The continued focus on improving energy
efficiency and sustainability remains a
major driving force for innovation in
mechanical and electrical engineering
arenas. Improved speed control using
variable speed drives (VSDs) has produced
significant energy savings, but the next
step needs to be much bolder – the
implementation of a direct current (DC)
grid within industrial premises has the
potential to reduce operating costs and take
advantage of renewable energy sources.
The German Electrical and Electronic Manufacturer’s Association
[Zentralverband Elektrotechnik und Electronikindustie (ZVEI)]
initiated the DC-INDUSTRIE research project along with 21 industrial
companies and four research institutes. Together they are jointly
working on the project to implement the energy transition in industrial
production and, therefore, bring more energy efficiency and energy
flexibility into industrial production.
Among those involved is Bauer Gear Motor, which is part of the
Altra Industrial Motion Corporation, and Karl-Peter Simon, Bauer’s
Managing Director, who is taking a leading role in the research.
Karl-Peter Simon: “This research project has the potential to
benefit a large number of manufacturing industries, with one large
automotive company already planning to implement some of the
recommendations in a new test facility. Bauer is keen to use its
expertise to help deliver this innovative vision and make a significant
contribution to improving energy efficiency.”
In the industrial sector, electric motors account for about 70%
of electricity consumption and are thus the most significant load of
electrical energy. Reducing the energy requirements of these drive
systems by increasing their efficiency contributes to an equivalent
reduction in CO2
emissions.
Since January 1, 2017, all new 3-phase motors sold in Europe
with rated power from 0.75 to 375 kW must conform to energy
efficiency class IE3, or alternatively, IE2 for use in frequency inverter
operation. These efficiency classes are specified for three-phase
asynchronous motors operating at nominal speed and nominal torque.
However, experience has shown that an energy efficiency regulation of
a component can only sustainably reduce energy in certain operating
modes.
With this in mind, the DC-INDUSTRIE project, by means of
direct current networks, aims to support both the energy transition
and energy efficiency, as well as Industry 4.0. The project is
sponsored by the Federal Ministry for Economic Affairs and Energy
[Bundesministerium für Wirtschaft und Energie (BMWi)] and has a
term of three years.
Inefficiencies in Speed Control
Bauer’s Managing Director, Karl-Peter Simon,
has been heavily involved in the ZVEI research
project
The advantage of using a frequency inverter is the continuous
adaptation of the motor speed to the actual need, which can very often
also lead to energy savings. A frequency inverter is supplied with the
alternating current, which is first converted into direct current using a
rectifier. The direct current is then converted into alternating current
with variable frequency and voltage through a voltage feed inverter in
order to electronically change the speed of a three-phase motor.
However, if the three-phase motor is operating in the braking
mode, e.g. in a crane that is in lowering mode, the energy flow changes.
But, this energy cannot be fed back into the grid by the frequency
inverter because the input rectifier only allows the energy
to flow in one direction. Therefore, the energy that is fed
back must be dissipated via the direct current voltage
circuit of the frequency inverter.
For this purpose, a brake chopper is connected to the
intermediate circuit. This monitors the intermediate circuit
voltage with regard to the voltage level. If the intermediate
circuit voltage exceeds a set threshold value, the brake
chopper switches the braking resistor between the positive
and the negative pole of the intermediate circuit. This is
usually an additional external braking resistor that converts
the braking energy into heat energy./p>
Reducing Harmonics Issues
The increasing use of frequency inverters to control
motor speeds has led to problems with mains effects,
causing harmonics and distorting the voltage. There is no
standard solution for harmonics, since each grid and its
electrical load are very different. Ultimately, the operator
is responsible for the voltage quality of its own production
facilities. If frequency inverters or other devices with power
electronics are increasingly installed, grid effects will
increase.
The challenges presented show that a further increase
in the use of inverters for the flexible control of electric
motors is desirable and very often even necessary. This
is the only way to improve both production processes
and energy efficiency. However, line perturbation due to
harmonics and equipment costs limit the increase.
In order to achieve significant progress in energy
efficiency and system cost optimization, new approaches
are needed. To enable energy efficiency, energy transition
and Industry 4.0, new grid structures are required.
Creating the Solution
The new network structure is based on an alternating
current supply, which provides the direct current power
supply for production plants via a central rectifier. Active
grid filters are integrated into the central rectifier to ensure
the voltage quality harmonic requirements.
The direct supply of the frequency inverter with direct
current means that all decentralized energy conversion is
no longer needed. Since central energy conversion (from
AC to DC) is significantly more efficient, conversion losses
are significantly reduced.
Through the direct supply of all electric motors via
a frequency inverter with direct current power supply,
all installed motors are connected via a common direct
current voltage grid. Furthermore, a direct current voltage
network essentially only causes ohmic transmission losses.
Compared to an alternating voltage network, the capacitive
and inductive line losses are eliminated.
In addition, the central direct current voltage network
offers the possibility of integrating photovoltaics directly at
the direct current voltage level. In this case also, conversion
from DC to AC is not required to be done by an inverter.
This grid infrastructure offers the possibility of optimizing
the purchase of energy and to stabilize the grid.
Through the elimination of the input rectifier and the
grid filter with frequency inverters, these can be designed
more cost-effectively and more compactly. This simplifies
integration into the motor, which can significantly increase
the degree of acceptance. Variable speed motors allow for
a reduction in variants and energy savings. They provide
status signals from all DC-fed drivers, which are of great
importance for flexible and safe production control.
Grid management makes it possible to optimize
operational management in terms of energy costs. The
accessible information enables preventive production
control measures to significantly increase the availability
of production. This is a prerequisite for the successful
implementation of Industry 4.0.