MOTORS | MEDICAL
HIGH-SPEED MMOEDTIOCRASL Advanced medical applications
The electric motor plays
a significant role in the
modern medical industry.
The growing worldwide
demand for medical analysis and
testing services has created a
niche for equipment with greater
throughput and high reliability.
To accomplish this goal, drive
train components must provide more
torque over expanded speed ranges
and at higher duty cycles. As medical
equipment grows smaller, lighter,
there is a trend for high-speed motor
drives for power density increase,
volume and weight reduction and
higher efficiency.
High-speed brushless permanentmagnet
motor technology is the
most suitable choice for these kinds
of advanced medical applications
due to a variety of merits. They offer
substantial reduction in size and
thermally excellent high-power
density, which reduces the running
costs with good performance and
reliability.
Several considerations must be
addressed when choosing highspeed
motors for medical devices.
Torque pulsation has an important
role where the medical application is
very sensitive to vibration and noise
such as devices and equipment or
patient care facilities in hospital. The
motors used are required to comply
with low noise level standards to
endorse patient comfort and reduce
anxiety. Torque pulsation can excite
a system that is not mechanically
well damped, and this may lead to
destructive consequences.
There are two main sources for
torque pulsation in advanced medical
equipment: the current pulsation in
the stator winding and the cogging
make very specific demands
of high-speed motors. Dr. Sab
Safi of SDT Drive Technology
looks at the challenges.
torque. Careful consideration must
be given to all aspects of motor
design when evaluating the design
of selected motor topology that
minimise or eliminate these pulsation
sources and thus low audible noise
generation which are very important
criteria for medical devices.
Since reliability of medical device
products is critical, the motor needs
to be designed to be durable and
versatile. Many parts have integrated
functions to help reduce the number
of components and ensure the
product is compact and robust. They
should be designed to endure hostile
environments, caustics fluids, steam,
elevated temperatures, vacuum,
vibration and mechanical impact.
In order to achieve high power
density and reliability, it is not
enough to only optimise the cooling
capability and the electrical design.
All aspects of the motor’s design
must be optimised. Thus during the
design phase, it is important to take
into account the complete system, i.e.,
blower or pump, motor, driver and
feedback.
Finally, OEMs want
medical devices that are
a compact design
and energyefficient.
Power
and space
are limited
in portable
equipment. Most devices also need to
be light and easy to manoeuvre. One
way to accommodate these trends
is to use high-speed motors that can
deliver the necessary power and
performance within the specified
footprint.
CHHIGAHL-LSEPNEGEEDS FOR
A number of design challenges
arise as a consequence of the
motors becoming smaller and faster,
designers must consider the potential
effects of electrical drive frequencies,
mechanical stresses on the quickly
rotating shaft (rotor dynamics),
magnetic eddy currents, laminated
steel core loss, and the inverter.
First, the mechanical integrity of
the rotor becomes an issue. Keeping
the rotor radius small enough should
guarantee operation below critical
speed. The introduction of a ring
in the air-gap will ensure magnet
retention. This results in an increase
of the effective air-gap length.
Another design issue
occurring at high-speeds
is that of increased rotor
losses. However, through
careful selection of the
permanent magnet material
and through proper thermal
management, the rotor
losses can be minimized and a
successful design can be obtained.
32 WWW.EUREKAMAGAZINE.CO.UK | JULY 2020
/WWW.EUREKAMAGAZINE.CO.UK