# Rotational speed

Rotational speed (also called speed, or speed of rotation) can be quantified as the number of revolutions a rotating system makes within a defined period of time. The unit used for rotational speed is s–1 (rev/s); pump speed is generally given in min–1 (rpm).

The rotating frequency of the pump shaft therefore characterises a pump's rotational speed (n). It should not be confused with specific speed (ns) and is always defined as a positive figure.

The pump direction of rotation is specified as clockwise or anti-clockwise and is separate to the defined direction of rotation of the impeller, which, when turning to the right with respect to the direction of inflow, is clockwise.

The selection of pump rotational speed is closely related to the characteristics of the pump hydraulic system (circumferential speed, impeller, specific speed), as the overall strength and economic efficiency of the pump and drive system need to be taken into account.

Most pumps operate at rotational speeds between 1000 and 3000 rpm but frequently reach in excess of 6,000 rpm with special gearing and turbine drives.

Larger centrifugal pumps (e.g. cooling water pumps for power stations), however, are typically mated to slow-running electric drives that are very costly. Reduction gears between the drive and pump maintain today's low pump speeds of just 200 rpm.

Rotational speed (n) is proportionate to angular velocity (ω), the latter of which is more conducive to physical calculations and is the quotient of the plane angle and time interval. The unit is rad/s. The rad (radiant) is equal to the plane angle (57.296 degrees), which intersects an arc of 1 m in length as the centre angle of a circle with a 1 m radius.

This is represented with the number 1 in practice. The following relationship exists between rotational speed (n) and angular velocity (ω):