Which type of protection should provided to motor to prevent fault

Different types of Induction motor protection given below:

Quick links
 
1)Thermal Protection
2)Thermal Model
3)Frequent Startup Supervision
4)overload protection
5)overheating protection
6)Protection against Insulation Failures

Introduction of motor protection:

Electric motors are exposed to many kinds of disturbances and stress. Part of the disturbances is due to imposed external conditions such as over voltage and undervoltage, over frequency and underfrequency, harmonics, unbalanced system voltages and supply interruptions,  Other possible causes of external disturbances are dirt in the motor, cooling system and bearing failures or increase of ambient temperature and humidity. Stress on motor due to frequent successive startups,overload situations including mechanical stress. The above stress and disturbances deteriorate the winding insulation of the motor mechanically and by increased thermal ageing rate, which may eventually lead to an insulation failure.

The purpose of the motor protection is to limit the effects of the disturbances and stress factors to a safe level, for example, by limiting overvoltages or by preventing too frequent startup attempts. If, however, a motor failure takes place, the purpose of the protection is to disconnect the motor from the supplying network in due time

1)Thermal Protection:

Motor overload condition is mainly a result from abnormal use of the motor, harmonics or unbalanced supply voltages. They all increase the motor losses and cause additional heating. As the temperature exceeds the rated limits specified for the insulation class, the winding insulation deterioration startup. This will reduced the expected lifetime of the motor and may lead to an electrical fault in the winding. Thus, the thermal overload protection can be
considered being the most important protection function

2) Thermal Model 
The thermal behavior of the stator and the rotor during startups and during constant overload situations differs significantly from each other. Due to this fact, the dynamics of the motor heating and cooling is typically designed separately for the stator and for the rotor of motor. Designing the thermal overload protection in this way, it can be set to follow the thermal state of the motor optimally, and good and accurate protection against both short and long-time overload conditions can be accomplished, which allows the full use of the available capacity.

3) Frequent Startup Supervision:

In order avoid, shorten the expected lifetime of the motor, there must be an adequate time interval between successive startups of motors. Therefore, a certain starting frequency, that is, the number of startups per hour specified for the motor, must not be exceeded.Especially during successive startups, the temperature of the rotor rises and drops rapidly whereas the temperature of the stator changes much more slowly. At rated load, the temperature of the rotor is much lower than the temperature of the stator.If after a startup the motor is running for some time before stopping, the rotor has enough time to cool down. Then whether a restart can be done or not depends totally on the stator temperature, which can be a limiting factor. On the other hand, if the initial start had been done from a cold condition and it failed for
some reason, then whether a restart can be done or not depends now totally on the rotor temperature, which can be a limiting factor instead. If the initial start had been done from hot condition, then the limiting factor is again the stator temperature. this kind of thermal behavior simulated with the two-time-constant model for the stator and for the rotor.

4) Overload Protection
A minor overload does not cause a motor failure immediately but it will eventually shorten the expected lifetime. On the other hand, a constant overload can be a sign of some kind of disturbance in the process in which the motor drive is being used. Thus, a two-stage overload protection is preferred. The alarming stage gives an indication that the rated load of the motor with a possible margin has been exceeded. This function can be implemented by a pre-warning temperature level setting in the thermal model. The pre-warning alarm gives the operator some time to find out the possible source of the overload and to attempt to remove it. If the overload becomes higher, for example 10-15%, the tripping stage starts and trips the motor feeder in due time unless the source of the overload has disappeared before that. According to the thermal model,tripping takes place when the estimated thermal level exceeds 100%. shows an example of the thermal behavior when the motor is running on a constant cyclic overload. These curves have been simulated using the two-time-constant model for the stator and for the rotor. In this case, the prewarninglevel is exceeded and the operator is notified. As a result, the tripping is prevented as the loading of the motor becomes suitably reduced. It can also be concluded from that a comprehensive overload protection in fact requires the use of the two-time-constant model for the rotor and for the stator, and in this way, the full utilization of the available capacity of the motor is ensured. However, adequate protection can also be implemented using the single-time-constant model, which is set to allow the normal use of the motor drive with a suitable margin
 
5)Overheating Protection

The motor temperature can rise above the rated values allowed for the stator and for the rotor even if the motor is not actually being overloaded. Possible reasons for this are dirt in the motor, failures in the cooling system or harmonic currents. Especially in case of variable frequency drives, overheating may take place if the motor is being loaded with rated current simultaneously with low rotating speed because of the increased losses and reduced cooling.

6)Protection against Insulation Failures

i) Stator Earth-fault Protection

As the winding insulation deteriorates due to ageing processes, instantly imposed severe thermal or mechanical stress may exceed the withstanding level, leading to an insulation breakdown. Usually, this results in an earth fault, where the fault current flows through the stator iron plates. If the current is less than 10A,only alarming protection can typically be considered. With a higher fault, current tripping is recommended as the damages to the stator iron may become substantial. 

ii) Short Circuit and Inter-winding Fault Protection:

Sustained earth fault in the motor can develop to a short circuit fault between the phase windings. Another possible cause is the out-of-phase re-energizing during autoreclosing when high mechanical stress may shake and vibrate the windings so that the insulation between the windings breaks. This kind of fault must be tripped as fast as possible to minimize further damages. Usually a high-set overcurrent stage is used for the protection. This function also constitutes the short circuit protection of the supplying cable. Also with bigger machines, differential protection can be considered if the neutral point with suitable CTs are available.