Tech Briefs Magazine - October 2021 - ST-17

designed to be in direct contact with an
outer bearing race. This mounting provides
the most accurate temperature
measurements. The bearing housing
must have features designed in that will
accommodate the temperature sensor
and provide close access to the bearing.
Temperature sensors are also used to
measure the temperature of stator eletrical
windings. These are the coils that
generate the magnetic fields to propel
the rotation of the armature. If these
windings are damaged by impact or corrosion,
the resistance of the wire increases,
and their ability to conduct current
diminishes. This causes the affected
wires to heat. They can get to temperatures
that will melt insulation and even
cause a fire.
Figures 5a and 5b show temperature
sensors (TE Connectivity Sensors
Business Unit) being built into stator
windings on a motor. The sensors
become a permanent part of the device.
As with the vibration sensors, initial
operating temperature is measured and
recorded. As time passes, the temperatures
are taken periodically and compared
to the baseline. If the temperatures
begin to deviate from normal, it's
an indication of mechanical problems
for the bearings or electrical problems
for the stator windings. Again, maintenance
can be scheduled before a major
failure occurs, preventing an unexpected
line shutdown.
Current Sensors
While vibration and temperature sensors
can report the physical health of a
motor, a current sensor can check its
SHUNT
M
50mΩ
electrical health. Current sensors come
in two basic configurations: shunt or
inductive designs. Each has advantages
and disadvantages. The amount of current
a motor draws is affected by many
things: the applied voltage, the motor
speed, the load on the motor shaft, and
the condition of the armature brushes.
Out-of-range currents can signal power
voltage problems, shorts in the stator or
armature windings, worn brushes, or
problems with attached tooling.
Vdd
OUT
Figure 6. Shunt current sensor schematic. (TE Connectivity)
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Tech Briefs Magazine - October 2021

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