Thermal Overload Relays
The most common overload protection methods for three-phase motors are thermal overload relays, which are especially suited to provide long-term protection against overloads for small and medium motors. The core principle is the thermal expansion characteristic of the bimetal strip used for the measurement of current changes in the motor. Assuming that the current exceeds the set value, it heats the bimetal strips, due to which they bend and consequently break the circuit to stop the running motor to avoid overheating.
The general setting is in the range of 1.05 to 1.25 times the rated motor current to avoid nuisance trips under normal operating load conditions. However, its response is relatively slow, so this type of relay will be more suited for overload situations with the motor having a slightly longer overload period than for the momentary high current draw applications—a condition experienced at startup or under short-circuit conditions.
One disadvantage of thermal overload relays is that it is affected by ambient temperature. If the motor operates in a high-temperature ambient, it may cause the relay to trip prematurely; if the ambient is colder, it slows down its response. Therefore, in practice, technicians usually adjust the settings according to the operating environment and the load on the motor so that it cuts off power at the right time to prevent damage to the motor.
Magnetic Overload Protection
Magnetic overload protection, mainly applied to the treatment of very high instantaneous currents at the instant of short circuits or motor startups, acts on a change in current through an electromagnet. When the current increases beyond the set value, the magnetic field rapidly builds up and acts to trip the circuit breaker for the protection of the motor. Magnetic protection is extremely fast-acting, producing effects in milliseconds compared with the response time of a thermal relay. This makes it especially suited for loads that have brief, high current surges.
Talking about heavy industrial equipment or motors with very high startup currents, such as large compressors and hoisting equipment, magnetic overload protection may become essential. The motor current at the time of startup can be as many as six to eight times the rated current. Hence, magnetic overload protection allows for the normal startup of the motor without damage from the current surge.
The protection against magnetic overload is very often combined with thermal overload relays in complete protection capable of managing both long-term overloads and short-term surges of current. The combined type is widely utilized in industrial equipment, pumping systems, and large motors where starts are quite frequent.
Electronic Overload Relays
The modern electronic overload relays have very precise current sensors and digital processors that monitor the real-time status of the operating motor. Because these are more sensitive compared to conventional thermal and magnetic relays, electronic overload relays can set accurate trip points and times according to particular load conditions.
It trips when the load current of the motor rises to 1.1-1.5 times its rated value, offering effective protection against overloads for the motor. Because it is independent of temperature changes in making its judgment on overload, the electronic overload relay enjoys good stability even after the ambient temperature shifts notably. Moreover, its tripping time and current threshold can be adjusted according to actual needs to ensure that the best protection under different working conditions can be guaranteed.
Current and load status at the time of a trip are recorded, making it easier for technicians to find the problem source after a motor fault occurs. Precise monitoring and diagnostic capability extend the applications to modern industrial automation systems, precision production equipment, and applications with high reliability and real-time monitoring requirements.
Overcurrent Relays
The overcurrent relay has been designed to protect the motors from deteriorations that result from excessive current. They detect the magnitude of the current to decide whether to cut off the power. It is also applicable for motor overloads and short circuits. Overcurrent relays can be categorized into two types, including definite-time overcurrent relay and inverse-time overcurrent relay.
The operating principle of a definite-time overcurrent relay is to trip with a fixed time delay when the identified current signal is higher than its set value. It finds good application in cases when the motor operates under slight overload, which prevents spurious trips due to momentary fluctuations in current signals. On the contrary, in inverse-time overcurrent relay trips, the trip time is inversely proportional to the magnitude of the current. Thus, it is especially suitable for dealing with serious electrical faults such as short circuits.
In practice, the trip current of an overcurrent relay in industrial applications is usually set to 1.5 to 2 times of rated current of the motor so that power cut can be done on time in order to avoid motor damage due to short circuit or heavy overload.
Fuse-Based Protection
The fuse is relatively simple in design as a motor protection device. Mainly, it has found application in the protection of motors against damage from short circuits or instantaneous overcurrent. Fuses protect the interruption of power by melting a metal wire inside the fuse. When the current exceeds the rating of the fuse, the metal wire quickly melts, thereby breaking the circuit and protecting the motor against excessive current.
Fuse selection is usually based on the motor’s rated current and is usually set within the range of 1.25 to 1.5 times the rated current. Another disadvantage with fuses, unlike other protection devices, is that they cannot be reset once they operate; instead, they have to be replaced with new ones. This means that once a fuse trips, the motor operation stops until the fuse is replaced.
Although they are used only once, fuses are comparatively low in cost and easy to install, so they have the advantage in small motors and for backup protection. However, most modern industrial equipment requires both high up-time and high reliability, so fuses are normally installed with other resettable protection devices, such as motor protection circuit breakers, for more comprehensive protection.
Motor Protection Circuit Breakers
An MPCB is an integrative, multifunction unit that protects for thermal, magnetic, and phase failure conditions. These high-quality motor protection circuit breakers are designed to meet motor protection needs against overload, short circuits, and phase faults. Unlike traditional fuse systems, they offer comprehensive protection features and the possibility of manual or automatic resetting, reducing downtimes in this equipment.
For motor overloads, the MPCBs are designed with thermal protection characteristics that cut power when the current exceeds 1.05 to 1.2 times the rated value. Under short-circuit conditions, the magnetic protection capability of the MPCB can respond within milliseconds to cut the current and prevent burnout of the motor due to short circuits.
The phase failure protection works in such a way that it immediately on the failure of any of the phases in the three-phase power supply to the motor. This prevents the occurrence of motor damage due to operation under phase-loss conditions. Capability for multiple functions makes MPCBs the modern choice for industrial motor protection systems and find wide application in main equipment and factory automation systems.
Phase Failure and Imbalance Protection
Three-phase motors rely on the balance of power feeding between their three phases for normal operation. During phase failure or phase imbalance, there is a danger of excessive current in one phase due to overheating, reduced efficiency, and possibly irreversible damage to the motor. Phase failure protection devices can give a very prompt cut to the power supply of the motor when one phase current disappears in order not to overload the motor during conditions of lost phase.
Phase imbalance protection detects the balance of the three-phase currents or voltages. When this imbalance exceeds a threshold value, usually within the limit of 2% to 5%, the protection device trips automatically to prevent the motor from running in an imbalanced state, which can result in overheating or mechanical stress.
Studies indicate that with every 1% increase in phase imbalance, the rise in motor temperature can increase by 6% to 10%, which will reduce the motor’s life. As such, phase failure and imbalance protection are critical for maintaining motor reliability and stability due to complex power grid conditions, especially in those industries where power supply is unstable.
