WIND ENERGY SYSTEMS - Cd3wd

Chapter 7—Asynchronous Loads 7–13
Figure 5: Pump characteristics of head, efficiency, and input power as a function of capacity
at constant speed.
head point. The power input is now being used to overcome pumping losses and ultimately
appears as a temperature increase in the water flowing out of the pump.
The actual curve of the input power P m versus the capacity will vary with the specific
speed of the pump. At low specific speeds, P m will increase with capacity. It may peak at
around the maximum efficiency point, as shown in Fig. 5, or it may continue to increase until
the maximum capacity point is reached. At a specific speed of approximately 4000, the pump
power input becomes nearly constant, independent of capacity. At still larger specific speeds,
the pump shaft power may actually decrease with increasing capacity.
Suppose now that our pump is operated at some other speed n 2 . A new head versus
capacity curve will be obtained as shown in Fig. 6. It can be shown that equivalent points on
the two curves are found from the relationships
Q 2
Q 1
= n 2
n 1
(7)
h 2
h 1
= n2 2
n 2 1
(8)
If the efficiency remains the same at equivalent points, the input shaft power variation is
given by
P m2
P m1
= n3 2
n 3 1
(9)
Wind Energy Systems by Dr. Gary L. Johnson November 21, 2001