Operation with a static frequency converter
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Operation with a static frequency
converter
WILO products can be operated with standard frequency converters. These are normally pulse-
width-modulated converters. However, the following points must be observed when operating the
converter:
Selecting the motor and
converter
Any standard WILO motor can be used.
If the rated voltage is above 415 V, you must consult the
manufacturer.
Because of the additional heating caused by harmonics, the rated power of the
motor should be around 10% more than the power requirement of the pump. For converters with a
low-harmonic output
, it may be possible to reduce the 10% power reserve. This is normally done
by using output filters. Ask the converter manufacturer.
The size of the converter depends on the rated motor current. Selecting the motor power in kW can
cause problems, since submersible motors have
different specifications
to standard motors.
Waste
water motors are marked with the appropriate rated power
(type sheet power in catalog).
Minimum speed for
submersible pumps
(well pumps)
Submersible pumps have water-lubricated bearings. A minimum speed is required in order for a
lubricating film to form.
Continuous operation at frequencies below 25 Hz (30 Hz 4-pole) must be avoided
, since the lack
of lubrication and possible occurrence of mechanical vibrations may cause damage to the bearings.
The lowest speed range (up to 12.5 Hz) should be passed though within
2 seconds.
In practice, the speed should only be lowered to the extent that a pump flow of at least 10% of the
maximum flow remains. The precise value depends on the type – ask the manufacturer for details.
Minimum speed for waste
water and sewage pumps
There is no prescribed minimum speed for waste water and sewage pumps.
However, make sure that the motor operates without jerking or vibrating, especially in the lower
speed range. Otherwise, the mechanical shaft seals could be damaged and start leaking.
Operation
It is important that the pump motor operates across the entire control range without vibrations,
resonance, oscillation or excessive noise (ask the manufacturer if necessary).
Increased motor noise caused by the harmonics of the power supply is normal.
When configuring the converter, always make sure the quadratic curve (U/F characteristic) for
pumps and fans is correctly set. This ensures that the output voltage at < 50 Hz frequencies is
adjusted to the power requirement of the pump. Newer converters feature an automatic power
optimization function which achieves the same effect. See the converter operating manual for this
setting and other parameters.
Max. voltage peaks and
rise speed
Submersible motors with water-cooled windings are more susceptible to voltage peaks than dry
motors.
The following thresholds may not be exceeded:
Max. voltage rise speed: 500 V/µs
Max. voltage peak to earth: 1250 V
These values apply for well pumps < 1 kV and can usually be attained by replacing a sinusoidal filter
or dV/dt filter. For motors > 1 kV, the permissible values can be obtained from the plant. It remains
the case that the lowest possible pulse frequency should be set.
