Are the main contact parameters of an AC contactor coil-dependent?

Table of Contents

    As an electrical worker, there should not be many people who do not know about AC contactors. AC contactors are mainly used as control appliances for low-voltage motors.

AC contactor coil

Picture 1: Outline and internal structure of an AC contactor

The following figure shows a specific circuit diagram of an AC contactor used to control a motor:

Picture 2: AC contactors in motor control circuits

In Picture 2, the KM is the AC contactor. We see the main contacts of the AC contactor in the main circuit on the left side of Picture 2, the coil of the AC contactor at the bottom of the auxiliary circuit on the right side, and the auxiliary contacts of the AC contactor at the top of the auxiliary circuit on the right side.

When we press the control button SB1 in the auxiliary circuit, the coil of the AC contactor is energized, the contactor enters into the suction phase, and the main contact closes, and at the same time, the auxiliary contact of the AC contactor connected in parallel with SB1 closes, realizing the self-holding function. The so-called self-holding refers to the fact that the auxiliary contact can maintain the AC contactor coil charged after the closing control button SB1 returns with the release of the hand. Thus, the motor enters into the starting and running state.

When we press the control button SB2 in the auxiliary circuit, the AC contactor coil loses voltage, the contactor releases, and the main contact opens and returns, and the motor loses the supply of electrical energy and enters into the stop running state.

The above process for electrical workers should be more than familiar, now we come to scrutinize some, see if we can dig out what.

 

First: the main contactor contact belongs to the main circuit, the contactor coil belongs to the auxiliary circuit.

The so-called main circuit, refers to the transmission and control of electrical energy circuit. The so-called primary circuit is also called the primary circuit; the so-called auxiliary circuit refers to the implementation of the control, signal transmission, signal amplification and electric parameter acquisition circuit. The auxiliary circuit is also called the secondary circuit.

 

Second: what happens when the contactor main contact opens and closes?

When the main contactor contacts closed instantly, what forces exist on the main contacts? Let’s look at picture 3:

Translated with DeepL.com (free version)

 

AC contactor coil

Picture 3: Analysis of the forces acting on the contacts of the AC contactor at the moment of closure

In picture 3, we see that the main contact has the electromagnetic closing suction force Fx given by the coil, the reaction force Ff given by the reaction spring, the repulsive force Fc exerted by the C-shaped structure of the conductive rod of the contact, the Hohm repulsive force Fh generated by the contraction of the current of the contact to the contact point, and the force Ft exerted by the rebound force of the magnetic circuit armature and the iron core suction impact acting on the main contact, which can be seen, the process of the main contact’s suction closure is not only related to the coil, but also the reaction force Ft. related to the coil.

The time for the impact rebound force to act is about 5 milliseconds. When the contacts are contacted and then bounced off, arc ablation occurs between the movable and static contacts. The contact material melts and evaporates as a result, producing metal vapor. The presence of metal vapor intensifies the arcing action.

As the rebound effect disappears and the contacts close again, the Holm force begins to appear.

Picture 4 is a diagram used to analyze the Holm force:

AC contactor coil

The distribution of the current lines in the contacts can be seen in the left panel of picture 4, and the magnetic field produced by the current line on the right side can be seen in the right panel of picture 4 along with an analysis of the force.

In the lower right panel is the current in the static contact, we note the current line Ix, and using the right hand helix rule we can determine the direction of the magnetic field it produces, noting that the right hand side is the direction into the paper. Looking at the current line Is in the moving contact, we can use the left hand rule to determine the direction of the electromotive force it is subjected to as Fs in the diagram.Fs can be broken down into a horizontal force Fsx and a vertical force Fsy.We see that the horizontal force is canceled out by the force generated by the other symmetrical current line, but the vertical force is enhanced. This vertical force is the Holm force, which was discovered by Holm, an electrical engineer at Siemens.

Holm force is manifested as a repulsive force, in general its value is not large, but in the presence of large currents (such as motor starting inrush current and short-circuit current) when the Holm force can reach a large value, and make the contact is repulsive open, and repulsive open after the disappearance of the Holm force, the contact is closed again, and then again because of the Holm force and repulsive open. After several passes, the contacts are subject to spattering and evaporation of material due to arc ablation, and even fusion welding of static and dynamic contacts. For this reason, the effect of the Holm force is known as the dynamic stability of switching appliances.

These roles to participate in my series of articles: so – low voltage electrical contacts before and after the closure of a number of physical phenomena (1) one to five.

Third: Arcing, an important factor affecting the operating performance of the main contacts

Let’s look at picture 5:

First, we use Kirchhoff’s second law, KVL, to write the equation of the diagram in Figure 5bis:

E=LdIhdt+RIh+UhE=L\frac{dI_h}{dt}+RI_h+U_h , equation 1

Notice that E in Fig. 5 is the DC power supply and K in the figure is the main contact of the contactor. The circuit has a resistance R and an inductance L.

For diagram 1 of Fig. 5, let the current In of the circuit be unchanged, then in the case of neglecting the DC resistance of the inductor, because dIh/dt = 0, the voltage across the inductor L is zero, and the arcing voltage Uh = 0. So the current Ih = In = ERI_h = I_n = \frac{E}{R} , which is the value of the current that flows through the contactor’s main contact that has been closed.

Looking at Figure 2 again, let the current Ih=0 in the circuit, and the arc voltage Uh=E.

Let’s look at picture 6:

The straight line EK in the figure is the DC load line plotted according to the above conditions.

We put the volt-ampere characteristic curve H1 of the arc into picture 6, see the red H1 curve in picture 6. Arc volt-ampere characteristic curve is characterized by its negative resistance characteristics. That is, the higher the arc voltage Uh the smaller the arc current Ih, and conversely the smaller Uh the larger the arc current Ih.

No change, then in the case of neglecting the inductor DC resistance, because dIh/dt = 0, so the voltage across the inductor L is zero, the arc voltage Uh = 0. So the current Ih = In = ERI_h = I_n = \frac{E} {R} , which is the value of the current that flows through the contactor has been closed to the main contacts.

In diagram 1, the main contact of the contactor is closed. In diagram 2, it is open, so there is an arc between the contacts, the arc voltage is Uh, and the current flowing through the line is naturally Ih.

We know that the reverse electromotive force across the inductor is UL=-LdILdtU_L=-L\frac{dI_L}{dt} , where we have omitted the negative sign in order to take it into account in the expression.

So does DC current affect the arc extinguishing ability of a contactor’s main contacts?

The answer is yes.

We all know that DC current is very difficult to extinguish in DC systems because there is no zero crossing process.

If the main contacts of the contactor are used in a DC system, the main contacts need to be connected in series to achieve arc extinguishing. See picture 5:

The purpose of this is to lengthen the arc and achieve rapid cooling and extinguishing of the arc.

However, another problem arises here. We have from equation 1,:

Uh=E-LdIhdt-RIhU_h=E-L\frac{dI_h}{dt}-RI_h , Equation 2

In Eq. 2, if the arc is extinguished too quickly, i.e., dIh/dt is larger, the reverse electromotive force appearing at the two ends of the inductor is too large, and the system will experience severe overvoltage.

Therefore, the arc extinguishing should not be too fast either.

 

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