Motor 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 responsibilities:

Stator faults
Rotor Faults
Overloads
Unbalanced supply voltages, including single phasing
Under voltage
Reverse or Open-phase starting
Loss of synchronism ( in the case of synchronous motors only)

Stator Protection of Motor

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 a high current. Fast overcurrent relays supplied from CTs are provided for motors of larger ratings (Usually more than 50 HP).

Phase – two high-set instantaneous relay elements provide fault protection; 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 usable instantaneous relay within approximately 30% of the motor’s full load current in the residual circuit of three CTs. Operation of the relay due to CT saturation during the 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 are shown in the figure below. When an engine operates on an unearthed neutral system, E/F relays, as shown in the figure, are useless, and neutral displacement equipment must be applied.

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

Rotor Protection of motor

Any unbalance in the supply voltage or the loading pattern will cause negative sequence currents to flow in the stator, including high-frequency currents in the rotor. These currents in the rotor are (2-S) times the nominal supply frequency. The rotor heating due to the positive sequence component of the stator current is proportional to the DC resistance value. In contrast, the heating effect on the rotor windings of the negative sequence components is proportional to (2-S)f (approximately 100Hz) ac resistance value.

The heating effect of the negative phase sequence current is greater than that of the positive phase sequence current. Therefore, motor protection must consider this 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 a 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.

Overload Protection of motor

The wide diversity of motor duties and motor designs makes it very difficult to cover all types and ratings of the motor with a given characteristic curve. The overload protection is so designed that it matches as closely as possible the heating curve of the majority of engines. The protection characteristic should lie just below the -engine-protected heating curve. The protection should preferably have adjustable attributes so that it may be adapted to different designs of engines and other duties. The defense should not allow the engine to be restarted after tripping while the winding temperature is still high, as this may have dangerous consequences. To be an effective safeguard, ideal protection should not allow the motor to be restarted after tripping.

At the same time, the winding temperature is still high, as this may have dangerous consequences. To be an effective safeguard, ideal protection should match the heating characteristic of the rotor and its cooling feature. It must also be ensured that the relay must not operate under heavy starting currents up to six times the 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 15 to 20 minutes; hence, the relay should have this for protection from overload.

When a motor fails, a current equal to the starting for protection from overload 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 the motor is protected against such damage.

Induction overcurrent relays having characteristics of the type shown in the 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 location is 120% of the full load, but a starting current of 6 times the 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 location.

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

Unbalanced supply voltages, including single phasing

Unbalanced supply voltages, including single phasing, pose significant challenges in electrical systems and can lead to various issues. When the supply voltages provided to an electrical system are not evenly balanced, the voltages across different phases are unequal. This situation can occur due to various factors, such as faulty connections, broken power lines, or issues with the distribution transformer.

One common problem associated with unbalanced supply voltages is single-phasing. Single phasing refers to the condition where one of the phases in a three-phase system is lost or disconnected. This results in an imbalance in the electrical load across the remaining two phases, which can lead to severe consequences.

In electrical motors, for instance, unbalanced supply voltages can cause an increase in the current flowing through the motor windings. The unstable current distribution can lead to overheating and subsequent damage to the motor.
Unbalanced supply voltages can also impact other electrical equipment, such as transformers, generators, and electronic devices. The unequal voltages can result in increased stress on the components, leading to premature aging, increased losses, and decreased lifespan.

Under voltage

Under voltage refers to a situation where the voltage supplied to an electrical system or device drops below the expected level. Various factors can be the reason for its occurrence, such as power grid fluctuations, equipment malfunction, or high electrical demand exceeding the capacity of the power source. Under voltage can have significant implications for electrical equipment and systems operation.

When under voltage occurs, it can lead to reduced performance and efficiency of electrical devices. Many electrical appliances and machinery require a specific voltage range to function optimally. Devices may not operate as intended if the voltage drops below this range. Motors, for example, may experience reduced torque and speed, leading to decreased performance and potentially affecting industrial processes or equipment functionality.

Under voltage can also cause overheating in electrical equipment. Devices may draw higher currents when the voltage is lower than normal to compensate for the reduced power. The increased current flow can result in elevated temperatures within the equipment, leading to overheating and potential damage. Over time, repeated exposure to voltage conditions can lead to accelerated wear and tear, reduced lifespan, and increased maintenance and replacement costs.

Reverse or Open-phase starting.

Reverse or open-phase starting refers to a situation where a three-phase motor is initiated with one or more phases connected in reverse or completely disconnected, resulting in improper motor operation. This condition can occur due to wiring errors, faulty connections, or damaged components. Reverse or open-phase starting can harm the motor and the connected equipment.

When a motor is started with reversed or open phases, the normal rotation direction of the engine can be reversed. This can lead to mechanical stresses on the motor and the driven equipment, potentially causing damage to gears, belts, or other transmission components. Furthermore, the reversed rotation can result in a loss of efficiency and reduced motor performance, leading to increased energy consumption and decreased productivity.

Loss of synchronism

Loss of synchronism refers to a condition that occurs specifically in synchronous motors when the rotor speed becomes out of sync or fails to maintain synchronism with the rotating magnetic field generated by the stator. Synchronous motors are designed to operate at a specific synchronous speed determined by the power supply frequency and the number of poles in the engine.

The loss of synchronism causes the rotor to fall out of step with the rotating magnetic field, resulting in erratic operation and decreased motor performance. One common cause of loss of synchronism is a sudden change in load. If the motor experiences a sudden increase in load or an abrupt reduction in torque demand, it may not be able to maintain the required speed and can fall out of synchronism. This can lead to mechanical stresses, increased vibration, and potential motor or driven equipment damage.