vibration may be acceptable or excessive, depending on the modal damping associated with the corresponding mode. Additionally, drive-induced torque harmonics may cause resonance conditions with torsional rotor dynamic modes. However, such conditions are usually correctible or preventable. Variable speed vertical pumps are more likely than horizontal machines to exhibit operational zones of excessive vibration. This is because such pumps’ lower natural frequencies are more likely to coincide with running speed. Small, vertical close-coupled and multistage pumps normally do not present this type of problem.
3. Additional Considerations for VFDs: The introduction of VFDs requires additional design and application considerations. VFDs can be fitted to most existing motors in Europe and other areas, which use a 400 Volt (V) network. However, this is generally not the case in the United States, and other areas where network voltages exceed 440 V. Hence, reinforced insulation “inverter duty” motors are often needed. The high rate of switching in the PWM waveform can occasionally lead to problems. Voltages can be induced in the shafts of larger motors, potentially leading to circulating currents, which can destroy bearings.
33