and using a little less iron will be cheaper every time.

Gerald J.

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old

12-09-2006 21:01:31




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 Re: 5 hp motor in reply to MF294-4, 12-09-2006 19:53:33  
Well lets say you have a compressor brand X. If you use a 1750RPM motor on it then you need say a 14 inch pulley on the compressor and a 2 inch on the motor. Now if you have the same compressor but use a 3450 RPM motor on it then you need say a 20 ich pulley on the compressor and a 1.5 inch pully on the motor. If both motors are of the same HP. So no theres not one betterthen the other other then the faster motor may burn out its bearings faster the the slower one just because of the speed it turns. Just my $0.02 worth

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Gerald J.

12-09-2006 20:44:06




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 Re: 5 hp motor in reply to MF294-4, 12-09-2006 19:53:33  
The higher speed motors have advantages when you need high speed work, but for slow speeds (like 900 RPM for a compressor) you need a bigger ratio of pulleys and sometimes its hard to accomplish the speed reduction in one stage without a 24" low speed pulley.

Gerald J.



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