Grundfos MTR 1s Instrucciones De Instalación Y Operación página 20

10.5 The pump runs but gives no liquid, or the pump performance is not constant
Cause
The pump strainer is partly blocked by impurities.
The liquid level in the tank is too low.
The pump draws in air.
10.6 Leakage from the shaft seal
Cause
The shaft seal is defective.
10.7 Noise
Cause
Cavitation.
The pump does not rotate freely because of an
incorrect pump shaft position (frictional resistance).
Frequency converter operation.
Related information
6.2 Frequency converter operation
20
Remedy
Clean the strainer.
Increase the liquid level.
Check the inlet conditions.
Remedy
Replace the shaft seal.
Remedy
Check the inlet conditions.
Adjust the pump shaft.
Frequency converter operation may cause increased
acoustic noise from the motor. Make sure to protect the
motor against voltage peaks and dU/dt according to IEC
60034-17.
For noise-critical applications, fit an output filter between
the motor and the frequency converter to reduce the
voltage peaks. For particularly noise-critical
applications, fit a sinusoidal filter.
See Grundfos recommendations in the section on
frequency converter operation.