Comprehensive Guide to Overload Relays: Motor Protection, Components, and Testing Explained
Introduction
In industrial automation, when a motor is working to handle a load, it pulls current from the network, naturally creating some heat. Most of the time, this warmth isn’t anything to worry about – it’s just part of how the motor works!
Sometimes, a motor has to work extra hard, and things can get a bit heated—literally! For instance, if the shafts of the motor and the load aren’t aligned correctly or the rotor gets jammed because something’s physically blocking it, it can lead to excess current draw and overheating. Other situations that might overwork a motor include when it’s constantly starting and stopping, when the motor is too small for the load it’s trying to handle, when one of the phases in a three-phase system fails, or when the surrounding temperature is high while there’s not enough ventilation. Even damaged bearings (bearings support the motor’s shaft) can cause extra friction and make the motor overheat. All these issues can put the motor under stress, leading to overheating and possible overload.
Running a motor in overload for an extended stretch means it’s working harder than it’s built to handle, creating a lot of heat. This extreme temperature can wear down its more sensitive parts and may end up causing permanent damage.
The fix for this is to install an overload relay. This device is hooked up to the contactor, and it makes sure the motor stops running if it starts pulling too much current for an extended period and risking damage.
Prerequisites
Prior familiarity with the ‘Industrial Relay Control System’ will prepare you to engage more effectively in this tutorial.
What is an Overload Relay?
An overload relay is like a safety net for the motor. It allows for brief inrush currents, like the surge of current that occurs when the electric motor starts up, without triggering the protection mechanism.
However, it also monitors the motor’s current draw and steps in if it detects excessive current for an extended period, preventing the motor from overheating and ensuring it doesn’t get damaged.
Overload Relay Protection
How exactly does an overload relay protect a motor? If the overload relay senses that an overload has happened for a while, it trips and lets the Contactor know. The contactor then cuts off the current flow and shuts off the motor to avoid further issues.
Overload Relay Components
It’s time to dive into the components you will find commonly in overload relays. One of the most important is the Current Limit Adjuster. This dial allows you to choose the level of current that needs to be drawn by the motor for a considerable time before it’s flagged as an overload. In other words, this is where you can set the motor’s Full Load Amps, or FLA, which is the maximum current it should pull during regular operation.
Up next is the Overload Signal Light. It’s a helpful component that lets you see the overload relay’s status. If there is an overload condition, this indicator will light up to display that it has been triggered.
Next, you have two Auxiliary Contacts inside the overload relay. The first pair, labeled 95 to 96, is Normally Closed (NC), which means they stay closed when everything runs smoothly. The other pair, 97 to 98, is Normally Open (NO), meaning they remain open under normal conditions.
When an overload occurs, the internal switches react by opening the Normally Closed pair (95 to 96) and closing the Normally Open pair (97 to 98). It provides a clear signal about the issue.
The overload relay has a Trip Simulation Button right on the front, which is truly convenient. This button helps you test if the overload relay is working correctly.
When you press it, the overload relay opens the normally closed contacts (95 to 96) and closes the normally open contacts (97 to 98), just like it would in an actual overload situation. This simple action simulates an overload trip, helping you confirm the relay’s reliability and ensuring the system is protected against overloads as it should be.
Finally, you have got the Reset Button! This button isn’t just for show – it lets you pick between manual and automatic reset modes, and of course, it’s the one you’ll press to reset the overload relay.
Picture this: your overload relay trips due to an issue, but it’s in automatic mode. As soon as the overload problem is cleared, the overload relay resets itself without you having to lift a finger. Handy, right?
If the overload relay is set to manual mode instead, it’s a different story. Even after you’ve solved the overload issue, you’ll need to go over and press the Reset Button yourself. It’s a great way to have that little extra control over the process when required.
Overload Relay Wiring Diagram
It's time to jump into the schematic view to understand the inner workings of an overload relay within a motor circuit. In Figure AAA, you have a three-phase power setup and a straightforward control circuit to help you see how all connects and functions. Here, NEMA symbols make it easy to notice where each component, like the overload relay, fits into the big picture. The three-phase power supply hooks up to the inputs of the contactor, and from there, the outputs of the overload relay channels power straight to the motor, creating a reliable and safe path for power.
When it comes to controlling the coils of the contactor, you've got options with the overload relay's contacts! You can use the normally open or closed contacts, but this control circuit sticks with the normally closed contact. This setup helps ensure the coil only gets power when conditions are right.
The moment you hit the start push button, things get rolling! The contactor's coil powers up, snapping the contacts in the power circuit shut. It opens the path for current to reach the motor, with the overload relay in place as a safeguard. It's like flipping the switch to give the motor the go-ahead, but with built-in protection.
When the overload relay picks up on a constant overload, it automatically opens up its normally closed contact. It cuts power to the contactor's coil, which, in turn, opens the primary contact in the power circuit and shuts off the motor. The overload relay is like a built-in safety switch that keeps the electric motor from overheating!
Overload Relay Testing
It's time to test the overload relay using the multimeter set to measure resistance (ohms). Remember, the overload relay is deactivated, so you are starting from a neutral point.
It's vital to have continuous connections between L1 and T1, L2 and T2, and L3 and T3 on your overload relay.
If the overload relay is not tripped, here's what should happen: the normally open contact (97 to 98) should be open, showing 'OL' on your multimeter.
Meanwhile, the normally closed contact (95 to 96) needs to stay closed, which will read as 0 ohms on the meter.
You can now press the test button to simulate an overload and see how the overload relay reacts in a tripped condition. The connections L1-T1, L2-T2, and L3-T3 remain intact, although when overload happens, the overload relay doesn't disconnect power to the motor.
With your multimeter in hand, check the auxiliary contacts: you will notice the normally closed contact is open (OL on the multimeter), and the normally open contact is now closed (showing 0 ohms).
Conclusion
In conclusion, you’ve gained a solid understanding of how an overload relay operates and protects a motor from overheating due to excessive current draw. You explored various situations that could lead to a motor overload. Each of these situations can cause a motor to work beyond its capacity, potentially leading to excessive wear and, eventually, permanent damage if not addressed. You also learned about the main components of an overload relay. Finally, you saw how an overload relay fits within a motor control circuit, wiring considerations, and how to test its functionality using a multimeter. By understanding these elements, you are better equipped to implement, maintain, and troubleshoot overload relays in motor control applications, ensuring your motor systems are safeguarded against unexpected overcurrent.
