Introduction
Both contactors and relays are switching devices used to control electrical circuits. They look similar in construction and working principle, but their applications, ratings, and functionalities are different.
Understanding these differences is essential for designing motor control circuits, automation panels, and household control systems.
Both relays & contactors contains an electro-mechanical arrangement. In this, electromagnetic coil, silicon steel stamping, NO-NC contacts & spring mechanism is arranged in a particular pattern.
For further discussion about relays & contactors , We should need to discuss SPDT first.
SPDT
SPDT stands for “single pole double through switch.” It is a type of changeover mechanism consists of 3 contacts NO, NC & common contact.
- NO stands for normally open contact. At forced state, Common contact get connected with NO contact.
- NC stands for normally closed contact. At normal state, Common contact is connected with NC contact.
A Schematic diagram of SPDT is shown below. Where contacts & electromagnetic coil is arranged.
Working of SPDT
- At normal state, the spring & mechanical arrangement stays at normal uplift position due to spring tension. In such condition, Common contact is connected with NC contact.
- When an electromagnet get energized, it pulled the contacts downward. Which results common contact get connected with NO contact leaving behind NC contact disconnected.
- Again when electromagnet get de-energized, contact get pulled to its initial original position.
Difference Between Contactor and Relay
Next difference between contactor and relay are discussed one by one.
1. Basic Definition
Relay:
A relay is an electro-mechanical switch used for low-power control signals. It controls small loads such as lamps, sensors, alarms, logic circuits.
PCB mounting type relay is shown in an image below. Which contains only one single relay .
For better understanding, 14 pins general purpose relay is discussed. This relay contains 14 pins, In which 2 pins are power pins; operating voltage is 24 V DC. While rest 12 pins are control pins.
- Since a single SPDT contains 3 contacts. That 12 pins represents 4 SPDT, Working altogether simultaneously.
- Normally, When coil is not energized all the 4 common contacts are connected with NC contacts.
- When coil get energized, total 4 common contacts get connected to NO contacts.
In an image below, General purpose 8 pins relay is shown. In which 2 pins are power pins While rest 6 pins are control pins. 6 pins represents 2 SPDT working simultaneously all together.
contactors :
A contactors is a high-powered switching device, Also a type of electro-mechanical switch. contactors is used to control heavy loads such as motors, heaters & compressors.
For better understanding, ML-3 contactors by L&T is discussed briefly next. !
In an images below, It is seen that :
- Contacts of the contactors are attached with silicon steel stamping.
- Base of the contactors also contains a silicon steel stamping. While electromagnetic coil is placed in between the stamping.
- A spring is sandwiched in between both the stampings, “Stamping of contacts” & “stamping of electromagnetic coil”.
In a collage image below, It is seen that :
- The top most image, there is clear Gap in between the contacts.. It is the normal initial state of the contacts.
- While in an image below, Both the contacts are connected, There is no gap in between. It is the state when electromagnet get energized.
2. Current Rating
3. Voltage Rating
- Relay : Coil Voltage can be up to 230 V AC/DC. Control voltage can also be up to 230 V AC/DC.
- Contactor : Coil voltage can be up to 500 V AC/DC. Control voltage can be up to 1000 V AC/DC.
4. Application Area
- Relay : Control signals, automation, logic switching
- Contactor : Motors, pumps, HVAC, industrial machinery
5. Arc Suppression
- Relay : Minimal or no arc suppression needed
- Contactor : Comes with arc chute to break high current safely
An image of MO C 5 contactors by L&T is shown below. This special capacitor operating contactor contains a series damping resistors as an add on block. This damping resistors attenuates the generated transient during switching operation.
6. Auxiliary Contacts
- Relay : May have few NO/NC contacts
- Contactor : Often includes multiple auxiliary contacts for interlocking & signaling
7. Overload Protection
- Relay : Does not have overload provision
- Contactor : Often used with thermal overload relay for motor protection
8. Physical Size
9. Noise & Coil Consumption
- Relay : Lower coil power consumption
- Contactor : Higher coil consumption + mechanical noise (clack sound)
10. Mechanical Life
Both devices have high mechanical life, but:
- Relay : switches smaller loads but are also durable
- Contactor : Designed for high switching cycles under load
11. Base
Practical applications of contactor & relay
Example 1: Domestic Application
If you want to control a 230V lamp using a small 12V control signal → relay is used.
Example 2: Industrial Motor Control
Starting a 5 HP pump motor from a panel → Contactor + Overload Relay is used.
Example 3: Automation Panel
PLC controlling solenoid valves or small actuators → relay interface module.
Comparison Table of contactor & relay
| Parameter | Relay | Contactor |
|---|---|---|
| Usage | Low power control | High power load |
| Load Type | Signal & small loads | Motors & heavy loads |
| Rating | Up to ~10A | Up to 1000A+ |
| Arc Control | Not required | Required |
| Protection | No overload | Works with overload protection |
| Auxiliary NO/NC | Limited | Available |
| Application | Home / PLC / Logic | Industrial motors / HVAC |
| Size | Small | Bigger |
| Cost | Cheap | Costly |
| Base | Required a Base | Do not required base |
Relay vs Contactor – Simple Shortcut
➡ Small Loads = relay
➡ Large Loads = contactors
Conclusion
Both devices work as an electrically operated switches, but their applications differ drastically. Relay are preferred for low current logic circuits. Contactors dominate industrial motor control tasks. They are suitable due to their high current handling, arc suppression, and auxiliary features.
FAQs for contactors & relays
Q1. Can a contactors be used instead of a relay?
Not ideal. contactors are meant for heavier loads and consume higher coil power.
Q2. Can a relay run a motor?
Small motors, yes. Industrial motors, no—use a contactor.
Q3. Why do contactors make noise?
Due to strong coil magnetization and spring return force.
Q4. Do contactor need overload protection?
Yes, especially for motors. Usually paired with a thermal overload relay.
Q5. Is relay cheaper than contactor?
Yes, relays are more economical for small control tasks.