Principles Of Inverter Operation; Introduction; Variable Speed Drives - Siemens MICROMASTER Eco Reference Manual

ECO REFERENCE MANUAL
3.

PRINCIPLES OF INVERTER OPERATION

INTRODUCTION

This section of the manual is intended to help
first-time users of variable speed drives to
understand basic principles of operation, and gain
some insight into successful installation and
application of the MICROMASTER Eco range of

Variable Speed Drives.

VARIABLE SPEED DRIVES
A Variable Speed Drive (VSD) system consists of
a Motor and some form of speed controller.
History
Early electric VSDs consisted of AC and DC
motor combinations which were used as rotating
AC to DC converters. The DC supply was used to
drive the DC motor at variable speed by
controlling the field current in the DC generator to
vary the DC voltage to the DC motor.
The first electronic controllers used Thyristor
(SCR) Rectifiers which controlled the voltage, and
therefore the speed of DC motors. These DC
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Simplified Induction Motor - Cross Section
VSDs are still widely used and offer very
sophisticated control capability. However, the DC
motor is large, expensive and requires periodic
brush maintenance.
Present day
The AC induction motor is simple, low cost,
reliable and widely used throughout the world. In
order to control the speed of an AC induction
motor a more complex controller, which varies
the frequency as well as the voltage, usually
called an inverter is required.
Induction motor
In order to understand how an inverter works, it is
first necessary to understand how an induction
motor works.
An asynchronous induction motor works like a
transformer. When the stator (the fixed, outer
winding) is connected to a three phase power
source, a magnetic field which rotates at the
frequency of the supply is set up.
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Stator windings
Air gap
Rotor
Shaft
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