Page 296 - Industrial Power Engineering and Applications Handbook

Protection of electric motors 121275
12.1 Purpose Note Sometimes when there are perennial wide voltage
fluctuations at certain locations/installations the manufacturers
of the contactors on demand from users may design their
An electric motor must be adequately protected against holding coils for even lower pick up and higher drop out
all unfavourable operating conditions and internal or voltages than noted to save the feeders from unwanted trips,
external faults. We have classified these conditions into the user making extra capacity provision in the motor or getting
three categories to identify the most suitable protection: it redesigned for special voltages to surtain the wide voltage
fluctuations.
1 Unfavourable operating conditions (iii) Reverse rotation This may occur due to a wrong
2 Fault conditions phase sequence. While the motors are suitable for
either direction of rotation, the load may be suitable
3 System disturbances and switching surges (for HT for one direction only and hence the necessity for
motors)
this protection. A reverse rotation means a reverse
rotating field and is prevented by a negative phase
12.2 Unfavourable operating sequence, i.e. a voltage unbalance or single-phasing
conditions protection. Moreover, this protection is also of little
significance, as once the motor is commissioned
with the required direction of rotation, it is rare that
Operating conditions that may overload a machine and the sequence of the power supply would reverse.
raise its temperature beyond permissible limits may be (iv) Protection from harmonic effects Motors are
called unfavourable. This overheating, however, will be influenced less with the presence of harmonics. This
gradual (exponential), unlike rapid (adiabatic) heating is due to the benign effects of harmonics on inductive
as caused during a locked rotor condition. The machine loads, on the one hand, and the motor providing no
now follows its own thermal curve and therefore a path to the third harmonic quantities on the other, as
convcntional thcrmal protcction dcvicc can be used to it is normally connected in delta. In HT motors,
protect it from such conditions. These conditions may however, which are normally star connected, the
arise due to one or more of the following: neutral may be left floating to provide no path to the
third harmonics.
(i) Otzerloading Due to excessive mechanical loading. Higher harmonics increase the harmonic reactance
(ii) Undervoltage Low voltage results in forced over- and have a dampening effect (Section 23.5.2(B)). A
loading due to higher slip losses and higher current motor circuit, LT or HT, possesses a high inductive
input to sustain the same load requirement. An impedance due to interconnecting cables and its own
unstable sub-distribution network, a number of small inductance, and provides a self-dampening effect to
LT loads on a long and already overloaded LT the system’s harmonics. There is thus no need,
distribution system, or inadequate cable sizes may generally, to provide protection against harmonics
cause an excessive fall in the receiving end voltage. specifically, except for high no-load iron losses.
See also Section 23.3, where we analyse the effect If, however, high contents of harmonics exist, as
of low power factor on the terminal voltage. when the machine is being fed through a static power
The effect of small voltage drops (Section 1.6.2)
is taken care of by the standard overcurrent protection inverter (Section 6.13), they will produce magnetic
used in the motor’s switching circuit. But for installa- fields, rotating in space, proportional to the individual
harmonic frequencies. These fields may be clockwise
tions where the voltage available at the motor or anti-clockwise, depending upon whether the
terminals may fall below the permissible level, say, harmonic is positive or negative. The fields produce
below 90% of the rated voltage, overheating or even different torques, which may also be clockwise or
stalling may occur if the voltage falls below 85%. anti-clockwise. The net effect of all this is a pulsating
In such a condition, depending upon the severity of torque. In a six-pulse thyristor circuit, for instance,
the voltage fluctuation and the load requirement, a the harmonic disorder is -5, +7, -1 I, +I 3, -17 and
separate undervoltage relay may also be used. +I9 etc. giving rise to clockwise and anti-clockwise
The ‘no-volt’ coil of the contactor or the fields.
undervoltage trip coil of the breaker, used for the To reduce the no-load iron losses caused by such
motor switchings, are designed to pick up at 85% of harmonics the machine core may be formed of thinner
the rated voltage. These coils drop out at a voltage low-loss laminates (see also Section 1.6.2(A-iv)).
between 35% and 65% of the rated voltage and would When the machine has already been manufactured
not protect the motor against undervoltages. In normal and there is a need to suppress these harmonics,
service con-ditions the system voltage is not likely filter circuits may be employed along the lines
to fall to such a low level, particularly during running. discussed in Section 23.9.
Thus, the protection will not prevent the closing Excessive harmonics may also make the protective
of the contactor or the breaker when the supply devices behave erratically or render them inoperative.
voltage is *85% or more, nor will it trip the motor Filter circuits would suppress the harmonics and
until the voltage falls to a low of *65% of the rated eliminate these effects.
voltage. In both cases, therefore, separate
undervoltage protection will be essential. This Nore The protective devices which meature r.m.s. values of
problem, however, is a theoretical one, as an industrial current also detect the harmonic contents and provide automatic
power system would seldom fluctuate so widely. protection for the machine against harmonic disorder\. But in