Electrical engineering is a complex yet thriving industry, with roughly 153,000 electrical engineers in the United States. In the world of electrical engineering, electrical relays and electric contactors are extremely important. The trouble is, many people get confused between which is which and what applications they should be used for.
To help you avoid this issue, we’ve put together a guide on the difference between an electric relay and an electrical contactor. Let’s take a look!
What Is a Contactor?
A contactor is a device that switches an electrical circuit off and on. Contactors can range in size and form factor depending on their design and classification (NEMA vs IEC). Contactors typically posses a set of Normally Open (NO) contacts that provide power to the load when the coil of the contactor is energized.
You’ll most often find contactors in 3-phase motor applications where they are sized to safely connect and disconnect power to the motor.
Contactors can handle currents greater than 500 amperes and can also handle power of more than 100 kW. That makes them perfect for controlling heavy motor currents that produce power arcs since they can easily break these currents.
There are several different types of contactors, and each one has its own features, applications, and capabilities. Let’s take a look at a few common types of contactors you might come across.
Knife Blade Switch
Knife blade switches originated in the 1800s, and are one of the first contactors used to control electric motors. These contactors were made up of a metal strip that dropped onto a contact.
The switch featured a lever that you could either pull down or push up. To operate the contact, you’d have to level the knife blade switch by standing next to the switch and manually maneuvering it.
Knife blade switches had a few issues associated with them, mainly that they would wear out quickly. Plus, as the size of motors grew over the years, the switches were no longer sufficiently rated to switch high currents.
Once engineers determined that the knife blade switch was too dangerous to use, they developed a new type of contactor. This contactor, called a manual controller, offered several features that the knife blade switch didn’t.
For one, the manual controller was safe to operate and was a non-exposed unit with proper encasing. Additionally, it was a smaller device that used double-break contacts.
The way the manual controller works is that the double break contacts open the circuit in two different places at once. They divide the connection to form two different contacts.
Then, when an operator activates the manual controller, the power circuit engages. That carries electricity to the load.
The last type of contactor to be aware of is a magnetic contactor. Humans do not need to interact with this contactor at all because it requires electro-mechanics to operate.
Magnetic contactors have one of the most advanced designs, and we can even operate them remotely through the use of programmable logic controllers. Because of that, they are much safer than other contactors and help keep staff out of danger.
Today, magnetic contactors are the most common contactor you’ll find on the market. They’re typically found in industrial control applications.
What Is a Relay?
Now that you have a solid understanding of contactors, let’s talk about relays. Relays, like contactors, are switches that open and close circuits. They do so both electromechanical and electronically.
Controls engineers use relays to control how circuit contacts open and close on an electronic unit. Once the engineer supplies an electric charge to the relay, the status changes.
When the relay contact is NO, or open, it isn’t energized with the open contact. When it is NC, it isn’t energized with the closed contact.
It’s important to be aware that there are a few different types of control relay coils out there. Let’s take a look at a few common relays to be aware of.
Engineers that need to power components for a specified amount of time often use time-delay relays. These relays can turn components off and on for specified periods of time.
There are two different categories of time delay relays: on-delay timers and off-delay timers.
On-delay timers start when an input is applied and power the second circuit after a specified length of time. A few examples of on-delay timers include alarms and warning systems.
Off-delay timers wait for a trigger after energizing the input and energize the output after removal of the trigger. A few examples of off-delay timers include washers and dryers or amusement park rides.
Another key type of relay to be aware of is sequential relays. These can power multiple different components in a specified order.
Many times, sequential relays support powering up multiple systems or sets of lights. For example, runway lights and power supply sequencing often use sequential relays.
In automotive situations, engineers can use relays for just about any application you can think of. Many times, automotive relays let manufacturers add electrical conveniences and advanced safety features to vehicles.
A few examples of relays in automotive systems include the following systems in vehicles:
- Interior lighting
- Warning systems
- Gas Valves
- Windshield wipers
What’s the Difference Between Contactors and Relays?
Now that we’ve discussed what contactors and relays are, it’s time to talk about the difference between the two. After all, both switch a circuit and have similar functions. Let’s take a look at the differences between these devices.
The first difference between relays and contactors is the load capacity. Contactors can carry loads greater than 10A while relays can only carry loads 10A or lower.
Open/Closed Contact Standards
Contactors typically, and almost exclusively, work with open contacts. On the other hand, relays can work with both types of contacts depending on what function they’re serving.
Contactors tend to be outfitted with either NC or NO auxiliary contacts that can carry out additional functions. Relays don’t need these auxiliary contacts since they are already designed for both NO and NC functionality.
Contactors typically include spring-loaded contacts to help break the circuit when it’s not energized. This is an added safety situation that prevents contacts from welding together.
Relays don’t need spring-loaded contacts since they don’t work in such high-power situations. It’s rare to find relays that include this feature.
Contactors commonly include arc suppression since they work with very high loads. This feature works by extending the path that an electrical arc travels.
By extending the distance, they suppress the arc. Relays aren’t designed for high loads, so they don’t typically include arc suppression.
Contactors connect to overloads that stop the circuit if it rises above a selected threshold for a certain period of time. Typically, the time period is between 10 and 30 seconds. The reason for this is that they protect the equipment from getting damaged.
Relays don’t often have this feature. That’s because overloads don’t often occur in relays, eliminating the need for this feature.
Choosing Contactors vs Electrical Relays for Your Application
It can be tough to determine whether you should use electrical relays or contactors in an application. However, there are a couple of general rules that you can follow to help you decide which you should use.
When to Use a Relay
You should use relays when you are dealing with currents of 10A or less. They cannot handle high-volume currents the way that contactors can.
Additionally, relays usually only have ratings up to 250VAC. You should not use them for applications higher than this number.
Finally, relays are perfect for single-phase applications. The common contact in a relay connects to a neutral position for a single-phase application.
Remember, if you’re switching low power or don’t need additional safety features for your application, a relay is a great choice, too.
When to Use a Contactor
Many times, manufacturers create contactors for 3-phase applications. You can use contactors if you have multiple applications, as well as for single applications.
On top of that, contactors don’t have a common circuit between them. They join two poles together, which is part of why they are great for multiple applications.
Next, contactors have ratings of up to 1000VAC. And, they are suitable for 9A or more current. You can use them for much larger applications than relays.
Remember that you should also check the function of your project. If you are dealing with a situation where you could experience overload conditions or other dangerous conditions, contactors are the best choice since they have safety features.
Know Your Relays and Contactors
With this guide to electrical relays and contactors, you have all the information you need to successfully choose between the two. Knowing these key differences is important for finding the correct solution for your application.
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