How to Select a Right AC Contactor

How to Select a Right AC Contactor

Table of Contents

    The selection of contactors shall meet the requirements of the controlled equipment.
    Besides that the contactor’ s rated working voltage shall be the same as that of the controlled equipment, the load power, using sorts, operating frequency, operating life, installation mode, size and economy of the controlled equipment all are the basis for selection.

1) Selection of AC contactor for controlling electrical heating appliance

    This kind of equipment includes resistance furnace, temperature regulating heater, etc. The current fluctuation range in this kind of load is very small. According to the using sorts, it belongs to AC-1. It is easy to control this kind of load by contactor, and the operation is not frequent. Therefore, when selecting a suitable contactor, the only thing should be considered is that its agreed heating current Ith shall be equal to or greater than 1.2 times of the working current in the electric heating equipment.
Example 1: try to select a contactor to control a three-phase Y-type resistance furnace with 380V and 15kw.
Solution: first calculate the rated working current Ie of each phase.
Ith=1.2Ie=1.2 × 22.7=27.2A
Therefore, any type of contactor with agreed heating current Ith ≥ 27.2a can be selected.
Such as: CI20-25, CJX2-18, CJX1-22, CJX5-22 and other models.

2) Selection of contactors for controlling illumination equipment

    There are many types of illumination equipment. Different types of lighting equipment have different starting current and starting time. This kind of loads refer to using sorts AC-5a or AC-5b. If the illumination equipment’s starting time is very short, the contactor’s selection should be considered that its agreed heating current Ith can be equal to 1.1 times of the working current Ie in the lighting equipment. If the illumination equipment’s starting time is slightly longer and its power factor is low,  the contactor’s selection should be considered that its agreed heating current Ith is larger than the working current of the lighting equipment, as information shown in Table 1.

Table 1 Selection Principle of Contactor for Controlling Illumination Equipment

Table 1 Selection Principle of Contactor for Controlling Illumination Equipment

3) Selection of contactor for controlling welding transformer

    The welding transformer has a abruptly large current due to the electrode’s short circuit on the secondary side, and has a large current on the primary side. Therefore, the selection of contactor must on the basis of the transformer’s rated power, rated working current, short-circuit current and welding frequency on the primary side at the time of electrode’s short circuit. This type of load belongs to AC-6a. Table 2 is the selection reference table.

Table 2 reference list for selecting welding transformer’s contactor

Table 2 reference list for selecting welding transformer’s contactor

4) Selection of contactor for AC-3 asynchronous motor

    Motors include asynchronous motor and wound-rotor induction motor, and their using sorts are AC-2, AC-3 and AC-4 respectively. Therefore, contactors with different structures are selected for motors of different models and usage.
    The starting current of asynchronous motor is about 6 times of the motor’s rated current Ie, and the breaking current of contactor is the motor’s rated current Ie. Its using sort is AC-3, such as water pump, draught fan, wire drawing machine, boring machine, printing press and hot cutting machine in steel plant. Here, direct-acting AC contactor can be selected.
    The selected methods include look-up table method and curve method. The rated working current and power of controllable motor under different rated working voltages are directly listed in the product samples to avoid the conversion of users. At this time, the contactor can be selected by look-up table method according to the power or rated working current of motor .

5) Selection of contactor for AC-2 wound-rotor induction motor

    Under such kind of load, the connecting current and breaking current of the contactor both are 2.5 times of the rated current Ie in the motor— AC-2 using sort. Generally, rotative type AC contactors is more suitable.
    Because its electrical endurance is higher than that of direct acting type, and it is easy to overhaul. When selecting, you can check the table according to the rated current of the motor. (Note: CJ12B should not be selected for occasions with the high number of operating cycles per hour). Direct-acting AC contactor can also be selected, but its electrical endurance is not as long as that of rotative type. When AC-3 type direct acting contactor with an electric endurance of 1.2 million is used in AC-2 type, its electric endurance comes to 100000 times.

6) Selection of contactor for AC-4 asynchronous motor

    When the motor is inching, or in reverse operating or reverse connection braking, the load is different from AC-3, and its connecting current is 6 Ie, belonging to AC-4.
    Under the condition of rated voltage 380V in AC-4 type, the rated working current of the contactor is given. Based on this, the controllable motor power Pe of AC-4 can be calculated.
    For example, under the condition of rated voltage 380V n AC-4 type, the rated working current of CJX1-9 AC contactor is 3.3A. We know that the rated output power of the motor =3UeIeCOSΦη.
    Assuming that the COS of the motor = 0.85, η= 0.9, substitute the rated current value 3.3A under the condition of AC-4 in the above contactor into the above formula, then =3×380×3.3×0.85×0.9=1.66KW
    That is, under the condition of rated voltage 380V in AC-4 type, the controllable motor power Pe of the three-phase asynchronous motor that CJX1-9 AC contactor controls is about 1.66kw or less.
    If the service endurance of contact is allowed to be shortened appropriately, the rated working current of AC-4 can be increased appropriately. When the on-off frequency is very low, the contactors of AC-4 can be selected according to AC-3 type.

7) Selection of contactor for mixed load motor

    In many cases, contactors are mixed under the conditions of AC-3 and AC-4 or AC-2 and AC-4, that is, they are used in a mixed way under normal on-off and inching operation modes. The service endurance of contact X for mixed use can be calculated by the following formula:
X=A/[1+0.01*C*(A/B-1)]
    For example: in a 37KW three-phase asynchronous motor, COSΦ=0.85,380V,Ie=72A. It uses 3TB48 type contactor for switching operation under mixed working modes, and its inching (AC-4) accounts for 30% of the total number of switching operations. Try to calculate the service endurance of contact X .
Check the life curve of 3TB48 type contactor and get that the electrical endurance at AC-3 is A = 1.2 ×106 times, the electrical endurance at AC-4 is B = 5 × 104 times
    C = 30%, so the service endurance of the contactor in the mixed working mode is: X = 15.2 * 104

8) Selection of contactor for capacitor

    As mentioned above, the contactor is generally required to reduce the capacity because there will be a high closing inrush current when the contactor is connected to the capacitor bank and the contactor will be burned seriously during the closing process.
    TIK1 series contactor for switching capacitor is specially designed for switching capacitor, and this series products adopt series resistance to suppress inrush current.

Precautions for selection of AC contactor

1) operation of contactors in series and parallel connection

    Many electrical equipment are single-phase loads, so the poles of multipole contactors can be used in parallel connection, such as resistance furnace, welding transformer, etc.
    When the several poles are used in parallel connection, it can choose contactors with smaller capacity. However, it must pay attention to that the agreed heating current of the contactor in parallel connection is not completely proportional to the number of poles in parallel connection. The resistance values of circuits from the moving and static contact are not exactly the same, so that the current flowing through is not evenly distributed.
    Therefore, when two poles are connected in parallel connection, the current can only increase to 1.8 times. While the current can only increase to 2 ~ 2.4 times when three poles are connected in parallel connection. In addition, it should be pointed out that the connecting and breaking capacity cannot be improved, since the contacts of each pole cannot be connected and disconnected at the same time in the parallel connection.
    Sometimes, the poles of the contactor can be used in series connection. Due to the increase of contact points, the arc can be divided into many sections, which improves the arc extinction ability and accelerates the arc extinction.
    Therefore, the working voltage can be increased when the poles are connected in series connection, but it can not exceed the rated insulation voltage of the contactor. In series connection, the agreed heating current and rated working current of contactors will not change

2) Influence of power frequency

    For the main circuit, the change of frequency influences the skin effect. When the frequency is high, the skin effect increases. For most products, 50 Hz and 60 Hz have little effect on the temperature rise of the conductive circuit. However, it becomes different when it comes to attraction coil. When the attraction coil designed at 50 Hz is used at 60 Hz, its magnetic flux of the magnet wire will be reduced, and its electromagnetic attraction will also be reduced. So, whether it can be used depends on the margin of its design. In general, clients should use it according to its certified value, and make orders according to the operating power frequency.

3) Influence of operating frequency

    It should think highly of that the number of operating cycles per hour of the contactor has a great impact on damaging contact. The applicable operating frequency is given in the technical parameters of the contactor. When the actual operating frequency of electrical equipment is higher than the given value, the contactor must be used with the reduced capacity.

Precautions for installation and usage of AC contactor

The service endurance and efficiency of the contactor depend not only on the technical performance of the product itself, but also on the proper use and maintenance of the product.
Pay attention to the following points in installation and adjustment:

1. Before installation

1.1Check whether the data on the nameplate and coil of the product (such as rated voltage, current, operating frequency and load factor) meet the actual use requirements.
1.2 For the moving parts of the opening and closing contactor, the product shall be flexible without jamming.
1.3 If anti-rust oil covers the pole area of the contactor’s iron core , it shall be wiped off before using, to prevent the contactor from being powered off and discharging due to oil dirt sticking.
1.4 Check and adjust the working parameters of the contact (clearance between open contacts, overrun, initial pressure and terminate pressure, etc.), and make contacts at each pole connected at the same time.

2. Installation and adjustment

2.1 When installing the wiring, pay attention to not omit the screws, washers, terminals and other parts, so as not to cause jamming or short circuit in the the contactor. When installing, the screws shall be tightened to prevent loosening in vibration.
2.2 After checking that the wiring connection is correct, the attraction coil shall be energized, making it opened and closed for several times under the condition that the main contact is not charged. The product can be put into use only after checking whether the action of the product is effective.
2.3 For contactors used for reversible conversion, in order to ensure effective interlocking, mechanical interlocking mechanism shall also be installed, besides the electrical interlocking.

3. Use

3.1 In the process of use, all parts of the product shall be checked regularly. It is required that the moving parts are not stuck and the fixed are not loose. If somewhere is damaged, it shall be replaced in time.
3.2 The contact surface shall be protected and cleaned frequently, and oil is not allowed to be used on it. When prills are formed on the contact surface due to arc, they shall be removed in time. When the contact is seriously worn, the contact shall be replaced in time.
However, it should be noted that the black oxidation film has low contact resistance, which is generated on the surface of contacts made by silver and silver based alloy when breaking arc. It will not cause poor contact, so it is not necessary to file the black oxidation film, otherwise the service endurance of the contacts will be greatly shortened.
3.3 The original contactor with arc-extinguishing chamber must be used with the arc-extinguishing chamber to avoid short circuit accident. The arcing cover made by clay is fragile, so it should avoid collision. If broken, it shall be replaced in time.

How to troubleshoot AC contactor?

AC contactor Immobility or futile action

Qusetions:The power supply voltage is too low or fluctuates too much
Answer:Adjust the power supply voltage
Qusetions: The power capacity of the operating circuit is insufficient or there is disconnection, error connection and
poor connection of control contact
Answer: Increase the power capacity, and correct and repair the control contact
Qusetions: The control power supply’s voltage is inconsistent with that of the coil voltage
Answer:Replace the coil
Qusetions:The product itself is damaged (under following circumstances: the coil is broken or burned, and the moving parts of machine are stuck, the rotating shaft is skewed, etc.)
Answer: Replace the coil to eliminate the jamming fault
Qusetions:The pressure and overrun of the contact spring are excessive
Answer: Adjust the contact parameters as required
Qusetions: The power supply is too far from the contactor and the connecting wire is too thin
Answer: Replace the thicker connecting wire

AC Contactor No discharge or discharge slowly

Questions: Pressure of contact spring is excessive
Answer: Adjust contact parameters
Questions: Contact is welded
Answer: Eliminate the welding fault and repair or replace the contact
Questions: The moving parts of the machine are stuck and the rotating shaft is skewed
Answer: Remove the stuck fault and repair the damaged parts
Questions: The reaction spring is damaged
Answer: Replace the reaction spring
Questions: There is oil stain or dust on the pole area of the iron core
Answer: Clean the pole area of the iron core
Questions: The service time of E-core is too long, so that the air gap disappears for demagnetization in the circuit, and the residual magnetism increases, making the core not discharge
Answer: Replace the iron core

AC Contactor Coil overheated or burnt

Questions:The power supply voltage is too high or too low
Answer: Adjust the power supply voltage
Questions: The technical parameters (such as rated voltage, frequency, load factor and applicable working mechanism) of coil are inconsistent with the actual service conditions
Answer: Replace the coil or contactor
Questions: The operating frequency is too high
Answer: Select other suitable contactors
Questions: The coil is poorly manufactured or due to mechanical damage, insulation damage, etc
Answer: Replace the coil and eliminate the fault caused by mechanical damage to the coil
Questions: Special environmental conditions: humid air, corrosive gas or high ambient temperature
Answer: use the coil with special design
Questions:The moving parts are stuck
Answer: Eliminate the jamming fault
Questions: The polar area of AC core is not smooth or the air gap for demagnetization is too large in the circuit
Answer: clean the polar area or replace the iron core
Questions: The two coils derived from the DC operation of AC contactor overheat because the normally closed interlocking contact is not charged by fusion welding
Answer: Adjust the parameters of interlocking contact and replace the burnt coil

Loud noise of electromagnet (AC)

Questions:The power supply voltage is too low
Answer:Increase the operating circuit voltage
Questions:Pressure of contact spring is excessive
Answer:Adjust the pressure of contact spring
Questions: The magnetic system is skewed or mechanically stuck, so that the iron core cannot be attracted flatly
Answer:Eliminate the mechanical jamming fault
Questions:The pole area is rusty or foreign matters adhere to the pole area of the iron core (such as oil stain and dust)
Answer: Clean the pole area of the iron core
Questions:The short-circuit ring is broken
Answer:Replace the iron core or short-circuit ring
Questions:The iron core polar area is excessively worn and not smooth
Answer:Replace the iron core

Welded Contact

Questions:The operating frequency is too high or the product is overloaded
Answer: Replace the appropriate contactor
Questions: Load side meets short circuit
Answer: Remove the short circuit fault and replace the contact
Questions:The pressure of contact spring is too small
Answer: Adjust the pressure of contact spring
Questions:There are metal particles bulging or foreign matters on the contact surface
Answer: Clean the contact surface
Questions: The voltage of operating circuit is too low or mechanically stuck, resulting in stagnation in the process of attracting, and the contact stops at the position just contacted
Answer: Increase the voltage of operating power supply, eliminate the mechanical jamming fault, and make the contactor attracted effectively

Contact overheated or burned after eight-hour operation

Questions: The pressure of contact spring is too small
Answer:Increase the pressure of contact spring
Questions: There is oil stain on the contact, or the surface is not smooth, and there are the metal particles bulging
Answer: Clean the contact surface
Questions: The ambient temperature is too high or it is used in a tightly closed control box
Answer:Reduce the capacity of contactor
Questions: Copper contact is used for long-term working
Answer: Reduce the capacity of contactor
Questions: The overrun of the contact is too small
Answer:Adjust the overrun of contact or replace the contact

The contact is worn excessively in a short time

Questions:The selection of contactor is improper, and the capacity is insufficient in the following occasions:
(1) Reverse connection braking
(2) Too much close connection operations
(3) Operating frequency too high
Answer: Reduce the capacity of contactors or use contactors suitable for heavy tasks
Questions:The three-phase contacts do not contact at the same time
Answer: Adjust the contacts to be contacted at the same time
Questions: Load side meets short circuit
Answer: Remove the short circuit fault and replace the contact
Questions:The contactor cannot be attracted effectively
Answer: See solution for futile action

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