Rated power and shaft power of the pump

It should be said that the shaft power is related to the power of the motor used

The rated power is the power required to transport the medium at the rated flow rate and head, and it is also equal to the shaft power at that point

The shaft power can correspond to the power at multiple flow and head points, but there is only one maximum value

At a certain flow rate and head, the power delivered by the prime mover to the pump shaft per unit time is called shaft power. Shaft power is a technical term commonly used in pumps, referring to the power with which the shaft transfers power (motor power) to the working component (impeller). The power value is less than the motor’s rated power. Essentially, shaft power is closely related to the coupling; the motor is connected to the impeller of the pump head through a coupling. When the motor rotates, it drives the coupling, which in turn connects to the impeller inside the pump head, thereby causing the impeller to rotate. Due to the presence of the coupling, the motor power cannot be fully converted into the actual efficiency of the impeller’s rotation; therefore, the shaft power is less than the motor power (rated power).

The power of the motor should be selected based on the power required by the manufacturing machinery, with an effort to operate the motor under its rated load. The following two points should be considered when making a choice: 1. If the motor power is chosen to be too low, it will result in a situation where a weak motor has to drive a heavy load, causing the motor to be overloaded over time. This leads to damage to the motor’s insulation due to heat, and in severe cases, the motor may even burn out. 2. If the motor power is chosen to be too high, it will result in a situation where a powerful engine is used for a task that requires a less powerful engine. Its output mechanical power cannot be fully utilized, resulting in low power factor and efficiency, which is detrimental to both the users and the power grid. It also leads to waste of electrical energy. To correctly select the power of a motor, the following calculations or comparisons must be carried out: P = FV / 1000, where P is the calculated power (in KW), F is the required pulling force (in N), and V is the linear velocity (in m/s). Additionally, the analog method is most commonly used to choose the power of a motor. The so-called analogy method involves comparing it with the power of motors used in similar production machinery. The specific approach is to find out what power rating the motors used in similar production machinery at one’s own unit or at other nearby units have, and then select motors with a similar power rating for testing. The purpose of the trial run is to verify whether the selected motor is suitable for the production machinery. The verification method is as follows: use the motor to drive the production machinery, measure the operating current of the motor with a clamp ammeter, and compare the measured current with the rated current indicated on the motor’s nameplate. If the actual operating current of the electric motor is not significantly different from the rated current indicated on the nameplate. This indicates that the power of the selected motor is appropriate. If the actual operating current of the motor is about 70% lower than the rated current indicated on the nameplate. This indicates that the power of the motor has been chosen too high; a motor with lower power should be used instead. If the measured operating current of the motor is more than 40% higher than the rated current indicated on the nameplate. This indicates that the power of the motor has been chosen too low; a motor with higher power should be used instead.