Siemens Simovert Masterdrives Compendium page 91

Instructions for Design of Drives in Conformance with EMC Regulations
3.3 The frequency converter and its electromagnetic
compatibility
3.3.1 The frequency converter as a noise source
Mode of operation
of SIMOVERT
MASTERDRIVES
3-4
SIMOVERT MASTERDRIVES frequency converters operate with a
voltage-source DC link.
In order to keep the power losses as low as possible, the inverter
switches the DC link voltage to the motor winding in the form of voltage
blocks.
An almost sinusoidal current flows in the motor.
Frequency converter
Mains
Rectifier DC
link
U
I
Fig. 3-1 Block diagram showing output voltage V and motor current I of a frequency
converter
The described mode of operation in conjunction with high-performance
semiconductor switching elements have made it possible to develop
compact frequency converters which now play a vital role in drive
technology.
As well as having many advantages, the fast semiconductor switches
also have one disadvantage:
A pulse-type noise current flows to ground through parasitic
capacitances C at each switching edge. Parasitic capacitances exist
P
between the motor cable and ground, and also within the motor.
Inverter
U
6SE7087-6QX70 (Version AD)
Compendium Motion Control
05.2003
Motor
I
M
3 ~
t
t
Siemens AG
SIMOVERT MASTERDRIVES