Transformer Dimensioning; Required Data - ABB ACS800 User Manual

Transformer dimensioning

Transformer selection is based on the demands of the application. DC components
together with the demands for starting torque form the need for voltage boost. A
special transformer must be used when the required starting torque is over 50% of
nominal torque, or if the motor cable length is over 300 metres (984 ft).
The transformer kVA rating must be at least equal to the apparent power (S) of the
motor.
An autotransformer is not allowed.
The following is a quick calculation for transformer dimensioning. The capacitive
reactance of the cable on the secondary side is not taken into account. Furthermore,
the voltage drop over the sine filter is estimated.

Required data

Motor:
Cable
Motor data:
Nominal impedance
Resistive component
Reactive component
Cable reactance at frequency f is calculated as follows:
ϕ
U I cos f
, , ,
n n n n
R L
, , length
c c
If cable characteristics are not known, typical values can be used:
Ω
------- -
R = 0,27
c
km
mH
-------- -
L = 0,33
c
km
U
n
----------- -
Z =
m
I 3
n
R = Z cos
m m
X = Z
m m
( ϕ )
n
2
( ϕ )
1 cos –
n
X = 2πfL
c c
Appendix – Step-up applications
35