An887, ac induction Motor Fundamentals

overall system loss. With continuous frictional loss,
the heating of the valve takes place, which brings down
it’s life considerably. The maintenance cost of the
valve adds to the operating cost of the pump. An
increase in the pressure head means higher power
input, which further increases the energy loss.
FIGURE 23:
CHARACTERISTIC OF CENTRIFUGAL PUMP WITH LOAD – WITH AND WITHOUT VFD
With use of the VFD, users can avoid all of the previ-
ously mentioned problems. First, the VFD can adjust
the speed of the pump to a new required speed in order
to get the needed flow. This process is like replacing
the present pump with the new pump having modified
characteristics (as shown in Figure 23). Reduction in
the speed means reduction in the pressure head and
reduction in the power consumption; no frictional loss
and hence no maintenance cost. The difference in
the pressure head (as shown in Figure 23) due to the
operating points of the pump, with and without the VFD,
leads to almost an 85% savings in energy. This
implies that there is no need to over design the pump
and a pump of lower rating can be installed (lower
investment cost). An occasional need for higher flow
can be taken care of by the VFD. Running the pump at
an overrated speed by the field weakening can meet
the higher load requirement.
Load Curve
Pump Curve
Flow (gallon/minute)
180
130
50
0
100
200
Load Curve
Pump Curve
Flow (gallon/minute)
180
130
50
0
100
200
Pump Curve
with VFD
Load Curve with
Throttled Valve
Pres
s
u
re H
e
ad 
(fee
t)
Point
Pres
su
re H
e
ad 
(fee
t)
Required Operating

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2003 Microchip Technology Inc.
DS00887A-page 19