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PLUMBING CONNECTION
SUMMER 2015
WHERE ARE THE EIGHT POLE CURVES?
T
his article describes how we can use standard
centrifugal pump catalogue curves to select other
running speeds. For example, you may have a low head
application and suspect that an 8 pole operating speed may
suit but the catalogue you have only shows 2 pole and 4 pole
speeds, and there is no range chart for 8 pole speed either.
POLES?
Most centrifugal pumps are driven by simple, rugged
and reliable squirrel cage induction electric motors, which
essentially run at a constant speed. Their speed depends
on the number of poles in the motor and the electric supply
frequency; usually 50Hz or perhaps 60Hz as in the Americas
and some parts of Asia.
For electric motors we are stuck with the typical fixed
speed options as shown below:
AFFINITY LAWS
As discussed in a previous article on variable speed, the
performance of a centrifugal pump at different speeds can
be predicted by the Affinity Laws as per Figure 1.
OUR PUMP EXPERT
RON ASTALL
EXPLAINS THE CONCEPT OF POLE CURVES.
PUMP SCHOOL
RON ASTALL
FIGURE 1
For Change Of Speed N
1
To N
2
New Head (H
2
) = H
1
x
N
1
N
2
2
New Flowrate (Q
2
) = Q
1
x
N
1
N
2
N
1
N
2
3
New Power (P
2
) = P
1
x
FIGURE 2
FIGURE 3
80 L/S at 10m at 8 Pole Speed
Becomes:
Duty “Speeded” Up To 4 Pole:
Select pump for 160L/S @ 40m
80L/S x 2 = 160L/S
10m x 4 = 40m
AVAILABLE MOTOR SPEEDS
Pump Running Speed - RPM
(assuming typical 20 RPM slip)
Number of poles
50Hz supply
frequency
60Hz supply
frequency
2
2980
3580
4
1480
1780
6
980
1180
8
730
880
10
580
700