Need some advice On Electrical Motore AC

Thread Starter

Mr Babu Mistry

Joined May 8, 2018
17
Hi Friends,
Need some advice On Electrical Motore AC.
On my compressor, the motor size is 7.5 KW RPM 1450. Three-phase. I was thinking of buying ba single-phase 240v two poll AC motor 7.5KW RPM 2880.
now I am getting is a Motor 5.5KW RPM 1450.
What do you suggest to me on this Motor 5.5KW 1450 will it work for my compressor. and which one is more durable.
Thank's.
Babu.
 

MaxHeadRoom

Joined Jul 18, 2013
21,629
The 3 phase motor is the better choice.
1450 will be the operating rpm for your compressor, it is not always wise to double the RPM, if it is not built for that size.
Obviously it seems that the compressor was made for the 10hp motor.
What is the reason for going 1ph 7.5hp when you have 3ph available?
Max.
 
My guess is you would have been better with the 2880 RPM other than having to figure out pulleys to get the RPM where the compressor turns at the right speed. 7.5 KW is a lot more power than 5.5 and your motor will probably strain as your pressure builds and eventually burn out the motor over time.
 

Thread Starter

Mr Babu Mistry

Joined May 8, 2018
17
My guess is you would have been better with the 2880 RPM other than having to figure out pulleys to get the RPM where the compressor turns at the right speed. 7.5 KW is a lot more power than 5.5 and your motor will probably strain as your pressure builds and eventually burn out the motor over time.
Thanks.
 
Actually Max has the better answer. It's possible you may have to upgrade wiring and such to convert to a single phase motor. Pretty much just meant to say you want the same power output as you originally start out with. The compressor design may have a little more motor power than really needed, but probably not 2 kW worth. You could probably change the pulley ratio and get away with the smaller motor, but you will loose compressor output. I'm thinking with this size motor to start with you don't have the time to wait for the tank to fill again.
 

shortbus

Joined Sep 30, 2009
8,048
Another problem with changing motor speed or gearing on compressors, is what type pump/compressor is it? One stage or more? The number of cylinders doesn't mean anything in this case, it's the diameter difference between cylinders. Many 2 cylinder pumps have the same diameter cylinders so they are single stage, and run faster. Some bigger(5HP and over) are two stage, a big diameter cylinder pumping into a smaller cylinder, to raise the pressure, they tend to use a slower RPM.
 
Be wary of reducing pump speeds to compensate for a smaller motor power rating. Air compressor pumps are designed to operate within a very particular range of speeds. Especially in the case of splash lubricated pumps, running them too slowly can starve the pistons, cylinder walls, wrist pins, crank journals, main bearings, etc. of oil. Look up the spec sheet for your pump before installing a smaller motor.

Likewise, running them too fast will cause excessive wear. Remember, piston pumps have reciprocating mass. A small increase in RPMs will have a profound effect on the internal forces at play. It will produce a lot more air... right up until it starts knocking and slapping.
 
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