## Mains frequency

Due to historical developments, a mains frequency of 50 Hz is used in Europe, Asia, Australia, most of Africa and parts of South America, while a mains frequency of 60 Hz has become established for the public power grids in North America. The specification of the mains frequency is necessary for the selection of the electric drive. For pumps that are operated with single-phase or three-phase motors without a frequency converter, the synchronous motor speed changes in the same ratio as the frequencies, i.e. from 50 to 60 Hz by a factor of 1.2. In accordance with the affinity laws, this also changes the pump’s performance data. A frequency converter is often used to adapt the performance data to the operating point. The electrical energy drawn at the mains frequency is converted into a voltage with a different frequency in order to change the motor speed.

## Speed – Affinity Laws

The following applies: 1. Model law 2. Model law 3. Model law Q – flow rate H – delivery head P – power consumption n – speed The indices relate to the respective speed. The affinity laws apply exactly to frictionless, incompressible flows. For technical applications, they are to be regarded as an approximate solution. In general, these affinity laws are independent of how the speed change is technically implemented. Traditionally, multi-speed functions of small and medium size pumps are realized by changing the motor windings configurations to achieve stepped speed variations. Meanwhile, these have been largely replaced by frequency converters. Slow-running electric drives are very expensive for larger centrifugal pumps, so that reduction gears are used for these cases. Combustion engines are used for some mobile applications. They are also variable in speed within a specified range.

## Duty Point

The point is composed of the volume flow Q and the flow rate H. To calculate the design point, the required volume flow (flow rate of the pump) is first determined. Depending on the application, this can depend on various variables (e.g. heat requirement for heating systems, volume of wastewater produced, etc.). The calculated volume flow is then used to determine the frictional losses of the pipeline, which together with the static head then gives the total head of the pump. If a minimum flow velocity is specified for the application and this is not reached for the calculated flow rate, the rated flow rate is adjusted so that the minimum flow velocity is reached. The pump then runs in off mode (intermittent). The duty point of the system is the required operating point for the pump selection. The standard pumps usually have a deviation between the desired duty point and the actual operating point. The permissible deviation depends on the field of application and is partly regulated by applicable standards. With speed-controlled pumps, the speed of the pump is modified so that the set operating point is approached exactly. Especially in systems that are operated in different load conditions (e.g. heating system), this enables efficient operation. Depending on the design of the pump, there are further possibilities for adapting the pump performance curve to the duty point. In addition to changing the speed, the following methods are widely used:
• Impeller trimming
• Blade angle adjustment for axial flow pumps
• Throttling
• Bypass