Offset Correction Via Xn1; Information On The Differential Input, Control Limits, And Grounding - Siemens SINAMICS DCM Operating Instructions Manual

6.6.5

Offset correction via XN1

For Sitor sets 6QGXX with shunt current measurement and both V/f and f/V conversion, the
potentiometer (R202 on module 6QM400 for SITOR thyristor set 6QG22) on the secondary
side should be calibrated to exactly match the zero point of the output voltage when the unit
is de-energized. Please refer to Chapter 6 for 6QG22, "Improving interference immunity and
drift problems of the analog actual current value for SITOR electronics." If this potentiometer
is not set correctly, not only will an offset error occur, but current measurement will be
subject to a scaling error. In order to make additional adjustments to the offset for a precise
zero current message, connector XN1 at power interface A7109 must be connected to an
analog output (±10 V) on the control electronics A7100. The offset can be minimized by
assigning output voltage parameters manually. This will not, however, rectify the scaling
error referred to above.
6.6.6

Information on the differential input, control limits, and grounding

If the difference amplifier is used with an amplification of 0.1, a differential input voltage of up
to ±30 V can be processed as a single-ended output voltage (up to ±3 V). The value ±3 V
represents the control limits of the analog digital converter on the CUD.
If the output signal from external armature current measurement is scaled to ±10 V at rated
current, it will not be possible to measure overcurrent. For example, if the output signal from
armature current measurement is scaled to ±5 V at rated current, a current of 2 × I
be measured.
It is vital for users of the SINAMICS DCM Control Module to realize that they need to check
whether the method used for current measurement is really able to map the desired
overcurrents and the expected peak values within the tolerance required. It is important to
check the current measurement circuit to ensure that current mapping remains linear right
through to the desired overcurrent capacity. If this is not the case, the capping of the actual
value will result in current overshooting and, ultimately, in blown fuses.
The most common risk is overloading of the last operational amplifier in the measuring
circuit. This problem can be avoided by scaling external current measurement to a
sufficiently low value. For example, R642 needs to be shorted in 6QG22. Similarly, R71
needs to be jumpered with an additional 100 kΩ in 6QG35. This ensures there is
controllability at anything up to 2 x rated current (±10 V).
The SINAMICS DCM Control Module is able to process load signals up to a peak value of
±3 V (= 3 × rated current). This means processing is possible up to a peak value of ±30 V
when the differential input is being used.
If the V-circuit is being used with a load resistor and there is no downstream amplifier, it is
advisable to dimension the load resistor for 1 V at rated current. The R701 or R704 internal
10 Ω resistor should be removed to minimize the influence of the supply line and terminal
resistance. Please take care when performing calculations if the resistor is still in place!
Existing load resistors for 10 V at rated current can have their previous value and power loss
reduced to a tenth. Terminals XB-1 and XB-2 are used for the connection, with XB-2 acting
as the ground connection. EMC requirements mean there should be no other connection
between the V-circuit and ground.
If a V-circuit for 10 V at rated current is to be connected without any changes, either
terminals XB-1 and XB-4 (with the difference amplifier at 0.1) or terminals X_I_IST are used.
In such cases, one end of the load resistor should be grounded for EMC reasons.
If an external measuring circuit (LEM transformer) is connected, its electronic ground should
be grounded for EMC reasons.
SINAMICS DCM Control Module
Operating Instructions, 06.2010, C98130-A7067-A1-02-7619
6.6 Measurement of the armature current
Connecting
can
RATED
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