Electrical systems must be strongly protected to ensure that they operate safely and continuously. The overload relay is one of the most crucial. These relays play a critical role in safeguarding electrical motors and circuits from the negative effects of overload. In this article, we'll look at the many types of overload relays, what they perform, where they're employed, and the protective concept that keeps the electrical system secure. We will also consider factors such as the cost of an overload relay and the quality of the overload relays available today from respected manufacturers.
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What is an Overload Relay?
An overload relay is an electrical device that is primarily meant to imitate the heating characteristics of electric motors while also interrupting the passage of current when the heat-detecting device in the relay reaches a fixed temperature. The design of an overload relay can be done using a heater and normally locked connections that unlock when the heater becomes too hot. This relay's connections can be made in series or between the motor and the contactor to prevent the motor from restarting when the overload is tripped.
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Overload Relay Image used courtesy of EEP |
Connection Diagram
The wiring diagram for an overload relay is
illustrated below, and the connections of an overload relay sign may appear as
two opposing question marks or as the 'S' symbol. The overload relay's operation/function is
detailed below. Although there are various varieties of overload relays on the
market, the most common is the "bimetallic thermal overload
relay". This relay may be designed
utilizing two different types of metal strips, which can be joined together and
grow at different rates when heated.
Whenever the strip is heated to a specific temperature, it can twist far
enough to break the circuit.
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Connection Diagram of Overload Relay Image used courtesy of elprocus |
When the current flow to the motor
exceeds the capacity of the heaters, the overload occurs within a few seconds.
Overload relays are classed into three categories based on the duration of the
relay's explore. Class 10, Class 20, and Class 30 overload relays can be examined
after 10 seconds, 20 seconds, and 30 seconds, respectively. One key security
feature of this relay is that it prevents the motor from restarting
immediately. For example, when the overload relay investigates within a
bimetallic relay, the NC (normally-closed) bimetallic connections will
release the circuit until the strip cools down. If somebody tries to press the
start button to shut off the contactor switches, the motor will not be turned
on.
Types of Overload Relay
These are classified into two types: thermal
overload relays and magnetic overload relays.
Thermal Overload Relay
A thermal-type relay is a protection device that is primarily meant to turn off the power when the motor consumes an excessive amount of current for a prolonged period. To do this, these relays have an NC (normally closed) relay. Once excessive current is supplied throughout the motor circuit, the relay opens due to enhanced motor temperature, relay temperature, or detected overload current, depending on the kind of relay.
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Thermal Overload Relay Image used courtesy of elprocus |
These relays are related to circuit
breakers in both structure and application; however, most circuit breakers
disrupt the circuit if an overload occurs, even for a brief period. These are
also intended to calculate the motor's heating profile; hence, overload should
occur for a whole duration before the circuit is broken up. Thermal overload
relays are split into two types: solder pot and bimetal strip.
Magnetic Overload Relay
Magnetic overload relays can be activated by measuring the magnetic field intensity produced by the passage of electricity toward the motor. This relay can be designed with a changeable magnetic core within a coil that stores motor current. The flux arrangement within the coil draws the core upward. As the core progresses far enough, it triggers a series of connections at the relay's summit.
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| Magnetic Overload Relay Image used courtesy of elprocus |
The main difference between thermal
and magnetic types of relays is that magnetic overload relays are insensitive to
ambient temperature. In general, they are employed in places where the ambient
temperature swings dramatically. Magnetic overload relays are split into two
types: electronic and dashpot.
Electronic Overload Relay
Electronic overload relays are commonly known as solid-state overload relays. There is no bimetallic strip on the interior of these relays. As an alternative, it incorporates current transformers or temperature sensors to detect the total flow of current toward the motor. This type of relay protects itself using microprocessor technology. Here, PTC plays an important function in detecting temperature and tripping the circuit when overload faults arise. Some overload relays have Hall Effect sensors and current transformers that sense the flow of current directly.
The main benefit of an electronic
overload relay over a thermal overload relay is that it lacks the bimetallic
strip, resulting in less heat loss in the relay. Furthermore, these relays
outperform thermal relays in terms of accuracy. Some electronic OLD
manufacturers build their products with additional features like as earth fault
and motor stall prevention. Electronic overload relays are utilized when motors
need to be started and stopped regularly. These relays can be designed to
resist the motor's starting current for a specific amount of time.
Eutectic Overload Relay
The eutectic overload relay consists of a winding heater, a eutectic alloy, and a mechanical device for activating the tripping mechanism. A eutectic alloy is a combination of two or more materials that melts and hardens at a certain temperature. In the OLR, the eutectic alloy is encased in a tube and used repeatedly via a ratchet wheel loaded with a spring to activate the tripping mechanism during the overload phase. Throughout the overload, the current in the motor provides the tiny heater winding, which heats the eutectic alloy tube and causes the alloy to dissolve, causing the ratchet wheel to revolve.
This act begins to open the OLR's closed auxiliary
connections. Once the tripping is complete, these relays can be manually reset.
So, typically, this reset may be performed using a reset button located on the
relay cover. The heating unit linked via the relay can be selected based on the
motor's full load current.
Fridge Overload Relay
A protective device, such as an overload relay, is employed in the refrigerator's compressor circuit. The power supply is provided to the compressor motor's windings via the overloading machine. This type of relay is mostly used to include the start winding into the circuit until the compressor is working at full speed.
The working process of an overload relay is based on an electro-thermal feature within a bimetallic strip. This may be arranged in the motor circuit in the same way that current can flow to the motor via its poles. When the current flow climbs over the predetermined value, the bimetallic strip heats up and bends. These relays always work with contractors.
When the bimetallic strips heat up, the contact trip is engaged, which cuts off the power supply to the contactor coil, deactivates it, and stops the flow of current to the motor. The time it takes to trip is always inversely proportional to the amount of current flowing through the relay. As a result, these relays are referred to as both current-dependent and inversely time-delayed. This relay may be connected in series with the motor, allowing electricity to flow towards it.
When the motor starts, the flowing motor
throughout the OLR will be present. When surplus current runs through the
relay, it will trip at a specific level, opening the circuit between the power
source and the motor. After a certain interval, this relay can be reset
automatically or manually. Once the overload has been identified and remedied,
the motor will be engaged again.
Application of Overload Relay
An overload relay can be used for the following purposes:
- It is often used to protect motors.
- It may be used to detect both overload and fault conditions and then generate trip orders for a protective device.
- This relay has evolved into microprocessor systems and solid-state electronics.
- When the gadget draws an unusually large amount of current, these relays turn it off.
Frequently Asked Questions (FAQ) About Overload Relay
1. What is an overload relay?
An overload relay is a protective device that protects electric motors from overheating due to prolonged overcurrent by disconnecting the motor when an overload condition occurs.
2. What is the main purpose of an overload relay?
The primary purpose of an overload relay is to:
- Protect motors from overload conditions.
- Prevent overheating of the motor windings.
- Extend motor lifespan.
- Reduce maintenance expenses and unexpected downtime.
3. How does an overload relay work?
An overload relay continuously monitors the motor current. When the current exceeds the relay's preset value for longer than the allowable time, it activates its normally closed (NC) auxiliary contact, opening the motor starter control circuit and disconnecting the motor.
4. What causes an overload relay to trip?
Common causes include:
- Mechanical overload
- Locked rotor
- Bearing failure
- Single phasing
- Low supply voltage
- High ambient temperature
- Incorrect relay settings
- Frequent motor starts
5. What is the difference between an overload relay and a circuit breaker?
| Overload Relay | Circuit Breaker |
|---|---|
| Protects against motor overload | Protects against short circuits and overloads |
| Requires a contactor to disconnect the motor | Can interrupt power directly |
| Operates with a time delay | Trips almost instantly during short circuits |
| Specifically designed for motor protection | General electrical circuit protection |
6. Can an overload relay protect against short circuits?
No. An overload relay only protects against overload conditions. Short-circuit protection requires a fuse, circuit breaker, MCCB, or MPCB.
7. What are the different types of overload relays?
The most common types are:
- Thermal overload relay
- Electronic overload relay
- Solid-state overload relay
- Eutectic overload relay (less common)
8. How do I select the correct overload relay?
Choose an overload relay based on:
- Motor full-load current (FLC)
- Motor voltage
- Motor power rating
- Starting method (DOL, Star-Delta, Soft Starter, VFD)
- Trip class (Class 10, 20, or 30)
- Ambient operating temperature
9. What is the overload relay setting?
The overload relay should generally be set to the motor's rated full-load current specified on the motor nameplate. Some applications may require slight adjustments according to the manufacturer's recommendations.
10. What is Trip Class in an overload relay?
Trip Class defines how quickly the relay trips during an overload.
- Class 10: Trips in approximately 10 seconds
- Class 20: Trips in approximately 20 seconds
- Class 30: Trips in approximately 30 seconds
Higher trip classes are used for motors with longer starting times.
11. Why does my overload relay keep tripping?
Possible reasons include the following:
- Motor overload
- Incorrect relay setting
- Single phasing
- Voltage imbalance
- Mechanical jamming
- Worn bearings
- Poor ventilation
- Frequent starts
Proper troubleshooting should identify the root cause before resetting the relay.
12. Can an overload relay be reset after tripping?
Yes. Most overload relays support:
- Manual reset
- Automatic reset (used in specific applications)
Manual reset is generally recommended for safety and troubleshooting.
13. What is the difference between thermal and electronic overload relays?
| Thermal Overload Relay | Electronic Overload Relay |
|---|---|
| Uses bimetal strips | Uses electronic current sensing |
| Lower cost | Higher accuracy |
| Less adjustable | Highly programmable |
| Affected by ambient temperature | Temperature compensated |
| Suitable for basic applications | Ideal for critical industrial systems |
14. Can an overload relay be used with a VFD?
Yes, but many Variable Frequency Drives (VFDs) already include built-in electronic motor overload protection. An external overload relay may still be required depending on the application and local electrical standards.
15. What happens if the overload relay is set too high?
If the setting is too high:
- The motor may overheat before tripping.
- Motor insulation may deteriorate.
- Equipment damage may occur.
- Fire risk may increase.
Always use the motor nameplate current as the reference.
16. What happens if the overload relay is set too low?
A low setting can cause:
- Nuisance tripping
- Production interruptions
- Reduced equipment availability
- Difficulty starting the motor under normal load
17. Is an overload relay mandatory for motor protection?
In most industrial installations, yes. Electrical standards and good engineering practice recommend using overload protection for motors to prevent overheating and equipment damage.
18. How do you test an overload relay?
Common testing methods include:
- Current injection testing
- Functional trip testing
- Simulation testing
- Measuring trip time against the relay's trip class
- Inspecting auxiliary contacts for proper operation
19. Where is an overload relay installed?
An overload relay is typically installed directly below the motor contactor in a motor starter assembly. It is connected in series with the motor so that it continuously monitors the motor current.
20. What is the lifespan of an overload relay?
The lifespan depends on operating conditions, switching frequency, and environmental factors. Under normal industrial use, overload relays can last 10–20 years with proper maintenance and periodic testing.
Conclusion
An overload relay protects a motor
from overloads, phase failures, and phase imbalances. Thermal and electrical
overload relays differ in their operating principles. Thermal OLR is
based on the deformation of a bimetallic strip during heating. The electronic
overload relay is a microprocessor-based device with several built-in features.
OLRs are used in conjunction with contactors. It opens the contactor when it
detects a malfunction. The time it takes them to open the contactor during
overloads is determined by their trip class. Overload relays cannot prevent
short circuits.
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