PuK - Process Technology & Components 2024

Components
Frequency converter
Frequency converter series SD4M for high-speed applications
Multilevel technology: What it can do
and what it enables
Markus Finselberger
Motor-driven and generator-driven
high-speed applications with high
output powers push the standard
converter technologies to their
limits. Especially in the field of renewable
energy as well as efficient
compressed air supply, the demand
for converters that enable
high rotating field frequencies
grows. Therefore, the high-speed
specialists at SIEB & MEYER AG
have developed a solution based
on three-level technology. With the
series SD4M, motor losses, electromagnetic
radiation as well as insulation
stress can be reduced significantly.
The following article answers
important questions regarding use
and function of the multilevel frequency
converters.
How does a multilevel frequency
converter work?
The majority of frequency converters
used in modern drive technology is
based on two-level technology. That
means, at first the converters rectify
the mains AC voltage into DC voltage
and then convert this DC voltage to
an AC voltage with variable frequency
and amplitude, which can be supplied
to motors with adjustable speed. The
AC voltage is generated with alternating
polarity – plus and minus – on
two levels. Many converters use the
modulation type PWM (pulse-width
modulation) for this purpose. Multilevel
converters use at least one more
intermediate voltage level, which
makes a quite different output stage
topology neces sary. A conventional
three-phase two-level converter requires,
for example, six electronic
power switches (transistors), whereas
a three-level converter requires
twelve switches.
For which applications are multilevel
converters suitable?
Multilevel converters enable, for
example, a significant increase in the
efficiency of turbomachinery such as
turbo compressors and blowers (e. g.
for wastewater treatment) and of rotating
energy storage units (flywheel)
as well as ORC systems for the conversion
of waste energy into electric
energy. The efficiency of these
systems increases with their speed.
However, until now the market hardly
offered any converters for output
powers >100 kW and rotating
field frequencies up to 2,000 Hz –
especially when sensorless control
of synchronous motors is required.
Multi level technology closes this gap.
What requirements do HS motors
demand of the converter technology?
The applications mentioned above
employ high-speed motors (HS motors)
that generate power via speed
and not via torque. As a general rule,
the rotor volume changes at the same
rate as the reciprocal of the speed increase.
That means, at 10 times of
the speed the rotor volume has decreased
to one-tenth. This in turn results
in limited heat dissipation. The
negative effects are amplified when
the motor is operated in vacuo or in
a gas with low thermal conductivity,
for example in flywheel applications.
Therefore, the used frequency converters
must reduce motor losses
and the resulting heat development
as far as possible.
What influence do high speeds have
on the motor design?
Fig. 1: Multilevel frequency converters support applications that require high rotating field
frequencies. They enable, for example, a significant increase in the efficiency of turbo
compressors and blowers that are used e. g. for wastewater treatment.
(Image © : Kletr_261344590 - adobestock.com )
The motor design must be adapted
according to the power/speed ratio
required by the application. Beside
the permissible circumferential speed
of the rotor, the bending-critical frequency
of the corresponding shaft
have to be considered. That means,
a synchronous motor with 100 kW at
60,000 rpm, for example, can reach
the required power density only by
means of a 4-pole motor design. With
96 PROCESS TECHNOLOGY & COMPONENTS 2024