Fundamental Principles and System Description
Engineering Information
SINAMICS Engineering Manual – November 2015
Ó Siemens AG
70/528
1.3.3 Features of standard transformers and converter transformers
Converter transformers are specially designed for use with converters. They are specially built to withstand the
increased stresses associated with converter operation.
Differences between converter transformers and standard distribution transformers
· The windings of converter transformers are designed with increased insulation strength. This makes them
capable of withstanding the extreme voltage surges which can occur during converter commutation.
· The laminated core and winding are specially constructed, e.g. with small radial conductor depth on GEAFOL
transformers, so as to minimize stray losses caused by current harmonics.
· The transformers are mechanically designed to achieve low short-circuit forces on the one hand, and high short-
circuit strength on the other. The high thermal capacity of the transformers means that they are easily capable of
withstanding frequent surge loads up to 2.5 times rated output, such as those typical of main drives in rolling mill
applications.
· A pulse imbalance in the converter (e.g. caused by an interrupted firing pulse to a thyristor in the rectifier under
full power draw and the ensuing DC components in the line current) can cause damage to the core and
laminated moldings on GEAFOL transformers as a result of overheating. Monitoring the temperature of the tie-
rod inside the core is an effective method of eliminating this problem and does not damage the transformer.
It is evident from this description of the features of converter transformers that they are designed for relatively
extreme operating conditions of a kind not generally encountered with SINAMICS drives. For standard applications
where the transformer power is adjusted to suit the converter rated output, it is therefore permissible, even in the
case of unit transformers, to use normal distribution transformers instead of converter transformers.
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