English (GB)
8
Frequency converter operation
In principle, all three-phase motors can be
connected to a frequency converter.
However, frequency converter operation often
exposes the motor insulation system to a heavier
load and causes the motor to be more noisy due to
eddy currents caused by voltage peaks.
In addition, large motors driven with a frequency
converter are loaded by bearing currents.
For frequency converter operation, observe the
following:
• The thermal protection of the motor must be
connected.
• Peak voltage and dU/dt must be in accordance
with the table below. The values stated are
maximum values supplied to the motor terminals.
The cable influence is not taken into account.
See the frequency converter data sheet
regarding the actual values and the cable
influence on the peak voltage and dU/dt.
• The minimum switching frequency is 2 KHz. The
switching frequency can be variable.
• If the pump is an Ex-approved pump, check if the
Ex certificate of the specific pump allows the use
of a frequency converter.
• Set the frequency converter U/f ratio according to
the motor data.
• Local regulations or standards must be complied
with.
Before installing a frequency converter, calculate the
lowest permissible frequency in the installation to
avoid zero flow.
• Do not reduce the motor speed to less than 50 %.
• Keep the flow rate above 1 m/sec.
• Let the pump run at rated speed at least once a
day to prevent sedimentation in the piping
system.
• Do not exceed the frequency indicated on the
nameplate as this may cause motor overload.
• Keep the power cable as short as possible. The
peak voltage increases with the length of the
power cable.
• Use input and output filters on the frequency
converter.
• Use a screened power cable if there is a risk that
electrical noise may disturb other electrical
equipment.
When operating the pump by a frequency converter,
consider the following:
• The locked-rotor torque can be lower depending
on the type of the frequency converter.
• The noise level may increase. See the
installation and operating instructions for the
frequency converter of choice.
1. Do not install pump controllers, Ex
barriers or the free end of the power
cable in potentially explosive
environments.
2. The classification of the installation site
must comply with the local rules.
3. On explosion-proof pumps, make sure
that an external earth conductor is
connected to the external earth
terminal on the pump by a secure cable
clamp. Clean the surface of the
external earth connection, and mount
the cable clamp.
4. The earth conductor must be minimum
AWG 12 type RHH, RHW, RHW-2 or
similar, rated for 600 V and minimum
90 °C, yellow and green.
5. Make sure that the earth conductor is
protected from corrosion.
6. Make sure that all protective equipment
has been connected correctly.
7. Float switches used in potentially
explosive environments must be
approved for this application. They
must be connected to the Grundfos
Dedicated Controls, DC, DCD or the
SLC, DLC controllers, by an
intrinsically safe barrier to ensure a
safe circuit.
If the power cable is damaged, it must be
replaced by the manufacturer, or his
service agent
Set the motor-protective circuit breaker to
the rated current of the pump +15 %
service factor. The rated current is stated
on the nameplate.
Maintenance and service work on
explosion-proof pumps must be carried out
by Grundfos or a Grundfos- authorised
service workshop.
Before installation and the first startup of
the pump, check the condition of the cable
to avoid short circuits.
