Why are contactors used in motor control

Contactors are basically beefed-up relays, but with some specialized features for handling high power loads. A previous post has already discussed control relays in detail. This blog post covers why contactors are used, how they work, the terminology involved, some key features and where they are typically installed. Contactors are used for high power applications. They allow a lower voltage and current to switch a much higher power circuit, so they are generally larger and more heavy-duty than control relays, enabling them to switch higher power loads on and off for many thousands of cycles Figure 1.

Standard control relays typically have contact ratings of 10A or less at Vac or less. Contactors, on the other hand, carry much higher contact ratings up to many hundreds of amps, and are commonly rated for operation at Vac. A very common category of electrical contactor includes devices designed to meet International Electrotechnical Commission IEC standards, which is predominant in Europe but also used in North America. IEC contactors feature a compact and modular configuration, with many electrical sizes available Figure 2.

End users always need to ensure that any components they select meet the application needs, carry any other required ratings such as UL, and comply with all applicable codes and regulations in service. Note that some control relays are rated at more than 10A, and some contactors are rated for less than 10A. Technically, a control relay may be able to switch some power loads, and a contactor could be used for control duty. However, best practice is to use devices suited for the intended service.

Standard contactors are electro-mechanical devices just like relays, with an electric solenoid coil arranged to close the mechanical contacts when energized. Typical contactors incorporate a spring mechanism just like relays do, but it is larger and more powerful to positively open the load-carrying contacts when the coil is de-energized.

Otherwise high current can cause a failure of the contacts to open if they weld together. Also, due to the larger loads there is more arcing as the contacts are opened.

Therefore, contactors have provisions to guide the arc away from the contacts to quickly quench or suppress the arc and preserve contact life. More specialized devices called vacuum contactors can be used for switching voltages higher than Vac because arcs extinguish quickly in a vacuum. Another difference between relays and contactors is in contact arrangements.

Control relays are available with varying counts of N. Contactors, on the other hand, are typically used to turn power on when energized. Therefore, contactors usually offer N. Certainly, there are contactors with N. Here are some contactor power pole arrangements, with a common application for each Figure 3 :.

Contactors may be used to turn loads on and off frequently. Other times, they are part of an emergency stop circuit where they may remain energized for long periods of time to provide main power to equipment, but will de-energize the equipment if the emergency stop circuit is activated. The circuit is then completed between the fixed and moving contacts by a normally closed NC contact allowing the current to pass through the contacts to the load.

When the current is stopped from passing through, the coil is de-energized and opens the circuit. The contactors contacts are able to rapidly open and close, which is why they are able to handle larger loads.

Since contactors are designed to rapidly open and close contacts, moving contacts may bounce as they rapidly collide with fixed contacts. Bifurcated contacts are used in many contactors to avoid bouncing.

Small amount of power is used up by the contactor coil during its operation. To reduce the amount of power drained by the contactor coil during operation, economiser circuits are employed.

Contactors with AC coils are equipped with shading coils. Otherwise, the contactor will chatter every time the AC crosses zero. Shading coils are able to delay demagnetization of the magnetic core to avoid chattering. DC coils do not need shading as the flux produced is always constant. When electric current pass through the contactor, it causes the electromagnet to create a strong magnetic field. This magnetic field pulls the armature into the coil, and this creates an electrical arc.

Electric currents flow in through one contact and into the device in which the contactor is embedded. Therefore, the function of the contactor is to switch an electrical circuit on or off. Overloading of the circuit can be prevented by adding an thermal overload relay. In the absence of electric current flowing through, the spring pushes the armature, thus breaking the connection. These are the most common types available and for good reason as they are more efficient than the previously mentioned types.

These contactors operate electromechanically and do not require human intervention. With their advanced technologies, they can be operated remotely, and this makes them safer and more efficient since they will not required to be operated manually. Only a small amount of current is needed by the magnetic contactor to open and close the circuit, so it is also energy efficient.

The knife blade switch contactors were introduced in the late s. It is safe to assume that they were probably the first types of contactors that were used. Their applications were mostly to control electric motors. They consisted of a metal strip which is designed to drop into a contact when operational. The switch was equipped with a lever for pulling it up or pushing it down. Back then, the contactors were so big; one had to stand next to the knife switchblade to level the switch into a closed position.

However, as with old technologies, this method of switching was not efficient enough, and there was functional problems that came with it.

The main problem was that it made the contacts wear out quickly. It was difficult to manually open or close the switch quick enough to prevent arcing; as a result, the soft copper switches underwent corrosion, this process made them more vulnerable to dirt and moisture which led to rusting.

As years passed and technology began to advance, larger motors were developed. The larger the motors, the more currents they require to work. But it is extremely dangerous to operate such high current carrying switches, so these type of contactors were no longer efficient. Even though technology was continuously improving, the knife blade switches could not be fully developed due to the problems and risks of operational hazards and short life of the contacts.

After discovering the dangers involved with the use of the knife switchblade, engineers and researchers came up with another contactor device which offered better safety and a number of features that were not available in the knife blade switch. This sudden interruption in current is defined as current chop and depending on its magnitude and frequency, it can cause serious insulation degradation over the lifetime of a motor.

By design, vacuum interrupters in contactors have a lower interrupting rating and are constructed to operate more frequently than the ones used in breakers. Vacuum interrupters in contactors are manufactured utilizing different metallurgy, which allows them to exhibit a lower chop current about 1 Amps than the chop current produced by a breaker interrupter about 5 Amps.

See below. Depending on the characteristic impedance of the electrical system, the transient voltage created by the chopping current could be detrimental to equipment insulation.

The waveform image presents the transient voltage created by a 1 and 5 amp chopping current in a simulated circuit with a characteristic impedance of 3, ohms. As shown, the 1 per unit overvoltage condition created by the 1-amp current chop of a vacuum contactor, is well within the rating of most MV insulation systems and will not compel design engineers to use surge arrestors.

Vacuum interrupters in circuit breakers on the other hand, have a 5-amp current chop increasing the voltage transient to levels above four times the system voltage. These higher levels of transient voltages often require the use of surge arrestors as a mitigator to protect insulation systems. A contactor is an electrically controlled switch used for switchging a power circuit with a lower power level control circuit. Learn from our experts at Eaton's Experience Center to understand contactors, how they compare to circuit breakers and the role they play in motor starting and other applications.

Motor starters using contactors have extended mechanical and electrical life compared to breaker-based starters. The impact here is substantial. For motor starting applications, contactors can require little, if any, maintenance for decades. In contrast, breaker-based solutions for motor starting could require repeated maintenance beginning within a year of operation.

As seen in the table below, vacuum breakers have a mechanical endurance of 10, operations with a manufacturer recommended maintenance period of operations. Vacuum contactors are commonly rated for 25, electrical operations before maintenance is recommended and a total electrical life up to , operation. Breakers are constructed to stop high levels of current in the event of a fault. They are excellent interrupters, and as such, they are better suited for power distribution applications instead of motor starting applications where the number of operations significantly increases.

For example, an application where only four motor starting operations are required per day, would lead to maintenance periods of 4 months if a breaker-based controller is applied. Using the same scenario, we find that the contactor will not require any recommended maintenance for more than ten years.