An887, ac induction Motor Fundamentals

VFD as Energy Saver
Let us have a look at the classical case of the centrifu-
gal pump and how the use of the VFD provides the user
the most energy efficient solution at a low cost. Any
centrifugal pump follows the Affinity laws, which are
represented in terms of the curves shown in Figure 22.
FIGURE 22:
TYPICAL CENTRIFUGAL PUMP CHARACTERISTICS
In simple terms, this means that the water flow, head
pressure and power are directly proportional to the
(speed), (speed)
2
and (speed)
3
, respectively. In terms
of mathematical equations, they are represented as:
EQUATION 5:
Flow
Pressure
Power
% Speed
%
100
0
100
3
1
2
1
2
2
1
2
1
2
1
2
1
2
;






=






=
=
Speed
Speed
Power
Power
and
Speed
Speed
Head
Head
Speed
Speed
Flow
Flow
Note: Subscripts (1) and (2) signify two different operating points.

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AN887
DS00887A-page 18

2003 Microchip Technology Inc.
Let us say that the user wants a centrifugal pump for
water flow of 100 gallons/minute for a pressure head of
50 feet continuously and occasionally needs a peak
flow of 200 gallons/minute. The curves of load and
pump are as shown in Figure 23. It can be observed
that for an occasional peak requirement of 200 gallons/
minute, the user is forced to go for an over designed
pump, which means higher investment cost. Also, if
the pump is run directly with supply, without any control
of flow, the pump continuously runs at a speed higher
than required. This translates into more power input to
the pump (Affinity laws) and hence a higher energy
bill. Also, the user does not have any control overflow.
For years, to control flow, the throttle value was imple-
mented. Closing this mechanical part partially, to regu-
late the flow, shifts the operating point to the left of the
curve and increases the pressure head (as shown in
Figure 23). But it adds to the frictional loss and the