World Grain - March 2024 - 66

TECHNICAL PROFILE
Asynchronous motors:
belt drive vs. direct drive
Making the right choice for plansifters
by Thomas Ziolko and Remo Friedrich
I
An asynchronous
belt drive
motor used in
Bühler's Sirius
MPAK plansifter.
n a modern flour mill, hundreds of machines are in
operation, the vast majority of which are equipped
with electric drives. Choosing the right type of motor
and the right overdrive is vital for the efficiency, operational
reliability, and maintenance requirements of
the relevant application. For some years now, leading
machine manufacturers have been using direct drive
for some of their machines. Until now, large plansifters
were an exception. They continue to be driven by an
asynchronous motor with belt drive - for good reason.
The Latin phrase " Quod licet jovi non licet bovi, "
which translated means " what is permissible for
Jupiter is not permissible for cows, " comes to mind.
Even the Romans knew that it could be a good idea
not to treat two things exactly the same way. If this
principle is applied to the drives of the machines and
equipment in a mill, this means that what is suitable
as a drive technology for a roller mill is not necessarily
the best solution for a plansifter.
Advantages of direct drive
The electric motor plays a central role in flour milling.
Various versions of this type of motor are used with
extremely diverse overdrive systems at every stage of
the milling process, from the intake of raw materials
through to the bagging of the end products. Over the last
10 years, leading mill builders have replaced the classic
asynchronous motor with belt drive in their machines
with a direct drive or gear motor. Today, these new drive
systems mainly are used to operate bran finishers, impact
detachers, and modern roller mills, for example.
These solutions can be divided into three categories.
In relation to a belt drive, a mechanical gear system
has a slightly higher efficiency of around 98%. It is
also easy to maintain and comparable in
price to a belt drive. However, the high
efficiency of the gear system is offset
by restrictions in terms of machine design.
In addition, explosion protection
approval causes problems for gear units
with high power and low speed.
An asynchronous motor with direct
drive has no loss of power in overdrive
because the power is transmitted directly
to the machine. For the most part, standard
motors are used. These are available
worldwide, operate efficiently, require
little maintenance, have an impressively
long service life, and undergo low wear.
The nominal speed of the motor must
be considered in the design. If the required
machine speed is outside the nominal
speed, a frequency converter must be used.
In addition, larger motors must be selected
for lower speeds. This applies to motors
used with a frequency converter as well as
to motors with low nominal speeds.
66
March 2024 / World Grain / World-Grain.com
Photos courtesy of Bühler
http://www.World-Grain.com
World Grain - March 2024

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