Brochure
104104
6. Pump tests
6.2.6 Power consumption
Distinction is made between measurement of the shaft power P
2
and added
electric power P
1
. The shaft power can best be determined as the product of
measured angular velocity w and the torque on the shaft which is measured
by means of a torque measuring device. The shaft power can alternatively
be measured on the basis of P
1
. However, this implies that the motor char-
acteristic is known. In this case, it is important to be aware that the motor
characteristic changes over time because of bearing wear and due to chang-
es in temperature and voltage.
The power consumption depends on the fluid density. The measured power
consumption is therefore usually corrected so that it applies to a standard
fluid with a density of 1000 kg/m
3
which corresponds to water at 4°C. Head
and flow are independent of the density of the pumped fluid.
6.2.7 Rotational speed
The rotational speed is typically measured by using an optic counter or mag-
netically with a coil around the motor. The rotational speed can alternative-
ly be measured by means of the motor characteristic and measured P
1
. This
method is, however, more uncertain because it is indirect and because the
motor characteristic, as mentioned above, changes over time.
The pump performance is often given for a constant rotational speed. By
means of anity equations, described in section 4.5, the performance can
be converted to another speed. The flow, head and power consumption are
hereby changed but the eciency is not changed considerably if the scaling
of the speed is smaller than ± 20 %.