Torque Characteristics

• The torque, or turning moment, characteristics of a pump need to be considered at the design stage. Continuous torque and low-speed torque effects impose demands on variable frequency drives.
  
  

Lateral Critical Speed

• Applying a variable frequency drive to a pump increases the likelihood of a rotating element reaching what is known as lateral critical speed. This occurs when the running speed coincides with the natural frequency of the rotor and produces excessive vibration; it can, however, be prevented by correct pump design.
  
  

Voltage Considerations

• In the United States, variable frequency drives cannot typically be fitted to existing pump motors because of the high network voltage. Typically, inverter duty motors with reinforced insulation are required for variable frequency drives where network voltage exceeds 440 volts.