Page 326 - Industrial Power Engineering and Applications Handbook

Protection of electric motors la305

R- with an accuracy of less than 1 "C and a response time of
the order of 0.2 second. Pure metals possess almost a
Y-
linear temperature coefficient of resistance p, over a wide
B- range and this characteristic is used in monitoring the
temperature of a particular object. In pure platinum
N-
R, = ~ ~ + pe) 1sz
(
sw
where R, = resistance at temperature 8°C
Ro = resistance at 0°C = 100 Q
8 = end temperature "C
3 p = 3.85 x per "C at 0-100°C
G
The exact resistance variations of a P, - 100, RTD over
a range of temperatures are given in Table 12.2, and not
very different from those calculated by the above equation
and drawn in Figure 12.41.
Application

All the above sensing and monitoring devices are basically
supplements to overload and single-phasing protection.
They are worthwhile for critical installations that require
more accurate sensing or monitoring of the operating
temperatures of the different vital components or likely
-I hot spots. They also eliminate any chance of tripping for
I
M I all operating conditions that can be controlled externally.
55 "D A few applications may be speed variation, affecting
cooling at lower speeds, frequently varying loads, frequent
PTC
thermistors starts, stops, plugging and reversals etc. In all these cases,
the other protections may not detect the heating conditions
of the vital components of a machine as accurately as a
Tripping unit temperature detector.

12.8 Monitoring of a motor's actual
Figure 12.38 A PTC thermistor protective circuit
operating conditions

large and critical motors as temperature sensors. They To warn of an unfavourable operating condition by the
are also being used as thermistor relays to trip a motor use of an audio-visual alarm or trip or both, schemes can
by converting the resistance change into a current change be introduced in the control circuit by means of a
of the control circuit. Figure 12.40 shows such a relay. temperature detector or other devices to monitor any or
all of the following internal conditions of a motor:
12.7.1 Embedded temperature detectors (ETDs)
Motor winding temperature
They are basically temperature monitoring devices and Motor or driven equipment bearing temperature
can indicate the temperature on a temperature scale. They 0 Coolant circuit inlet water pressure and temperature
may be one of the following types.
Moisture condensation in the windings
Motor speed
Thermocouples Vibration level
These are bimetal elements, consisting of a bimetal Safe stall time
junction which produces a small voltage proportional to Rotor temperature
the temperature at the junction. They are thus able to Any other similar condition, interlocking with other
feeders or sequential controls etc.
detect the winding temperature conditions when embedded
inside the motor windings. For more details refer to DIN
43710. To detect the above, sensing and monitoring devices
can be installed in the motor. Some of these are built-in
Resistance temperature detectors (RTDs) during the manufacturing stage and others are fitted at
site during installation.
These are normally of pure platinum wound on a ceramic Several types of sensing and monitoring devices are
or glass former and encapsulated in a ceramic or glass available that are embedded at a motor's vulnerable
shell, having an operating range of -250°C to +750"C, locations and the hot spots at the manufacturing stage