Technical aspects

A modern VFD is a compact, well-developed unit which is relatively easy to install. Unlike an old VFD, a modern version generally does not require as much power margin compared to ans old one since it uses higher switching frequencies that does not affect the induction motor to the same extent. The high switching frequency however, induces electrical transients which can lead to other problems such as nuisance tripping of control equipment. These transients are also more aggressive against stator insulation.

Regardless of the type of pump or VFD used, there are several technical aspects to consider:

 
Clogging: VFD-controlled pumps often run at reduced speeds, which mean that the energy available for the impeller to keep itself from clogging also is reduced. Therefore it is recommended to control the pumps in a way that long periods of running the pumps at low frequencies are avoided, especially in tough sewage applications.
 
Sedimentation in pipes: Avoid running at low frequencies that will result in velocities in the pipes lower than 0.7 metres per second to minimize the risk of having sedimentation.
 
Nuisance tripping: The high-frequency emission from a VFD can interfere with sensor control systems and other equipment. The use of shielded cables and appropriate filters are therefore recommended.
 
NPSH/power limit problems: The VFD does not dirctly have an impact on the NPSHrequired of a pump. However, decreasing speed with a VFD will in many cases significantly change the duty point for the pump. The new duty point could be at a position on the pumps perfromance curve where the NPSHrequired is much higher than at the duty point at full speed.

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Technical aspects VFD   (436 KB)