Condition Monitoring, Troubleshooting and Reliability in Rotating Machinery

17Operating Centrifugal Pumps with Variable Frequency Drives in Static Head Applications

By Robert X. Perez

Most centrifugal pumps in the industry are driven by reliable, squirrel cage induction, or electric motors, which run at essentially a constant speed (Figure 17.1). Their speed depends on the number of rotor poles and the electric supply frequency, which is typically 60 Hz in the US and 50 Hz in Europe. The most commonly used fixed speed electric motor options are 3600 rpm for 60 Hz power (3000 rpm for 50 Hz power), 1800 rpm for 60 Hz power (1500 rpm for 50 Hz power, and 1200 rpm for 60 Hz power (1000 rpm for 50 Hz).

In reality, induction motors operating under a load will never run at their synchronous speed, which is the rotating speed of the magnetic field inside the stator windings. They will always operate at a speed slightly less than the synchronous speed due to slip. Slip is defined as the difference between the rotating magnetic flux speed and rotor speed expressed in terms of per unit synchronous speed. For example, a 200 hp, 3600 rpm electric motor may operate at 3550 rpm at its rated load. This would mean there is (3600-3550)/3600 or 1.1% slip at full load. In practice, most designers and users assume that the magnitude of slip is minor and assume the motor speed is relatively constant from no load to full load condition.

While fixed electric motors are the norm in processing applications, it is increasingly more common for pumps to be operated by motors that ...