Once the pump configuration and rating (capacity and head) have been determined, as described in the three preceding sections, the next step in the selection process is to decide which pump speeds should be considered. It is quite often the case that two or more operating speeds may be commercially available for a particular pump rating and configuration. Each of these speeds results in a different sized pump, each having different first cost, operating cost, and maintenance cost.
The available motor speeds for standard alternating current (AC) electric motors are based on the following formula, for 60-cycle current:
rpm = 7200/N (at frequency = 60 Hz) (1)
where N is the number of poles.
Electric motors have an even number of poles, starting with two. The commercially available, constant-speed AC electric motors with 60 Hz electrical supply are as shown in below Table .
Designations for slower speed motors than shown in the above Table would follow Equation (1) . The actual operating speeds of motors are slightly less than the values shown in the above Table due to electrical slippage between motor rotor and stator. Thus, the operating speed of a two-pole motor is 3450 to 3550 rpm, the speed for a four-pole motor is 1750 to 1780 rpm, etc.
For 50-cycle current, which is common in Europe and some other parts of the world, Equation (1) is revised to
rpm = 6000/N (at frequency = 50 Hz) (2)
where N is the number of poles.
Accordingly, with 50-cycle current supply, commercially available AC electric motor speeds are 3000 rpm, 1500 rpm, etc.
The manufacturer determines which speeds will be offered for each pump type and size, based on a number of design and application considerations . In general, the larger the pump impeller (and pump capacity), the slower the pump runs. Also, certain types of applications such as abrasive slurries or paper stock require slower pump speed than clean services. Once the pump speeds to be considered have been determined, a centrifugal pump selection can be made. However, if a variable-speed pumping system is being considered for a system requiring a range of flow, additional information in the form of a system head curve must be developed before a determination can be made as to the required pump speeds for the various flow requirements. For the moment, it is assumed that a constant-speed pump is being selected.