When can the DC link voltage rise to unexpectedly
high values?
Answer
This may affect system installations in which devices
are installed that have hardly any base load in the DC link when switched off
andoperated on AC networks that do not comply with standards.
Devices that have hardly any load on the DC link when locked, i.e. without the
motor-side inverter in operation, can be identified by the
order code (e.g., i950AExxxF) on the type label. The F in the
order code indicates that the device is designed for a rated voltage range of 400/480V
.
In relation to the i950 device series, these are single-axis devices with
a power rating ≤ 4kW that have the following order code: i95AE155F; i95AE175F;
i95AE222F; i95AE240F.>
This device has no internal switching power supply, which
is supplied from the DC link and therefore has no base load.
If such a device is used without braking resistor is operated on an AC mains supply
that is subject to distortion from external consumers,
the Y-Cs (capacitors from the DC link to
PE) in the i950 devices, in conjunction with upstream filters or mains chokes, cause
the voltage in the DC link to rise above the expected level.
The effect is particularly noticeable on AC networks with high
rated voltage. On AC networks with a rated voltage of 480V,
taking into account the usual 10% voltage tolerance, a maximum value of
528VRMS. Due to simultaneous distortions in the
mains voltage, the reactive components present (Y-C's in the devices,
choke or filter components upstream of the device) cause the
intermediate circuit to be pumped up beyond the expected peak value of 750V. This
inflation of the intermediate circuit occurs very slowly (taking up to several minutes) due to the low capacity of the Y-Cs.
This inflation can be observed under index 0x6079:000
of one of the devices connected to the DC link. Theoretically, it is possible for the DC link voltage to rise to up to 800V.
A device failure due to this effect is not to be expected.
effect is not to be expected.
Possible measures
What measures can be taken to eliminate/minimize this
eliminate/minimize this DC link voltage increase?
Depending on the possibilities in an affected system, there are
various parking measures:
Measure 1:
Creation of a network that meets normative requirements.
Measure 2:
It is often sufficient to deactivate the Y-Cs present in all devices
.
Deactivate the existing Y-Cs in all devices connected to the common
intermediate circuit. Refer to the documentation for the device in question to find out how to do this.
In the case of the i950 mentioned in this example,
deactivation is carried out by loosening both IT mains screws in the
device.
The disadvantage of this approach is that it means that any statement on conducted
interference emissions is no longer based on the measurements generally made by Lenze
.
Measure 3:
Use of the device's internal brake chopper in conjunction with
a braking resistor. The brake chopper limits the voltage increase by
briefly chopping to the parameterized brake chopper threshold. The energy
that leads to this very slow increase in the DC link voltage is very
low. A base load of 10W/device on the DC link is sufficient to limit the increase
. This power must be supplied by the braking chopper output.>one
resistor to be installed in the DC link i950
.
Install the braking resistor according to the specifications in
the i950 product documentation.
