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.
The required shaft power of the pump is given as a performance curve depending on the flow rate. The performance curve changes when the speed of the pump changes in accordance with the affinity laws.
The shaft power of the pump is directly proportional to the density of the pumped medium. In the case of highly viscous media, the shaft power also depends on the viscosity.
Depending on the application and size of the pump, the drive is designed so that the motor power is either greater than or equal to the viscosity of the pumped medium.
- the shaft power at the operating point or
- the maximum power of the characteristic curve,
in each case plus a security surcharge of at least 5%.
The required safety margin depends on the required engine power. While the safety margin is reduced to up to 5% for larger motors, surcharges of over 20% are applied for smaller power values. In addition, the nominal motor power for standard motors must be converted to the ambient conditions.
P
2 is used as the symbol for the shaft power.
