A centrifugal pump is powered by a motor that has to produce adequate torque to start off the pump and take it up to the best speed. Without enough torque, the motor might not be able to start the pump at all or only have it function at a lower speed.
To determine the motor that is most appropriate for a pump, we use what is called the pump’s torque-speed curve. We can make this by vertically mapping the percentage of full load torque (FLT) against the percentage of horizontally mapped full load speed (FLS).
The torque-speed curve is common in all centrifugal pumps because though the pump torque changes with its squared speed, when the full load speed (FLS) is at zero percent or at rest, the full load torque (FLT) will never simultaneously be at zero percent. The motor must overcome static friction and inertia in order to start a rotating pump. This brings us to the question of how much full load torque is needed to do this. The answer — about 20% of full load torque. As the speed of the pump increases, the torque that is needed slowly decreases. (At 15% full load speed, about 5 to 10% full load torque is needed.)
Horsepower (HP) is a combination of speed denoted by RPM, and torque. The horsepower of a motor is a basic measure of its abilities. The torque of a motor is calculated with the following formula:
T = (5,250 x HP) / N
(T = Torque, and N = RPM)
When you need to select a new motor for your pump, there are three important values you need to look at. These are:
- Torque at zero flow
- Torque at rated flow
- Torque at maximum flow
Additionally, you will need to consider the inertia value of the pump at the motor shaft. The higher this is, the more time the motor will take to start off the pump and get it to the full load speed. This is important because the longer this is, the more heat is generated. The size of the motor must be suited to handle the inertia of the pump. Otherwise, the motor’s windings may get damaged.
In the case of the motor driving the pump directly, the inertia values of the pump-motor combination and the pump itself will remain constant no matter the speed of the pump. However, if both a gearbox and motor drive the pump, the inertia values will be affected greatly.
To choose the right kind of motor for your centrifugal pump, the above factors are important to keep in mind.
VEMC is an authorized Kirloskar dealer that supplies a range of industrial water pumps in Mumbai for various applications. We offer a diverse range of pumping solutions, including water pumps and process pumps. For more information on our products and help with selecting the most suitable ones for you, please contact us on +91 98199 07445. We would be happy to provide customized support. VEMC is ISO 9001:2015 certified and a pioneer in the field of electromechanical engineering products, allied equipment, and services.