Electric motor RPM vrs Load

[wment]

I replaced a 1725 rpm motor on a compressor with a 3hp 3450rpm 220vac single phase motor. I am trying to determine the pully size. I want to spin the compressor as fast as possible. As the load increases on the motor (larger pully) the rpm drops. What is the lowest RPM I can run the motor at without damage?

 

[DanDel]

Check the nameplate rating. If it says 3450RPM, then running speed below that will cause damage from overheating. Check the current draw as the motor speed reduces under load, you will see what I mean.
Motors have a built-in amount of slip and the nameplate RPM is based on the synchronous speed and the slip. The only way to change the speed significantly is by using a VSD.

 

[electricpete]

I would say it's better not to look at it as setting the motor's speed, but rather setting the compressor's speed.

The motor will run fairly close to it's syncronous speed, and the load will run higher by an amount determined by the pulley ratio. In most cases the mechanical load drawn by the load is dependent upon it's speed.

You would normally be safe setting pulley ratio in proportion to nameplate ratio of motor and compressor, assuming your motor horsepower equals or exceeds compressor horsepower at that speed.

IF you try to speed the load up further, you may overload the motor and also may have mechanical problems from overspeeding.

 

[GusD]

Hi WMENT

The motor nameplate indicates the motor RPM at full load.
If compressor load causes motor RPM to drop below the nameplate RPM, motor will overheat quite rapidly and the lower the speed the more heat you create in the motor/rotor.
I don't think you can go a lot lower than rated speed.
the rated slip at full load is approx 4.2 %.If you lower the RPM to 3400 rpm,for example, the slip will be 5.5 % slip.
MG10 says :the higher the slip,the lower efficiency,slip is a measure of the rotor losses. May not seem like a large speed drop,but the amps and heat will climb to very rapidly, inversely to motor speed. Any motor operation below its rated speed will stress motor /rotor to failure.

GusD

 

[jbartos]

Suggestion to wment (Electrical) May 16, 2003 marked ///\\I replaced a 1725 rpm motor
///Of which HP?\\ on a compressor with a 3hp 3450rpm 220vac single phase motor. I am trying to determine the pully size. I want to spin the compressor as fast as possible.
///Compressor data sheet or nameplate data is needed for the RPM design basis.\\ As the load increases on the motor (larger pully) the rpm drops.
///Only marginally on the induction motor. The slip is limited to several percents, depending on the motor NEMA Design letter.\\ What is the lowest RPM I can run the motor at without damage?
///The lower RPM are most effectively determined by the clamp on ammeter. The line current to the motor terminals should not read higher amperes in each line than the motor nameplate indicates. Preferably, some design margin should be there, e.g. 5 to 10%. Alternately, all may be calculated, analyzed and simulated by a suitable software.\\

 

[mdonner]

Raising reciprocating compressor's speed induces more mechanical stress on motion conversion parts.
Increased air/gas flow increases head pressure and compression ratio, thus operating temperature. None of these should exceed mfg's data sheet values.
Increased flow and pressure and decrease volumetric efficiency increases power demand from motor.

 

[wment]

Thanks for all the good suggestions. The compressor is rated for a 5 to 7.5 hp motor so I dont think I can run the compressor too fast with the 3hp motor. I will check the rpm and current draw under load and increase the diameter of the motor pully while trying to keep the rpm close to 3450 and not drawing more current than the rating on the spec plate.

 

[jbartos]

Suggestion to wment (Electrical) May 17, 2003 marked ///\\The compressor is rated for a 5 to 7.5 hp motor so I dont think I can run the compressor too fast with the 3hp motor.
///It depends. If the compressor is compressing to its rated PSI then you are correct; however, if the compressor is compressing to lower PSI, then it could potentially be harmed by RPM in excess of its rated RPM. See the postings above. There may even be increased mechanical vibrations.\\ I will check the rpm and current draw under load and increase the diameter of the motor pully while trying to keep the rpm close to 3450
///Good idea.\\ and not drawing more current than the rating on the spec plate.
///Good idea.\\

 

[bmodrow]

wment horsepower does not equate to motor RPM the change you are making is going to increase system RPM by a factor of two. The main concern you have in determining your pulley sizes is the maximum RPM that the compressor is capable of handling without damaging the compressor.

Your drive system must be designed with this in mind first. Then due a load test(current draw)on your 3hp 3450 RPM to ensure the motor can handle the load.