Page 304 - Industrial Power Engineering and Applications Handbook

Protection of electric motors 12/283

Note 1 Protection of small and medium-sized LT motors, up
1 It is for this reason that rotors, as standard practice, are to 300 h.p.
designed to withstand a much higher temperature of the 2 Protection of large LT motors, say, 300 h.p. and above.
order of 400450°C in LT motors and 300-350°C in HT This is to be decided by the user, based on the load
motors, compared to a too-low temperature of the stator. requirement and critical nature of the drive
This temperature is such that for almost all motor operating
conditions meticulous protection of the stator would also 3 Protection of HT motors
orotect the rotor. It is also observed that rotor failures are
therefore rare compared to stator failures. Small and medium-sized LT motors
2 Nevertheless, whenever the rotor is more critical, despite a
higher rotor operating temperature, rotor thermal curves
are provided by the manufacturer for facilitating protection Protection against overloading
for the rotor also through the stator. This can be achieved by an overcurrent relay. The basic
3 The rotor design, its cooling system or the motor size itself requirement of this relay is its selectivity and ability to
may have to be changed substantially for motors to be discriminate between normal and abnormal operating
installed in fire hazard environments to limit their tem- conditions. Three types of such relays are in use: thermal,
perature rise in adverse operating conditions within safe electromagnetic and static. Thermal relays are employed
limits (Table 7.6).
for motors of up to medium size and electromagnetic
and static relays for large LT and all HT motors, as
discussed in Section 12.5.
12.4 Protection The thermal relays in general use are of two types, i.e.

Bimetallic, and
12.4.1 Protection against unfavourable operating 0 Fusible alloy or eutectic alloy
conditions
Because of their spread between hot and cold
Protective devices and their selection characteristics, these relays allow a tripping time of less
than the starting time when a hot motor stalls, so a separate
We will now discuss protective devices, and their selection, stalling protection is normally not necessary. They detect
that will be essential to safeguard a motor against all the r.m.s. value of the current and thus account for the
unfavourable operating and fault conditions. A machine effects of harmonics, present in the current, drawn by
may be provided with a modest or elaborate protective the motor. They also take into account the heating, due
scheme, depending upon its size, application and voltage to previous running of the motor as they are also heated
rating. This enables savings on cost, where possible, and along with the motor. This feature is known as thermal
provides a more elaborate protection where more safety memory. These relays thus possess tripping characteristics
is necessary. Accordingly we have sub-divided the almost matching the thermal withstand capacity of the
protection as follows: motor.

Figure 12.11 Thermal overcurrent relay (Courtesy: L&T)