Running a pump without a gearbox

[Kokevi]

Hi,

I have a VFD controlled 55 kW motor running a centrifugal pump through a gearbox. The motor runs at 50Hz with the current setup. Not a vary variable duty, almost fixed speed with occasional slight variation (about 5Hz). Someone has suggested removing the gearbox (for a new stream, same arrangement) and running the motor at about 30Hz instead for the same duty.

As much as I would like to remove the gearbox, this sounds a bit too easy to me. Although it is the same hydraulic power required (about 40 kW), the motor duty is quite different. Without the gearbox the motor needs to produce more torque at a slower speed. Can this kind of thing be accommodated by a standard motor and VFD combination?

Thanks

 

[micalbrch]

Yes, generally you can remove the gearbox and couple the motor directly to the pump and slow the motor down to 30 Hz. But for that you need a coupling and perhaps a new pump frame. You can also use a v-belt drive instead of the gearbox. The question for me is: Is it worth that work? The gear does not seem to be that much expensive (1:1.6 ratio, 40 kW). It might be cheaper to leave everything as it is and buy a second gearbox for the new stream.

 

[jraef]

It will depend on the type of pump and the original design intent. If it is a centrifugal pump, then the gearbox might be unnecessary and in fact would be wasteful, but that depends upon whether or not the original design was factoring in the TORQUE at that speed. In a centrfugal pump, power required varies by the cube of the speed. So if you have a 1.67:1 gear box, at 3/5 speed the pump kW requirement will be roughly 1/5th of what it is at full speed. But that ASSUMES that the pump/gearbox/motor design was originally built around the motor running full speed, and someone LATER wanted to reduce the flow by adding the gearbox. Being that it ALSO has a VFD on it, I find that to be unlikely. It's entirely more likely that the pump was designed from the beginning to be using the gear box, and someone added the VFD to trim the speed a little.

So if you need 40kW from the motor AT THE SPEED IT IS RUNNING AT NOW through the gear box, then you likely have a load that requires the same torque as an 67kW direct drive motor at the pump at that speed. The gearbox is increasing torque at the expense of speed, but that VFD will NOT do that, you would have the SAME torque at 30Hz as you do at 50Hz. You will end up with insufficient torque at that speed in my opinion.



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[LionelHutz]

Forget even worrying about torque. The available output power of an induction motor running on a VFD varies in direct proportion to the % of rated speed. So, you will be lowering the motor speed to 60% of rated and that in turn means 60% of rated motor power. Won't work since the pump requires 40kW and the motor would be producing 24kW.

 

[Kokevi]

Thanks to all for the input.

 

[iop995]

Power required w/o gearbox is the same (slightly reduced because gearbox losses removed). You may use same motor but need a vectorial VFD to "impose" motor needed load torque. A V/f=ct VFD can't assure this.