Contactor vs Relay: Which is Best for Your Electrical Needs?

When it comes to electrical needs, choosing between a contactor and a relay can be a daunting task. Both contactors and relays serve the same purpose of controlling electrical circuits but each has its unique features. In this article, we will explore the differences between the two and help you determine which one is best for your electrical needs.

Differences Between Contactor and Relay

The most important differences between contactor and relay are:

1. The current switching capacity

The main difference between a relay and a contactor is that the contactor is used to connect and interrupt higher levels of power. Relays can switch currents up to 16 amperes, as well as relatively low-level voltage and current signals. Contactors are used to switch currents greater than a few amps and power levels of several hundred to several thousand watts.

2. Size

In general, contactors are intended to control higher levels of current. Therefore, the size of a contactor is larger than a relay. Contactors must have massive contacts, large in size and heavy in weight, supplied with additional powerful springs to provide the needed contact pressure, and with flexible band wire ways through which the current is fed to the contacts.

3. Usage in high voltages

Contactors are commonly rated for up to 1000V, while relays are usually rated at 250V. Besides, the insulation voltage of a contactor is higher than the relay.

4. Contact Configuration

Contactors are available in two poles, three poles, and four poles’ main contact configurations. Most contactors are normally open (Form A) although normally closed (Form B) contactors are also available for specialized applications.

Relays are available in many contact forms. (Form A and Form C)

Single-throw normally-open (SPST NO) / single-pole, double-throw (SPDT) / Double-pole, single-throw (DPST) / Double-pole, double-throw (DPDT) are the forms of relay contacts.

5. Auxiliary contact availability

Auxiliary contacts, which would have a lower current rating than the main contacts, are available with most contactors. The auxiliary contacts are often used to interlock in control circuits or for remote indication of contactor position. Relays do not have any internal and add-on auxiliary contacts.

6. Variants

There are numerous types and classifications of relays and contactors some of which are intended for very special and unique applications. The most common type of relay is the control relay. Other types of commonly used relays include time delay relays, protection relays, solid-state relays, and reed relays. Contactors are also available in different types. The most popular choice is the general-purpose power contactor. Others are capacitor contactors, modular contactors, mini contactors, and bar-type contactors.

7. Accessory options

The contactor has lots of electrical and mechanical accessories. The accessory options of the relay are limited compared to contactor. For example, you cannot interlock two relays with a mechanical interlock.

8. Applications

Typically, contactors control power to electrical gear such as motors, heaters, lighting, and capacitor banks. Contactors are required in those applications where electrical power must be repeatedly applied and subsequently interrupted. Relays are used in low-power controlling applications such as signaling, timing, and amplification. The large physical size and limited isolation (due to materials requirements and construction) make contactors a poor choice for signal switching. Contactors are used in main circuits; relays are used in auxiliary circuits.

9. Withstanding overloads and inrush currents

The inrush current of an apparatus is the maximum current that flows after being suddenly and fully energized. Generally, contactors can withstand high inrush currents. Relays cannot perform well in high inrush currents.

10. Switching speed

The operating speed of a relay is the rate at which the contacts may be cycled for reliable operation. It is limited by the actuation and release times. Actuation time is measured from when power is applied to the coil until the contacts have settled. The average switching speed of a relay is between 3 to 100 ms. The average switching speed of a contactor is between 20 to 250 ms. Relays are faster.

11. Lifespan

Relays have a longer lifespan compared to contactors. The life of a relay at rated load is around 10⁷ cycles. The life of a contactor at rated load is around 10⁵ cycles.

12. Coil power consumption

Coil power is the amount of power in watts needed to actuate the relay or contactor. The coil of a contactor and relay requires a set amp-turns product to actuate the contacts. Many turns of fine wire are used to keep the necessary current, and therefore power, to a minimum. The coil power consumption of the contactor is higher than the relay.

13. Electrical arc generation

When the contacts are opened, the current will be interrupted and may cause arcing. Arcing can damage the switching device or reduce its life. If the switched current is too high, the resulting temperature increase and contact arcing will degrade the device and shorten the contact life. Contactors generate more electrical arc compared to relays due to their high current carrying capacity.

14. Troubleshooting

Many designs of relays have manual operators that allow a person to manually shift the contacts from the normal position. Other relays have flags or small lights that indicate the position of a relay. Both of these features can be an aid in troubleshooting. Contactors do not have these features. Relays are easy to troubleshoot.

15. Costs and market size

Compared to contactors, relays are relatively inexpensive. And their market size is larger than contactors.

16. Humming level

Power contactors and relays make a humming noise when they are switched on. The sound level of the contactor is higher than the relay due to its big main contact construction.

17. Spare parts and maintenance

If selections are not made with due recognition of the capabilities and limitations of electrical devices, the result can be the premature failure of components. Generally, contactors need more maintenance. That’s why the coil and main contacts can be purchased as spare parts.

18. Inventors

Relay has a long history. American scientist Joseph Henry is often claimed to have invented a relay in 1835 to improve his version of the electrical telegraph. On the other hand, German engineer Hein Moeller developed the world’s first oil contactor in 1912.

In conclusion, choosing between a contactor and a relay depends on the specific electrical needs of the application. The main difference between the two is the current switching capacity, where contactors are used for higher levels of power, while relays are used for low-power controlling applications. Contactors are generally larger than relays and can switch currents greater than a few amps, up to several thousand watts. They are typically used in main circuits, while relays are used in auxiliary circuits. Both have unique features, such as auxiliary contact availability in contactors and the ability to withstand high inrush currents in relays. Ultimately, the choice between a contactor and a relay will depend on the application’s specific needs.