Contactors: 7 Answers You Should know

Contactors are almost used in all control panels I see all the time in my work. Starting from small lighting control panels to medium voltage motors control panels. No matter what voltage level the panel is, the contactor plays an essential role. In this article I will provide you 7 important answers you should know. Let’s get started directly.

Why exactly do we use contactors for the motors?

“We use contactors for motor control to prevent the damaging of the electrical circuits of the motor using the switching method. By using a combination of a contactor and an overload relay, it become easy to switch off the motor power in case of any fault resulting the motor to draw excessive current. As you know a contactor is a controllable switching device.”

Generally, the circuitries for electrical equipment or devices are the relays and the circuit-breakers in between supply and device to have control over input. The contactors are not that famous in small devices because of their use for the high current flow rates the ability to respond even in very high current flow through them where other safety circuits like relay and circuit-breakers cannot operate.

Just like all other electrical devices, motors also has point beyond which if the current exceeds, it damages the circuitry or the parts or even the whole motor may burnout.

Motors of high-power ratings need something to control the current if it exceeds the safety limit. High-power rating motors usually are not attached to the relay or circuit breaker due to their inefficiency. So, there comes the contactors.

When the current passes through the contactor, the voltage excites the electromagnetic coil the fixed ends of the normally closed contactor opens and the circuit starts conducting and vice versa. The arcs that are generated due to high current are dealt with the contactors very efficiently.

Can we use the AC contactor for the DC supply?

For the same value of current and voltage, No!

But in some cases, the answer is that it is “possible to run a contactor on AC and DC as well”. This is because the contactor name plate has two coil voltages, one for AC and the other is for DC. But must read the conclusion for avoiding the confusion of this method.

The AC contactor can be used for the DC supply. This is not an impossible thing. But the same value of the voltage of the AC supply, the DC voltage will burn the coil of the AC contactor.

The main reason behind such a reaction is that the AC changes its polarity according to the frequency (while changing polarity it comes to zero value 2 times per cycle) while the DC doesn’t come to zero at all. The frequency of the DC is undefined. Inductance, which is dependent on this frequency will play the role.

So, the constant flow of current will eventually lead to the heating and then melting of the internal circuitry of the contactor. It will work for a while but then the coil will burnout. Meanwhile,engineers use capacitors or inductors to control the flow of the current. For instance, the 240 AC voltage contactor is only feasible to be used with 24V DC voltage.

The number of turns in the coil of the DC contactor is way more than the AC contactor. Thus, they provide more reactance and when the AC is supplied and thus impose an infinite resistance to the current flow and will not work at all.

So conclusively, we can sum this whole thing up as if one wants to use the DC contactor in the place of the AC contactor, it is allowed but capacitors or inductors must be used to cater to the current flow.

Still, it is not recommended to use an AC contactor to replace a DC contactor.

In the other case, to use the DC contactor in the place of an AC contactor, the current will not flow through the circuit, so the device will not even start for which contactor was to be used.

In some cases the contactor is designed to operate on AC and DC voltages as well, in this case the contactor name plate has two voltages one DC voltage and the other is AC.

Why do contactors have thermal ratings?

The maximum amount of rated current of the contactor, when controlling a thermal load (i.e resistive load) is the contactor thermal rating. The thermal rating of the contactor is mentioned because the rating current of the contactor depends on the load type, inductive or resistive.

Contactor loads are categorized into:

Resistive loads AC1: These loads have almost unity power factor, like heaters and all thermal loads, so the contactor rated current for these loads is called “Thermal current Ith”.

Inductive loads AC3: These loads are motor loads, which have inrush currents. The contactor has to make this high starting current.

Contactor thermal current rating, of AC1 loads, is greater than the rated inductive load current of the same contactor i.e AC3

For example: If we have a contactor of Ith 40 A for AC1 loads, and rated current of 25 A for AC3 loads, this means that the contactor can control a heater up to 40A, and a motor up to 25 A only.

What is difference between the contactors and the circuit breakers?

ContactorsCircuit Breakers
A contactor use “connects” the device (load) with a high-power (voltage/current) supply.A circuit breaker “breaks” the device (load) from the power supply.
A contactor is a switching device/circuitry.A circuit breaker is used for the safety purposes.  
It has depends on the coil voltage to actIt can act automatically in case of faults
The contactor is a power handling/ controlling device.A circuit breaker is a piece of safety equipment.
The arcing and flashing are handled at high power using the contactors.The arcing or flashing in circuitry sets the circuit breaker to an OFF state.
Not a device for automatic use.The very need for circuit breaker is to automatically power off the system.
The basic rating for its use is its current rating.The circuit-breaker can sense the current as well as voltage.
We can’t use it to switch between states manually.It can be turned off/on manually.

Difference Between Contactors and Capacitor Contactor?

The power contactors, are the contactors that are used in the circuit to simply switch the circuit on or off, for any load type (inductive or resistive).

While capacitor contactors, are those type of contactors that are responsible for switching power factor correction capacitors for inductive loads i.e loads with low power factor like induction motors.

Capacitor contactors have a limited life. The reason for this limited life is the repeated switching of the capacitors.

When a capacitor undergoes a charging/discharging procedure, they produce a large value over-currents that may be a multiple of 100% more than the actual rated current capacitor. The capacitors get affected by these over-currents and so do the contact points of the contactor.

The capacitors release an overcurrent in the circuit that may exceed the rated value of the device (motors, appliances) or the contactors. This damages the contactor. To take care of this problem engineers had to use the Capacitor Contactor.

For damping these currents, the capacitor contactors have inbuilt resistors. When the connection is closed and the circuit starts conducting the resistor comes in series with the capacitors and thus damp the currents.

This results in the increased reliability of the system, extended life of the contactors as well as the load capacitors, and reduction in the Ohmic losses (losses caused by the high flow of current through ohmic conductors).

I have written a detailed article about power factor correction, you can read it here.


How does a vacuum contactor work?

Vacuum contactor, is a contactor which moving and fixed contacts are surrounded by vacuum inside a sealed tube, to prevent the arcing during the switching on and off. This contactor is very suitable to medium voltage applications, in dusty workplace.

Every fluid, liquid,or solid has an electrical permittivity. That means under some particular conditions the electrical currents can pass through them. Most commonly, we see metals(solids) for conduction purposes in the circuits. Liquids (except Mercury) are also conductors. Air also has an electrical permittivity of 1.0006.

While the vacuum has the electrical permittivity of 8.85418782 × 10^-12 (m^-3 kg^-1 s^4 A^2). This value is way smaller than that of the air.

It means if we want to pass current through the vacuum it is very difficult. The main function of the contactor is not to conduct or rise and arc at the lower voltages or while opening/closing of the circuit respectively.

When the contactor opens it should avoid the arcing between the terminals. It is not possible to reduce arcing to zero but to minimize it, we use vacuum contactors.

The vacuum contactors prevent the arcing as it is a very bad conductor and electric current is difficult to pass through it. The vacuum contactor does not have any kind of oil or air blast facility to avoid the arcing as it works exceptionally on its own.

There is no air or oil present in the vacuum contactor so there is no danger of arcing due to the ionization of air molecules due to high voltage. And also, the risk of an outbreak of fire or damaging nature due to leakage of oil is minimized when the vacuum is being used in the contactor.

What are the uses of vacuum contactors?

In my work we use vacuum contactor in a VSD panel of a medium voltage motor, its operating voltage is 3.3 KV.

The great benefits of the vacuum contactors over the general contactors have made them sought by all kinds of users whether they are the end-users or the market. Vacuum contactors are being used with:

  • Transformers
  • Generators
  • Feeders
  • Power transmitting units
  • Switch gears
  • Pumps (generally, with the motors and applications of motors.)

Vacuum Contactors with a power rating of less than 5kV came out to be the most famous and desired ones as more than 50% of the vacuum contactors market had their sales of this rating as they are desired by the end-users. Then comes the 5-10kV rating vacuum contactor as they are used in high-rated pumps and transformers.

Then the 10-15kV rating has even fewer customers and more than 15kV has the least number of customers looking for them as these ratings are used in marines, transmission systems, and waste water pumping systems (high-power rated motors).

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