Protection Against Electro-Chemical Way; Corrosion; Frequency Converter Operation - Grundfos AMG Series Installation And Operating Instructions Manual

6.7 Protection against electro-chemical

corrosion

Two different metals or alloys cause electro-chemical
corrosion if they are connected by an electrolyte. This
applies if more than one mixer or flowmaker are
installed in the same tank. Install one of the following
additional protection:
• galvanic separation of the earth conductor from
the neutral conductor
• galvanic separation of the main supply by isolation
transformer.
The earth conductor must be separated to ensure
that no direct current can flow through it. It must still
function as a protective conductor. This can be
achieved with a limiting unit (polarisation cell or anti-
parallel diode) or an isolation transformer.
2
3
Protection against electro-chemical corrosion
Pos. Description
1 Limiting unit
2 Earth conductor
3 Isolation transformer
WARNING
Electric shock
Death or serious personal injury
‐
When using an isolation transformer,
do not alter the ratio between starting
current and rated current (I
26

6.8 Frequency converter operation

All mixers and flowmakers are designed for frequency
converter operation for energy saving and for soft
start. However, frequency converter operation often
exposes the motor insulation system to a heavier load
and cause the motor to be more noisy than usual.
In this product range, only negligible amounts of
bearing currents occur during the use of frequency
converter.
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
L1
L2
actual values and the cable Set the frequency
L3
N
PE converter U/f ratio according to the motor data.
• Minimum switching frequency is 2kHz. Variable
switching frequency is acceptable.
1
• Set frequency converter for variable-torque
operation. Pulse width modulation should be
used.
• Local regulations or standards must be complied
with.
2
• Do not reduce the motor speed to less than 30 %.
• 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 mixer or flowmaker 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 selected
frequency converter.
Maximum repetitive
/I ).
peak voltage [V]
A N
1000
MaximumdU/dt [V/μs]
5000
Frequency converter use may reduce the
lifespan of the bearings and the shaft seal,
depending on operating mode and other
circumstances.