Fundamental Principles and System Description
Engineering Information
SINAMICS Engineering Manual – November 2015
Ó Siemens AG
183/528
1.14 Efficiency of SINAMICS converters at full load and at partial load
In many applications energy savings can be made by the use of variable-speed drives instead of conventional drive
solutions. Significant energy savings can be achieved with variable-speed drive systems in partial-load operation,
particularly if they are used to drive pumps and fans with a quadratic load characteristic. Systems of this type produce
very low losses over a wide speed range and are therefore very efficient. In order to be able to quantify these
savings, precise information is needed about the losses and efficiency of the converter and motor as a function of
load and speed.
For this reason, the converter losses and efficiency values for all SINAMICS units described in this engineering
manual are specified below for full-load and partial-load operation.
Note:
The calculation of partial-load losses and efficiency values is based on a calculation method specifically devised for
SINAMICS units. Using this method, it is possible to obtain a sufficiently accurate result for all SINAMICS devices
with just a small quantity of device-specific data and relatively little work. The result can be used as a reliable basis
for performing other configuring tasks such as calculating air-conditioning requirements. In its methodology, however,
this procedure deviates from the European standard EN 50598 which came into force in 2015 and outlines eco-
compatible design requirements (energy efficiency, eco-balance calculations) for electrical drive systems (motor
systems / Power Drive Systems (PDF)) in electrically driven machines in the low-voltage range. The results may
deviate in some respect from those defined by EN 50598.
Definition of efficiency
Efficiency is defined as the ratio between the active electrical power supplied at the output P
Out
and the active
electrical power drawn at the input P
In
. If the fact is taken into account that the active electrical power drawn at the
input P
In
is higher than the active electrical power supplied at the output P
Out
by a factor corresponding to the power
losses P
L
, then the following general formula can be applied to calculate the efficiency η:
LOut
Out
In
Out
PP
P
P
P
+
==
h
. (1)
1.14.1 Converter efficiency at full load
The converter efficiency η
100
at full load is calculated on the basis of converter operation with a motor which has been
matched to the rated data of the converter in terms of its rated voltage and rated current, and which is operating at its
nominal working point. In order to calculate the efficiency η
100
at this rated point, the active power at the output of the
converter P
Out-100
and the power losses of the converter P
L-100
must be specified.
The active electrical power P
Out-100
supplied at the converter output at full load is
MotOutOutOut
IVP
j
cos3
100100100
×××=
---
.
The output voltage V
Out-100
of SINAMICS converters in vector control mode is, when operating with pulse-edge
modulation, almost equal to the line supply voltage V
Line
on the input side. The output current I
Out-100
is the rated
output current I
Out-rated
of the converter and the power factor cosφ
Mot
is the power factor of a motor which has been
matched to the rated data of the converter in terms of its rated voltage and rated current, and which is operating at its
nominal working point. Thus, at full load, the active power at the output of the converter is
MotratedOutLineOut
IVP
j
cos3
100
×××»
--
(2)
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