Over load Relay Operating Principles | Types and Connections of OLR

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What is the use of Over Load Relay

Each motor must be protected from any possible faults to ensure long lasting and safe operation of the motors and to prevent damage from damage. Almost all industries rely on electric motors to control their processes and production. Therefore it is necessary to secure the failure of the motors. Overload relays are used as a device to protect the motor from damage from overload and overgrowth. The overload relay is connected to the connector, which you can see on the motor control panel and motor starters. An overload relay is a device that protects an electric motor against overload and power line failure.

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This helps in understanding the overload of the motor and impedes power to the motor, thus protecting the motor from overheating and damage. A part from overload, it can also protect the motor from electrical current loss / failures and power line imbalance. The overload relay is shortened and is usually called OLR.

An overload is a situation where the load exceeds the specified time for a motor exceeding its rated value. The maximum current a motor can carry is called its full load current (FLC).

Moving current above FLC is a common phenomenon, which may be the result of an increase in the temperature of the winding of the motor. This can happen for many reasons, so it is important to check the motor and return it to normal operation.

Operating Principle of OLR

A thermal overload relay operates on the electro-thermal principle of a bimetallic strip. The current flowing through the motor flows through the poles of the OLR. The dipole strip heats the stream directly or indirectly, and bends when the current exceeds the set value.

Overload relays are always connected to the contactor. When heating the bipolar strips, the electric current activates the contact, interrupts the power supply to the contactor, de-energizes the contactor and disrupts the supply to the motor. This tripping time is always inversely proportional to the current flow through the OLR. Therefore the overload relay travels as fast as flow. Therefore, a heat dependent relay is called a current dependent and inverted time-delay relay.

Operating a dipole thermal overload relay

As mentioned above, the dipole thermal overload relay works on the heating behavior of the bimetallic strip. In direct heating mode, the entire flow of the motor flows through the OLR. Therefore, it directly heats the electrode. In the case of indirect heating, however, the dipole strip is in close contact with the current heating conductor within the OLR. Excessive current flow to the motor heats the conductor so that currents do not flow through the strip as the dipole strip conductor separates.

Terminal Connections

Contactor input terminals are R, Y & B supply lines L1, L2 and L3 respectively. The overload relay can be mounted directly on the contactor. Overload relays can connect terminals T1, T2 and T3 to the motor.

OLR ampere range setting / Ampere range adjustment

The overload relay has a rotary knob. With this knob, you can set the rated current of the motor.
The current can be set between the upper and lower limits provided here. In the case of electronic overload relays, the class selector provides an additional knob for tripping.

Reset Button

There is a reset button on the overload relay to detect and fix the damage on the motor after one trip and reset the overload relay, by manual pressing.

Manual / Auto Reset selection;

Automatic mode / automatic reset selection
Using the manual / auto reset selection button, we can choose between manual and automatic reset of an overload relay. If the device is set to automatic mode, the OLR can be reset automatically after a certain time. In auto mode they can restart themselves without any motor malfunction. When the bipolar strip is cooled, these will automatically re-contact and exit the power supply.
It is usually better to put it in manual reset mode. This is because the mode is useful for manually resetting the motor.

OLR Assistant Contact

Two auxiliary contacts are provided in the overload relay - NO / NC and OL relay NO contact is always 97-98 and NC is always 95-96. Of these, NO is for contact trip signaling and NC is for start signaling contact. The NC contact contactor coil is capable of direct switching. When the tip is open, the force of the conductor coil is interrupted and the contactor is open. This will shut down without further damage to the motor.

What is the trip classes of overload relay?

The trip class specifies the time it takes for the contactor to open during overload. It is usually classified as class 5, class 10, class 20, and class 30 and so on. OLR trips are 5%, 10 seconds, 20 seconds and 30 seconds respectively, 600% of the full load of the motor. Class 10 and class 20 are the most commonly used. For the protection of high idle loads of motors, Class 30 overload relays and Class 5 overload relays are used for motors requiring fast tripping.

How to use an overload relay in a circuit?

Overload relays are always used in circuits in conjunction with contactors. It is connected to the motor so that the current flows completely out of contact. Below are a variety of connections for relay loading for single-phase and three-phase motors?


Reason for OLR tripping

There are four main causes of overloading, as discussed above:

1, overloading of the motor.

2, phase fault at the input level

3, phase imbalance.

4, Failure to maintain maintenance timely.

In addition to the above, some additional security measures may also be available. It is inherent in one manufacturer 1 varies from one device to another.

How does an overload relay prevent phase failures?

During normal operation, the electric current flowing from each pole of the motor overload relay will remain the same for a time. When one phase is interrupted, the current through the other two phases increases to 1.73 times the normal value. Therefore the overload relay heats up and remains open. It is also known as phase failure or phase loss.

Summary of OLR

Overload relay is a device that can protect a motor from overload, phase failure and phase imbalance. There are two types of overload relay, thermal overload relay and electronic overload relay. Thermal OLR is based on the principle of deformation of the dipole strip on heating. Electronic overload relay is a device based on microprocessors. Using OLR in conjunction with contactors. Whenever a motor breakdown occurs, it opens the electrode close contact (NC) to the contact coil. The time it takes for the contactor to open during overload specifies its trip class. Overload relay cannot protect the device from short circuit. Relays cannot protect a device from short circuits.




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