Answer MET Question 35
Question: Explain the matching of an induction electric motor to a pump
required for main circulating duty, with the aid of pump characteristic
and torque/slip diagrams.
Power drawn
$\displaystyle \small \mathrm{p= \frac{2\pi NT}{60}}$
N = rotation of shaft in RPM
T = Torque (N-m)
Thus the torque required to drive the pump is obtained.
Taking mechanical efficiency of the motor and coupling losses into account, the motor should provide the required torque at a speed closed to the pump driving speed.
The motor is now to be selected with the required speed and torque as well as its operating point should be within the stable speed range with a reasonable margin from stalling torque point.
Answer:
Say, for a given speed the characterstics of a centrifugal pump used for main circulating is obtained as shown above.
Depending
upon the requirement of head, H(pressure) and discharge (Q), The
operating point is selected, If possible, close to the maximum
efficiency point. Say for discharge capacity of 'o-a' in m-cube/min. The
power drawn is 'a-b' KW and head developed as 'a-c' Metres Power drawn
$\displaystyle \small \mathrm{p= \frac{2\pi NT}{60}}$
N = rotation of shaft in RPM
T = Torque (N-m)
Thus the torque required to drive the pump is obtained.
Taking mechanical efficiency of the motor and coupling losses into account, the motor should provide the required torque at a speed closed to the pump driving speed.
The motor is now to be selected with the required speed and torque as well as its operating point should be within the stable speed range with a reasonable margin from stalling torque point.
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