Motor sizing

[magoo5150]

Currently I have several centrifugal pumps controling and oil system. All the pumps have 30 hp motors with a max amp draw of 20 amps. Without knowing pump curves (older pumps) how can I tell wether I am oversized on the motors and how would I calculate that. Any help would be greatly appreciated.

Thanks

 

[bob]

Currently I have several centrifugal pumps controling and oil system. All the pumps have 30 hp motors with a max amp draw of 20 amps. Without knowing pump curves (older pumps) how can I tell wether I am oversized on the motors and how would I calculate that. Any help would be greatly appreciated.

Thanks

FLA 30 HP 460 volts = 40 amps 575 volts = 32 amps.(NEC 430.250) Check your name plate and see what your motor has for FLA.
From 430.250 a 20 hp 460 v = 27 amps and 25 hp = 34 amps

 

[Jraef]

Currently I have several centrifugal pumps controling and oil system. All the pumps have 30 hp motors with a max amp draw of 20 amps. Without knowing pump curves (older pumps) how can I tell wether I am oversized on the motors and how would I calculate that. Any help would be greatly appreciated.

Thanks

But what is your concern about being over sized? An AC motor is only going to draw as much current as the load demands. There might be a slight improvement in efficiency to lower the motor size, but given the cost of replacing the motors you would be hard pressed to realize a net savings in a reasonable amount of time unless those motors are running 24/7.

Some utilities still offer rebates for changing to energy efficient motors, which helps improve that ROI. If that's what you were considering doing, I would not try to second guess the original design engineer by assuming you can use a smaller motor. I have rarely seen that turn out well in the long run. A better approach would be to leave the motors as is and use a VFD if your goal is to reduce energy costs. It depends upon whether or not there is some sort of restrictive valve being used to control flow that you can eliminate by using variable speed instead. If there is, typical paybacks on that are 1-1/2 to 2 years, something the bean counters like to see. If your PoCo offers rebates for energy efficient motors, they probably aslo offer them for VFDs as well, and that can sometimes pay for the hardware.

 

[Besoeker]

But what is your concern about being over sized? An AC motor is only going to draw as much current as the load demands.

I understand your point but that shouldn't that be power rather than current?
Even completely unloaded, the motor would take around 30% of FLC.
Using a smaller motor might reduce losses some. Might.
What it would change to a significant extent is power factor.

 

[Jraef]

I understand your point but that shouldn't that be power rather than current?

Yes, I meant power. I was just in another discussion about current (another one of those "energy saver" scams where current is reduced so they think the power is too) and I failed to disengage my brain completely.
Thanks.