1. Originally Posted by Jraef
Simplified Centrifugal Pump rules for electricians:
1) Motor Load = liquid flow
2) Liquid flow = pump outlet pressure - head pressure - restriction (valves, pipe friction , elbows etc.)

So less restriction and/or less head pressure = more flow from the same pump, more flow then = more load on the motor. If a pump was designed INTO a system BASED ON an amount of head pressure and restriction, but you change that or don't connect it, the motor will often overload. The same is true if a pipe breaks by the way.
Simply head times flow is how I usually work it out
In SI units of course...........

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Originally Posted by Besoeker
Simply head times flow is how I usually work it out
In SI units of course...........
And there is the paradox (and in some cases error):

For a centrifugal pump (or blower) the time rate of useful work done by the system (to actually move air or water) is the product of the bulk flow and the resistance (head or back pressure), but the power used by the motor is proportional ONLY to the flow rate. The work done by the motor is that of accelerating the incoming water to the vane speed (moving in a circle) and depends only weakly on the back pressure or head.

(When playing with my parents canister vacuum cleaner as a child I was fascinated that the universal motor would speed up when I blocked the air flow.)

That is why the power used by the motor can be larger than the design value even when the mechanical head is zero (outlet at same height as inlet and pipe flow resistance negligible.)
Last edited by GoldDigger; 03-12-18 at 09:48 PM. Reason: Corrected time value issue.

3. Originally Posted by tom baker
A wise electrician I worked with said "its always an electrical problem", this is what the mechanics would say...when the overloads tripped from the pump being ragged up.
check the settings in your VFD, the ones I work on you set the motor FLA and the VFD won't allow more current than that.
What is the SF, most submersibles are 1.10?
My rule is that if I'm getting paid to fix it, it's an electrical problem, even if I solve it by fixing a mechanical issue...

If I have to eat it, I'll prove that it's a mechanical problem.

4. Originally Posted by GoldDigger
And there is the paradox (and in some cases error):

For a centrifugal pump (or blower) the useful work done by the system (to actually move air or water) is the product of the bulk flow and the resistance (head or back pressure), but the power used by the motor is proportional ONLY to the flow rate. The work done by the motor is that of accelerating the incoming water to the vane speed (moving in a circle) and depends only weakly on the back pressure or head.

(When playing with my parents canister vacuum cleaner as a child I was fascinated that the universal motor would speed up when I blocked the air flow.)

That is why the power used by the motor can be larger than the design value even when the mechanical head is zero (outlet at same height as inlet and pipe flow resistance negligible.)
I'm sure you know at power and work are different things but we'll leave it at that as we are getting off topic.

5. Originally Posted by just the cowboy
To add on, same thing with blowers running open ended without backpressure.
Centrifugal fan has mostly same characteristics as centrifugal pump. pressure doesn't load the motor, volume of air moving does. By putting a restriction in the outlet side you do cause a rise in pressure, but it is the reduced flow that is lessening the load on the blower/motor. Restricting the inlet also will lessen the flow and the load, but will increase vacuum on the inlet side

Originally Posted by Jraef
My rule is that if I'm getting paid to fix it, it's an electrical problem, even if I solve it by fixing a mechanical issue...

If I have to eat it, I'll prove that it's a mechanical problem.
I've had to prove many mechanical problems that the main symptom was tripping motor overloads. Sometimes even have to explain that turning up that motor overload though it does make it trip less often or not at all - is allowing the motor to work harder then it is designed for and can eventually destroy the motor.

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Originally Posted by Besoeker
I'm sure you know at power and work are different things but we'll leave it at that as we are getting off topic.
Fixed it, thank you.

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