Selection of AC Contactors in Printing Equipment

The selection of commonly used AC contactors in printing equipment is mainly considered through the structure and parameters of the AC contactor, the load of the AC contactor and the working environment.

The low-voltage AC contactor is mainly used for turning on and off the power of the electrical equipment and can remotely control the power equipment, and avoid personal injury when the equipment is powered on or off. The selection of AC contactors is very important for the normal operation of power equipment and power lines.

1 AC contactor structure and parameters

It is usually required that the AC contactor device has a compact structure and is easy to use. The magnetic blowing device for moving and static contact is good and the arc extinguishing effect is good. It is best to achieve zero arcing and a small temperature rise. According to the arc extinguishing method, the AC contactor can be divided into air type and vacuum type. According to the control mode, it can be divided into electromagnetic type, pneumatic type, and electromagnetic type.

Contactor rated voltage parameters are divided into high pressure and low pressure, low pressure is generally 380V, 500V, 660V, 1140V and so on.

The current parameters include the rated operating current, the prescribed heating current, the on-current and the breaking current, the conventional thermal current of the auxiliary contact, and the short-term withstand current of the contactor. The general contactor model parameter gives the rated operating current, such as Schneider's LC1-D25, the rated operating current of the main contact is 25A, and the rated operating current is related to the selected load current and voltage level.

The AC contactor coil is divided into 36V, 127V, 220V, 380V according to the voltage, and the number of contacts of the contactor is divided into 2, 3, 4, and 5 poles. The auxiliary contacts have several pairs according to normally open and normally closed, and they are selected according to the control needs.

Other parameters include the number of connections, the number of disconnections, the mechanical life, the electrical life, the maximum allowable operating frequency, the maximum allowable wire diameter, and the overall dimensions and mounting dimensions.

2 AC contactor selection principle

The contactor acts as a device for switching on and off the load power supply. The selection of the contactor should be carried out according to the requirements of the controlled device, except that the rated working voltage is the same as the rated operating voltage of the controlled device, and the load power, use category, and control of the controlled device. The method, operating frequency, working life, installation method, installation size, and economy are the basis for selection. The selection principle is as follows.

1) The AC contactor must have the same voltage rating as the load. The type of contactor used must be compatible with the load.

2) The calculated current of the load must meet the contactor's capacity level, ie the calculated current is less than or equal to the rated operating current of the contactor. The current of the contactor is greater than the starting current of the load, and the breaking current is greater than the breaking current required when the load is running. The calculated current of the load must consider the actual working environment and working conditions. For a load with a long starting time, the peak current of half an hour cannot exceed the prescribed heating. Current.

3) According to the short-term dynamic and thermal stability check. The three-phase short-circuit current of the circuit shall not exceed the allowable thermal and thermal stability currents of the contactor. When the contactor is used to disconnect the short-circuit current, the breaking capacity of the contactor shall also be verified.

4) The rated voltage and current of the contactor attraction coil and the number and current capacity of the auxiliary contacts shall meet the control circuit wiring requirements. To consider the length of the line connected to the control circuit of the contactor, it is generally recommended that the operating voltage of the contactor be able to operate at a rated voltage of 85% to 110%. If the line is too long, the contactor coil may not react to the closing command because the voltage drop is too large; because the line capacitance is too large, the trip command may not work.

5) Check the allowable operating frequency of the contactor according to the number of operations. If the operating frequency exceeds the specified value, the rated current should be doubled.

6) Short-circuit protection element parameters should be matched with contactor parameters. When you select it, you can refer to the sample manual. The sample manual generally gives the contact table and the fuse table. The cooperation of the contactor and the air circuit breaker shall be determined according to the overload coefficient of the air circuit breaker and the short-circuit protection current coefficient. The contractor thermal current of the contactor should be less than the overload current of the air circuit breaker. The contactor's on and off currents should be less than the circuit breaker's short circuit protection current, so that the circuit breaker can protect the contactor. Actual contactor in a voltage level agreement between the heating current and the rated operating current ratio between 1 ~ 1.38, and the circuit breaker overload inverse time coefficient parameters are more, different types of circuit breakers are not the same, so it is difficult to match between the two There is a standard that cannot form a coordination table and requires actual accounting.

7) The installation distance of the contactor and other components must comply with the relevant national standards and specifications, and maintenance and trace distance must be considered.

3 Selection of AC Contactors under Different Loads

In order to prevent the contacts from sticking and contaminating the contacts, the life of the contactor is prolonged, the contactor must avoid the maximum current of the load, and the unfavorable factors such as the start-up time must be taken into consideration. Therefore, the load of the contactor must be kept on and off. Analyze, according to the electrical characteristics of the load and the actual situation of the power system, calculate and match different load start-stop currents.

1) Selection of AC contactor for controlling heating equipment

This type of equipment has resistance furnaces, temperature control equipment, etc., and its wire resistance components used in the heating element load can reach a current of 1.4 times the rated current. If the power supply voltage is increased, the current will increase. The load current fluctuation range is very small, according to the use of categories belong to AC-1, the operation is not frequent, the choice of contactor, as long as the contactor's rated operating current is equal to or greater than the electric heating device operating current of 1.2 times.

2) Selection of motor contactor

The motor contactor can be selected from AC-2 to 4 according to the motor usage and motor type. For the starting current at 6 times the rated current, the breaking current can be AC-3 at rated current. For example, the fan and pump can be checked. The table method and the selected curve method are selected according to the sample and the manual, and need not be calculated again.

Winding motor current and breaking current are both 2.5 times rated current. In the normal start, resistors are connected in series in the rotor to limit the starting current and increase the starting torque. The category AC-2 is used, and rotary contactors are available.

When the motor is in jog, reverse running and braking, the on current is 6Ie and the use category is AC-4. The power of the motor can be calculated based on the current size listed under the use category AC-4. The formula is: Pe=3UeIeCOS¢η. Among them, Ue: motor rated current; Ie: motor rated voltage; COS¢: power factor; η: motor efficiency.

If the contact life is allowed to be short, the AC-4 current can be properly increased and changed to AC-3 at a very low switching frequency.

According to the requirements of the motor protection coordination, the current below the stall current should be switched on and off by the control electronics. The stall current of most Y series motors is ≤7Ie. Therefore, when selecting a contactor, it is necessary to consider dividing and blocking stall current. The specification stipulates that: When the motor is operated under AC-3 and the rated current of the contactor is not more than 630A, the contactor shall be able to withstand 8 times the rated current for at least 10 seconds.

For general equipment motors, the operating current is less than the rated current, and the starting current is 4 to 7 times of the rated current, but the time is short, and the contactor's contact is not damaged. The contactor has taken this factor into account in the design and is generally used. The contact capacity is greater than 1.25 times the rated capacity of the motor. For motors that work under special conditions, consider the actual operating conditions. Such as electric hoist is an impact load, heavy load start and stop frequent, reverse braking, so calculate the operating current to be multiplied by the corresponding multiple, due to frequent heavy load start and stop, use 4 times the rated current of the motor, usually reversed under heavy load The braking current is 2 times the starting current, so 8 times of the rated current should be used for this working condition.

4 AC contactor installation

The AC contactor has relatively large vibration when it is sucked and disconnected. When installing, the electric equipment with stricter vibration requirements should not be installed in a cabinet. Otherwise, it is necessary to use shockproof measures and generally try to install it in the lower part of the cabinet. The installation environment of the AC contactor must meet the requirements of the product, and the installation dimensions should meet the electrical safety distance and wiring regulations, and it is convenient to overhaul.

The selection of the AC contactor is not only related to the impedance parameters of the power system in the circuit where the load is disconnected and the contactor is located, but also related to the control method, the use environment and the usage requirements. Therefore, when selecting the AC contactor, it is necessary to fully consider and gradually calculate each of them. The parameter values ​​achieve reasonable and easy-to-use results.

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