# Shaft Power

The shaft power (also coupling power) is the mechanical power transferred from the motor to the pump.

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.

P2 is used as the symbol for the shaft power.

## More articles on this topic

### Mains frequency

Mains frequency (utility frequency) is the nominal frequency of the alternating voltage provided by the energy supplier.

### Speed – Affinity Laws

The affinity law describes the speed dependency of the delivery parameters of centrifugal pumps. It can be derived from the general similarity relationships as a special case.

### Pump Performance Curve

The pumping performance of a centrifugal pump is expressed in the form of an H-Q curve, depicting the Flow Q (e.g. in m³/h) and the Head H (e.g. in m) of the pump.

### Performance curve conversion for impeller trimming

Impeller trimming is the reduction of the impeller diameter of a centrifugal pump with the aim of adapting the operating point to the specifications.

### Number of Pole Pairs

The number of pole pairs determines the synchronous speed of the three phase motor.

### Required Pump Shaft Power

Accurate selection of the pump drive and calculation of operating cost or feasibility studies require the knowledge of the drive power required for the pump to operate at the required Duty Point.