# Small question regarding a motor-compressor gearbox

Greetings all

I have one question here. There is a centrifugal compressor that is driven by an the electric motor. the speed of the compressor is higher than the speed of the electric motor by using a gearbox with speed increaser.
suppose that the motor speed is 3600 rpm and the compressor speed is 7200 rpm

My understanding is that the input torque when we have such gearbox arrangement is lower than when we have direct motor coupled to the compressor. Because it takes more torque to keep the compressor running on 7200 rpm. The load here is the compressor, so we will supply the torque enough to rotate the motor (that is connected to load) at 3600 rpm ,then double the speed using gearbox. Is that correct ?

## Answers and Replies

Right .Just remember power is torque times RPM. And a perfect gearbox passes all the power; it never raises the power.

• sophiecentaur
sophiecentaur
Science Advisor
Gold Member
2020 Award
when we have direct motor coupled to the compressor
But that would require a different motor design if you wanted the same torque at the compressor. I imagine the motor would be an induction type.
The equation Power = torque times RPM (as in the above post ) is what counts and you have to compare like with like.

CWatters
Science Advisor
Homework Helper
Gold Member
Greetings all

I have one question here. There is a centrifugal compressor that is driven by an the electric motor. the speed of the compressor is higher than the speed of the electric motor by using a gearbox with speed increaser.
suppose that the motor speed is 3600 rpm and the compressor speed is 7200 rpm

My understanding is that the input torque when we have such gearbox arrangement is lower than when we have direct motor coupled to the compressor. Because it takes more torque to keep the compressor running on 7200 rpm. The load here is the compressor, so we will supply the torque enough to rotate the motor (that is connected to load) at 3600 rpm ,then double the speed using gearbox. Is that correct ?
Examples..

With the 1:2 gearbox...if it takes 10 Nm to turn the compressor at 7200rpm then the torque on the 3600rpm motor will be 20 Nm.

If you remove the gearbox and fit a 7200 rpm motor directly to the compressor the motor will need to generate 10Nm.

If you remove the gearbox and fit a 3600 rpm motor directly to the compressor the motor will need to generate less torque but how much depends on how the compressor behaves. If it's linear then it would have to generate 5Nm.

• Aptx4869
Right .Just remember power is torque times RPM. And a perfect gearbox passes all the power; it never raises the power.
Thank you

But that would require a different motor design if you wanted the same torque at the compressor. I imagine the motor would be an induction type.
The equation Power = torque times RPM (as in the above post ) is what counts and you have to compare like with like.
Yes, it will need a motor with different speed. Thank you

Examples..

With the 1:2 gearbox...if it takes 10 Nm to turn the compressor at 7200rpm then the torque on the 3600rpm motor will be 20 Nm.

If you remove the gearbox and fit a 7200 rpm motor directly to the compressor the motor will need to generate 10Nm.

If you remove the gearbox and fit a 3600 rpm motor directly to the compressor the motor will need to generate less torque but how much depends on how the compressor behaves. If it's linear then it would have to generate 5Nm.

So either you couple the motor directly or use a gearbox, power required to drive the compressor will be the same. The only difference will be the torque and speed depending on the gear ratio.

Thank you

• CWatters
sophiecentaur
Science Advisor
Gold Member
2020 Award
Yes, it will need a motor with different speed. Thank you
If you could even obtain one to work at 50 / 60Hz supply frequency.