There is a staggering variety of motor applications, each different, and each with particular overload protection requirements. Exactly how relays achieve their performance to match these requirements is what is critical and what, essentially, separates conventional relays from the newer, electronic models.
What is an electronic overload relay?
The electronic overload relay is a microprocessor-controlled device designed to protect three-phase and one-phase motors against excessive temperature rise from overload and phase failure.
Internal current transformers measure continuously the actual motor current in each phase. When the preset operating value is reached, a trip occurs. After the restoration of voltage, the relay reverts to its position before the interruption.
Electronic overload relays are the alternative to thermal overload relays. An electronic overload relay offers reliable and fast protection for motors in the event of overload or phase failure.
The electronic overload relays are three pole electromechanical devices. The motor current flows through build-in current transformers and an evaluation circuit will recognize an overload (overcurrent). This will lead to a release of the relay and a change of the contacts switching position (95-96 / 97-98). Contact 95-96 is used to control the load contactor. The electronic overload relay is self-supplied, which means no extra external supply is needed.
The overload relays have a setting scale in Amperes, which allows the direct adjusting of the relay without any additional calculation. In compliance with international and national standards, the setting current is the rated current of the motor and not the tripping current (no tripping at 1.05 x I, tripping at 1.2 x I; I = setting current). The relays are constructed in a way that they protect themselves in the event of an overload. The overload relay has to be protected against short circuits.
How does an electronic overload relay work?
Monitoring the current passing through a motor is the principal function of an electronic overload relay. An electronic overload relay is supposed to either provide a signal and/or cause disconnection of the load from the power supply at a preset value of overcurrent.
The relay is part of an overall protection scheme, put there to protect the motor, the branch conductors, and the motor control apparatus against sustained overloads by interrupting the power to the contactor coil, which disconnects the motor from the power line. It does not interrupt the motor power independently. Its role is to protect the motor, controller, and conductors through detection of overcurrents up to around 10..12 times the full load motor current.
An overload relay is expected to perform during a sustained current overload situation. A motor’s vulnerability lies in its insulation since motor heating increases with the square of the current and causes deterioration of insulation. Drawing excessive mechanical loads or operating with low line voltage increases the current drawn by the motor and increases the temperature. Overheating would result unless the overload relay performs its design function: interrupting the control voltage which in effect disconnects the motor from the line.
Three-phase voltage unbalance can cause current to unbalance up to 15 times the voltage unbalance, resulting in motor overheating. Overload relays are being assigned the task of monitoring and responding to these situations.
Single phasing can increase the motor slip. With excessive current in the uninterrupted lines amounting to 1,73 times the normal running current. Overload relays are being asked to deal with such conditions as well as severe duty cycling and long acceleration durations, which cause motors to be operated at below normal speeds.
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Unique features of electronic overload relays
Electronic relays do not contain bimetals. Thus, they do not have thermal memory. However, electronic relays do have built-in memory.
Electronic overload relays, in most cases, must be reset after a trip condition. Neither will allow restarting until the “cool-down” period has been reached (trip-free feature). This trip-free feature, however, does not come into play if the motor is turned off instead of tripping under overload conditions.
There are two reasons for this “waiting period.” First, it is desirable to wait until the motor comes to a complete stop before switching to start. Also, the cool-down period permits operating personnel to have time to think about corrective action before attempting a start.
The serviceability of the electronic relay is also refined over conventional types. Essentially, electronic relays can be tested at any time by simply pressing the TEST button.
Advantages of electronic overload relays
Using self-powered electronic overload relays has many advantages for the users such as:
- With external CTs, they can protect motors up to 1500A
- Reliable, accurate, electronic motor protection
- Easy to select, install and maintain
- Compact size
- Selectable trip class (10A, 10, 20, 30)
- Trip status indicator
- Remote monitoring capabilities
- Status LEDs
- Manual and remote reset
If you want to learn more about overcurrent, you can check the book "Electronic Overcurrent Protection" by Gilbert Seel