English (GB)
15
Fig. 21
Star-delta starting, four power cables
3.6 Frequency converter operation
In principle, all three-phase motors can be
connected to a frequency converter.
However, frequency converter operation will often
expose the motor insulation system to a heavier load
and cause the motor to be more noisy than usual
due to eddy currents caused by voltage peaks.
In addition, large motors driven via a frequency
converter will be loaded by bearing currents.
For frequency converter operation, please observe
the following:
•
Requirements must be fulfilled.
•
Recommendations ought to be followed.
•
Consequences ought to be considered.
3.6.1 Requirements
•
The thermal protection of the motor must be
connected.
•
The 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 has not been
taken into account. See the frequency converter
data sheet regarding the actual values and the
cable influence on the peak voltage and dU/dt.
•
Set the frequency converter U/f ratio according to
the motor data.
•
Local regulations and standards must be fulfilled.
3.6.2 Recommendations
Before installing a frequency converter, calculate the
lowest allowable frequency in the installation in order
to avoid zero flow.
•
Do not reduce the motor speed to less than 70 %
of rated speed or 35 Hz.
•
Keep the flow velocity above 1 m per second.
•
Let the pump run at the rated speed at least once
a day in order to prevent sedimentation in the
pipes.
•
Do not exceed the frequency indicated on the
nameplate. If you do, there is a risk of motor
overload.
•
Keep the power cable as short as possible. The
peak voltage will increase with the length of the
power cable. See the data sheet for the
frequency converter used.
•
Use input and output filters on the frequency
converter. See the data sheet for the frequency
converter used.
•
Use a screened power cable if there is a risk that
electrical noise can disturb other electrical
equipment. See the data sheet for the frequency
converter used.
3.6.3 Consequences
When operating the pump via a frequency converter,
please be aware of these possible consequences:
•
The locked-rotor torque will be lower. How much
lower will depend on the frequency converter
type. See the installation and operating
instructions for the frequency converter used for
information on the locked-rotor torque available.
•
The working condition of the bearings and the
shaft seal may be affected. The possible effect
will depend on the application. The actual effect
cannot be predicted.
•
The acoustic noise level may increase. See the
installation and operating instructions for the
frequency converter used for advice as to how to
reduce the acoustic noise.
TM
0
6
9
87
8 3
517
The earth lead is a green cable which is
marked yellow/green and labelled "PE".
To avoid overload of the motor of KPG LB
and KPG EB with bi-directional operation:
Before changing the direction of operation,
stop the pump and wait until the flow has
stopped. The delay of switching the pump
on again depends on the velocity of the
flow.
M
~3
U1
W1
V1
PE
U2
W2
V2
PE
U3
W3
V3
PE
U4 W4
V4
PE
Control cable
L1
L1
L3
PE
Maximum repetitive
peak voltage
[V]
Maximum dU/dt U
N
400 V
[V/
μ
sec.]
850
2000
