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.
Structured for DG Shipping MEO Class II examination scoring criteria.
The pump started with the discharge valve closed, so the pressure head is high with zero flow rate. As the discharge valve is opened, H decreases and Q increases.
Depending upon the requirements of head (H) pressure and discharge quantity (Q), the operating point is selected, if possible close to maximum efficiency point.
For example, the discharge capacity of OA in m3/min, the power drawn is AB in KW and the head developed in AC in meters. Power drawn P = 2πNT, at a given speed N, 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 close to the pump driving speed.
The motor is to be selected with the required speed and torque as well and its operating point should be within stable speed range with a reasonable margin from stalling torque point.