Hi, L have a question about the effect of changing the speed of motor that drive a centrifugal pump during the operation on the motor current and power
Usually this question is asked in relation to using a Variable Speed Drive where you are changing the actual motor speed. On a true centrifugal pump, power consumption varies by the cube of the flow, and flow follows speed for the most part. So for example at 60% speed, the power will be .60 3 that of consumption at full speed, or 21.6%. But it really depends on the pump performance curve and system head too, because some pump systems may not pump at all at 60% speed.
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As stated, the pump laws:
BHP~N^3
DP~N^2
Q~N
... describe the change in the pump curve only (not the operating point). They provide a 1:1 mapping from a point on one pump curve at speed N1 to another pump curve at another speed N2.... so you can construct one curve from the other.
If you think about it, the point that maps from curve 1 at speed N1 to curve 2 at speed N2 must also obeys the relationship (DP2/DP1)~(Q1/Q2)^2.
We can compare this to system characteristic curves. For a closed loop system, the curve may often be approximated as DP~Q^m. m=2 for turbulent flow, lower for laminar flow.
If m=2 exactly, then the mapping given by pump laws also matches the system characteristic curve, and we can predict change in operating point also as
BHP~N^3
DP~N^2
Q~N
If m<2 (more laminar flow), and we are scaling speed up (N2>N1), then the system curve will intersect the new pump curve at higher flow, lower dp (below and to the right on DP vs Q curve). So Q will be somewhat higher than predicted by Q~N and DP will be somewhat lower than as predicted by DP~N^2.
If you have a system which responds to change in flow (like temperature-control throttle valves in a cooling water loop), that is another thing to be considered.
The same rules apply if you wish to increase the speed too: power increases cubed and current/torque will increase squared until you reach the limitations of the motor/VFD(if used) and protection of your feed equipment.
