Civil Engineering Reference
In-Depth Information
energy, which is fed back to the network. Accurate braking control as well as
energy recovery are the main reasons for using this mode of operation both in
industry and in traction.
Electrical drives play an important role in energy saving when machines
such as pumps, compressors, and fans are controlled by speed variation
In addition, electrical drives are among the main components of industrial
automation systems and allow improvements in production quality and quantity,
thus generally determining a decrease in specific energy consumption per unit of
production. Interfacing with the plant centralized control, diagnostics and monitor-
ing are other factors that make electrical variable-speed drives attractive for indus-
trial applications.
7.4.3 Application Problems
Electrical variable-speed drives may be a source of thermal, mechanical, and
electrical malfunctioning which must be investigated carefully in order to guarantee
a safe operation of the drive itself and of the electric network.
The power factor at the drive feeding node changes according to the operating
mode: generally, the lower the actual load, the lower the power factor. That depends
on the internal structure of the power converter and on the current and voltage
harmonics, but it can be assumed as a general rule. A control system is usually
required to keep the power factor value within the accepted range.
Derating of electrical machines, when fed by power converters, is necessary
because of the increase in losses due to the harmonics and because of the reduction
in efficiency of cooling systems at low speed. A derating of 10-15 % depends on the
harmonics; higher derating, up to 50 %, is due to thermal phenomena caused by the
reduced speed range. In any case, an independent cooling system is installed,
generally a fan driven by an independent electric motor.
7.5
Electrically Heated End Users
Uses of electric energy for heating include metal treatments, boilers, induction
heating, electric arc furnaces, heat pumps, electromagnetic wave heating, mechani-
cal compression of steam, etc. They are different types of applications based on
different principles.
