There are several types of motors that can be used with centrifugal pumps, but the most common driver is the National Electrical Manufacturers Association (NEMA) Design B 3-phase alternating current (AC) squirrel cage induction motor. When 3-phase AC voltage is applied to the windings in the motor stator, the resulting 3-phase AC current creates a rotating magnetic field. As the rotating magnetic field cuts across the squirrel cage rotor, current is induced that creates a magnetic field in the rotor. The interaction of the stator and rotor magnetic fields develops torque on the shaft of the rotor that transmits power to the pump. The difference between the rotating speed of the shaft and the synchronous speed of the stator windings is often called motor slip. For this type of motor, slip is required to develop the torque on the shaft.
Image 1 shows a typical motor performance curve (Motor A) with torque, current and output power as a function of the shaft speed for a given motor on a 50 Hertz (Hz) electrical power supply. Image 2 shows the tested pump performance curve driven by this motor. The motor and pump operate in the speed range to the right of the peak value of torque.
To determine how the pump performance changes if a larger motor (Motor B) is installed, the affinity law for power must be applied from Motor A to Motor B on the motor power versus speed curves. This is shown graphically in Image 3, along with International Organization for Standardization (ISO) lines of constant proportionality for the affinity power rule.
All points on Motor A’s power curve that intersect the affinity ISO lines are projected along the ISO line to an operating point on Motor B’s power curve. This establishes the ratio of speeds to use in the affinity rules for flow rate and total head, resulting in the shift of pump performance shown in Image 4.
The Driver of the System
A fluid handling system consists of various equipment with hydraulic performances defined by various methods, including the use of the flow coefficient for control valves, the discharge coefficient for flow meters and the resistance coefficient of pipes, valves and fittings.
The hydraulic performance can also be defined by a performance graph developed by the manufacturer by operating the equipment in a test system most commonly seen with centrifugal pump performance curves. Electrical equipment also has defined performance characteristics, seen with the motor performance graphs.
Changing the performance of one component can impact the operation of interconnecting equipment and the overall system, as shown by using the affinity rules to evaluate the effect of changing the motor on a pump. Since the motor performance defines the amount of shaft power developed as a function of shaft speed, the pump’s power curve is the motor’s power curve. Changing the motor changes the power curve, which in turn changes how the pump distributes this power in the form of total head and flow rate, showing that the motor is truly the driver of the system.