Control Techniques Commander SE Advanced User's Manual page 156

Figure 11-9
The following calculations can be used to calculate the correct value of output line inductor.
Step 1
Estimate the cable capacitance - from one line to all others.
Typical values:
Multi-core cables, and screened armoured cables where there is a plastic sheath between the phases and the screen:
130pF/m
Screened cables with no plastic sheath between cores and screen, mineral insulated cables: 300pF/m
Add an allowance for the motor capacitance. This depends on the motor size, but we suggest a value of 1nF per motor is a
reasonable estimate. This will usually be rather smaller than the cable capacitance.
Step 2
Decide on the available charging current from the drive. This depends to some extent on the application and the drive. Here
is the typical procedure for Commander SE:
I = 1.41I
ch
Where:
I is the nominal rated rms drive output current
n
k is the acceptable short-term overload factor.
The drive is rated for 150%, i.e. k = 1.5, but increased cable driving capability is achieved if this can be reduced. For many
applications 1.25 is acceptable and for fan/pump applications, 1.1 may be acceptable.
In the case where the drive has been de-rated to allow for the long cable, i.e. a larger than normal drive is used, a lower value
of k will be appropriate. It should be the ratio of the expected maximum short-term torque to the (theoretical) rated torque
capability of the drive.
The factor 2.1 in the expression is the ratio of drive instant trip current to nominal output current, for Commander SE.
Step 3
Decide the maximum DC link voltage where the drive is required to produce full torque. This will normally be 1.41 times the
highest AC supply voltage.
Step 4
The minimum value of inductance (per phase) is given by the expression:
2C
------- -
L =
min
3
If using standard iron-cored chokes, the inductance at the high frequencies involved will be rather lower than the specified 50/
60Hz inductance. We suggest specifying an inductance of twice that determined by this calculation i.e. 2L
156
Thermal overloads
Output
line
inductor
( )
2.1 k –
n
2
Vdc
 
-----------
 
I
ch
min
Commander SE Advanced User Guide
Issue Number: 4