VFD driving motor at slower speed

[Randy2718]

I have a pump controlled by a VFD maintaining a constant flow with feedback from a flow transmitter.
I would like to use a larger pump at a slower speed with the same motor and VFD.
The pump would require the same HP for work done.
Can the motor supply the same HP at a slower speed? Or would it have less torque at the slower speed?
I know I can put a larger motor and VFD on it to make it work but at a huge cost.

 

[Joethemechanic]

I take it you know the curve for the new pump? I'm assuming centrifugal pump.

If you cut the speed, the horsepower is going to fall roughly in proportion.

 

[Besoeker3]

I take it you know the curve for the new pump? I'm assuming centrifugal pump.

Randy2718 says it is constant flow

 

[ActionDave]

I have a pump controlled by a VFD maintaining a constant flow with feedback from a flow transmitter.
I would like to use a larger pump at a slower speed with the same motor and VFD.
The pump would require the same HP for work done.
Can the motor supply the same HP at a slower speed? Or would it have less torque at the slower speed?
I know I can put a larger motor and VFD on it to make it work but at a huge cost.

What kind of pump? Centrifugal?
What kind of motor? How slow?
You can get constant HP from the VFD but if it is a centrifugal pump you are not going to get much flow control below a certain RPM and the motor needs to spin fast enough for the cooling fan to blow air over the windings.

 

[winnie]

Using a larger pump at lower speed and the same horsepower implies higher torque, just as @Joethemechanic describes in post 2.

A VFD can operate a motor at lower speed, but doesn't increase the torque capability of the motor. This means that when you reduce the motor speed you reduce the actual horsepower produced. If you keep the same motor then it won't provide the torque required by the new pump.

If the motor is connected to the pump using a pully or gearbox, then you can do what you wish if you change the gear ratio along with the pump. The motor spins at the same speed, producing the same horsepower, but the pump is operating at lower speed, with the gearbox delivering the necessary higher torque at the reduced speed.

Note that if the original motor is oversized for the load, you may be able to change to a new pump operating at lower speed and higher torque. This is simply using more of the existing (oversized) torque capacity.

-Jon

 

[Jraef]

I think

If you keep the same motor then it won't provide the torque required by the new pump.

I think it would be better said that it might not provide the torque needed by the new pump. Remember, the required flow is the same, but we don’t know what the torque requirement of the new pump is at that flow and head situation, and whether the (likely) reduced speed at that flow will allow the motor to get sufficient cooling. System performance curves are the real story here.