There is a wide range of motors in existence for various purposes. However, the fundamental problems affecting the choice of motor protection are independent of the type of motor and the type of load to which it is connected. The motor under discussion here are a.c. motors which include synchronous  and induction motors protection and its types of electrical faults.

 

 

Types of electrical faults in motors are similar to those of generators. Motors therefore in general are protected against the following faults:

The stator circuits can be either to earth or between phases. The protection from these faults is provided with the help of thermal or dash, pot type overcurrent tripping devices giving an inverse time-current characteristic and usually providing instantaneous tripping at high current. Instantaneous overcurrent relays supplied from CTs are provided for motors of larger rating (Usually more than 50 HP).

Phase – fault protection is provided by two high-set instantaneous relay elements; the setting is so chosen that it is well above the maximum starting current.

Earth fault protection for a motor operating on an earthed neutral system is provided by means of a simple instantaneous relay having a setting of approximately 30% of the motor full load current in the residual circuit of three CTs. Operation of relay due to CT saturation during initial high starting current should be avoided.

This is usually achieved by increasing the voltage setting of the relay by inserting a stabilizing resistance in series with it. Details of one such scheme applied to an induction motor is shown in figure below. When a motor operates on an unearthed neutral system E/F relays as shown in figure is of no use and neutral displacement equipment has to be applied.

 

 

Differential Protection is sometimes provided on very large and important motors in case of unearthed neutral systems.

Any form of unbalance either in the supply voltage or in the loading pattern will cause negative sequence currents to flow in the stator which will include high frequency currents in the rotor. The frequency of these currents in the rotor is (2-S) times the nominal frequency of supply. the rotor heating due to the positive sequence component of the stator current is proportional to DC resistance value while the heating effect on the rotor windings of the negative sequence components is proportional to (2-S)f (approximately 100Hz) ac resistance value. Obviously, the heating effect of negative phase sequence current is greater than that of the positive phase sequence current. Motor protection therefore must take this into account if it is to decide correctly what load the motor can stand for a given degree of voltage unbalance without overheating. Types of protection provided for unbalanced voltages will be discussed subsequently. On wound rotor machine some degree of protection against faults in the rotor winding can be obtained by an instantaneous overcurrent relay measuring the stator current.

The wide diversity if motor duties and motor designs make it very difficult to cover all types and rating of motor with a given characteristic curve. The overload protection is so designed that it matches as closely as possible the heating curve of majority of motors. The protection characteristic should lie just below the heating curve of the motor protected. The protection should preferable have adjustable characteristics so that it may be adapted to different designs of motors and different duties.

The protection should not allow the motor to be restarted after tripping while the winding temperature is still high as this may dangerous consequences. In order to be an effective safeguard an ideal protection should not allow the motor to be restarted after tripping while the winding temperature is still high as this may have dangerous consequences. In order to be an effective safeguard an ideal protection should therefore not only match the heating characteristic of the rotor but also its cooling characteristic. It must also be ensured that the relay must not operate under heavy starting currents up to say 6 times full load current which can last for a few seconds, half a minute or even longer in exceptional cases. The thermal time constant of most types of motors is of the order of 15 to 20 minutes; hence for protection from overload the relay should have a time constant slightly lower than this.

When a motor falls , a current equal to the starting current flows and serious damage results if it persists for a time longer than the starting time, Hence, the closer the characteristic of the overload relay matches the starting current curve the better is the motor is protected against such damage.

Induction overcurrent relays having characteristic of the type shown in figure below are best suited for such purposes. A typical setting required for overload protection is 120% of full load current. It can be seen from the figure that the current setting is 120% of full load, but a starting current of 6 times full load current for 30 seconds will not cause tripping. With the help of the time multiplier setting the operating time at high values of overcurrent can be adjusted to match the motor starting characteristic without changing the current setting.

 

 

One phase-connected relay element is sufficient for overload protection, but two provided single phasing protection as well.