SPEED LIMITATIONS BY VFD

[roker]

Hello,

What are the limits of speed increase of an existing pump (2900 rpm 50 hz),
can we increase the flow from 50 to 100 m3/hr?

regards,
roker

 

[alexit]

You say pump but this is really two piece question, pump and motor. You need to confirm with supplier(s) that motor could operate faster using VFD, many have limited bearing so speed could be <5000 rpm max.
Also can pump operated faster? Not cavitate or (if centrifugal as flow seems to indicate) fail due to overspeed if operating faster.

 

[hydrae]

Your biggest obstancal would be the power rating of the motor, its demand increses at the cube of the speed, while flow only increase directly to speed (at the same operating point) also the pressure would increase on the square of the speed. How your system curve responds to the increase in power will determine where on the operating curve the pump will fall into at the higher speeds.
Hydrae

 

[vanstoja]

The cubed increase in motor power with rotating speed cited by hydrae also applies (nearly) to motor current assuming a constant power factor. Since heat input to the motor winding varies as I^2*R where I is current and R is winding resistance, doubling of speed causes winding conductor heat input to increase by a factor of the sixth power of speed. This heat input is often the motor's limiting factor for speed increase. Knowing what the margin is between the winding insulation hotspot temperature at present running speed and the insulation class temperature rating coupled with data on running current versus winding hotspot temperature rise enables an estimate to be made of the allowable increase in electrical line frequency where winding heatup is the controlling factor. The effects of shaft speed increase on rotordynamics of the driveshaft is another possible limitation on speed increases by VFD.

 

[Ashereng]

roker,

Is this an estimate before you try it?

Do you have an opportunity to speed the pump up and measure the flow to see what happens?