Each motor must be protected from all possible failures to ensure long and safe operation, as well as time loss caused by breakdowns. Almost all industries rely on the electric motor to control their processes and production. Therefore, it is necessary to make the motor fail-safe. The most vulnerable part of a motor is its winding insulation. Winding insulations are at high risk of being damaged by excessively high temperatures. The overload relay is one such device that protects a motor from damage caused by overloads and overcurrents. It is used with contactors and can be found in motor control centers and motor starters. An overload relay is a device that protects an electric motor against overloads and phase failures. It detects motor overload and interrupts the flow of power to the motor, thus protecting it from overheating and winding damage. Apart from overloads, it can also protect the motor from phase loss/failure and phase unbalance. They are very commonly known as OLR
A thermal overload relay is a relay designed to protect electrical systems from overheating conditions. It does not remove power from the circuit, but instead senses when the current has reached a high enough level and opens, allowing the motor to continue running. Thermal overload relays protect motors, transformers and other electrical devices from overheating. These relays are often installed at points where there is an electrical circuit with various devices. If one of these devices overheats, it can damage itself or other parts of this circuit. The thermal overload relay has three main functions:
Electronic overload relays are generally referred to as solid state overload relays. The interior of this type of relay does not contain a bimetallic strip. Alternatively, include current transformers or temperature sensors to sense the sum of current flowing to the motor. For protection, this type of relay uses microprocessor-based technology. Here, the PTC plays a key role in sensing the temperature and tripping the circuit once overload errors occur. Some types of overload relays come with Hall effect sensors and current transformers to sense current flow directly. The main benefit of an electronic overload relay over a thermal overload relay is that it lacks the bi-metal strip, resulting in less heat loss from the relay. Also, these types of relays are more accurate compared to thermal relays. Electronic overload relays are used where frequent starting and stopping of motors is required. The design of these relays can be made in such a way that they withstand the initial current of the motor for a restricted period.
Thermal overload relays are wired in series with the motor, so current flowing to the motor also flows through the overload relay. When the current reaches or exceeds a predetermined limit for a given time, the relay activates a mechanism that opens one or more contacts to interrupt current flow to the motor. Thermal overload relays are classified by their trip class, which defines the amount of time the overload can occur before the relay responds or trips. Common travel classes are 5, 10, 20, and 30 seconds. Timing, as well as current, is important to consider for AC induction motors because they draw significantly more than their full rated current (often 600 percent or more) during startup. So if the relay trips immediately when the overload current is exceeded, the motor will have a hard time starting.
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