How to select ac contactor (selection method)
Select the type of contactor
Ac contactors are generally classified into one, two, three and four types according to the type of load, which are respectively denoted as AC1, AC2, AC3 and AC4.The corresponding control object of an ac contactor is non-inductive or micro-inductive load, such as incandescent lamp, resistance furnace, etc.Two kinds of ac contactors are used for starting and stopping wound induction motors.The typical use of the three types of ac contactors is the running and breaking of squirrel cage asynchronous motors.Four types of ac contactors are used for starting, reverse braking, reversing and inching of cage asynchronous motors.
Select the rated parameters of the contactor
According to the controlled object and working parameters such as voltage, current, power, frequency and working system, the rated parameters of contactor are determined.
1) the coil voltage of the contactor should be generally lower, so that the insulation requirements of the contactor can be reduced and it is safer to use.But in order to facilitate and reduce equipment, often according to the actual grid voltage selection.
2) the operating frequency of the motor is not high, such as compressor, water pump, fan, air conditioner, punch, etc., and the rated current of the contactor is greater than the rated current of the load.Contactor type can choose CJl0, CJ20, etc.
3) for heavy task type motor, such as machine tool main motor, lifting equipment, capstan, crusher, etc., whose average operating frequency is more than 100 times /min, and running in starting, inching, forward and backward braking, reverse braking, CJl0Z, CJl2 contactor can be selected.In order to ensure the electrical life, the contactor can be used to reduce the capacity.When selected, the rated current of the contactor is greater than the rated current of the motor.
4) for heavy task motors, such as printing presses and boring machines, the operating frequency is high, up to 600 ~ 12,000 times /h, and they are often operated in starting, reverse braking and reverse state, etc. The contactor can be selected roughly according to the electrical life and starting current, and the contactor type number is CJl0Z and CJl2, etc.
5) when the capacitor in the ac loop is put into or removed from the power grid, the contactor selection should take into account the closing shock current of the capacitor.Generally, the rated current of the contactor can be selected according to 1.5 times of the rated current of the capacitor.
6) when using the contactor to control the transformer, the magnitude of inrush current should be considered.For example, ac arc welder, resistance welder, etc., the contactor can be selected according to 2 times of the rated current of the transformer. The models are CJl0, CJ20, etc.
7) for electric heating equipment, such as resistance furnace and electric heater, the cold resistance of the load is small, so the starting current should be relatively large.Contactor can be selected without considering (starting current), directly according to the load rated current selection.Models can choose CJl0, CJ20, etc.
8) due to the high starting current and long starting time of the gas discharge lamp, for the control of lighting equipment, ac contactor can be selected according to the rated current 1.1 ~ 1.4 times, and models can be CJl0, CJ20, etc.
9) rated current of contactor refers to the maximum allowable current of contactor under long-term operation, whose duration is less than or equal to 8h, and it is installed on the open control board. If the cooling condition is poor, the rated current of contactor should be selected according to 110% ~ 120% of the rated current of load.For long time working motor, the oxidation film has no chance to be cleared, so that the contact resistance increases, resulting in the contact heating exceeding the allowable temperature rise.In practice, the rated current of contactor can be reduced by 30%.
Ac contactor selection principle
The voltage level of ac contactor should be the same as the load, and the contactor type should be suitable for the load.
The calculated load current shall be less than or equal to the rated working current of the contactor.The turn on current of the contactor is larger than the starting current of the load, and the breaking current is larger than that of the load when the load is running. The calculation current of the load should take into account the actual working environment and working conditions. For the load with a long start time, the peak current in half an hour should not exceed the agreed heating current.
Check dynamic and thermal stability for a short time.The three-phase short-circuit current of the line should not exceed the dynamic and thermal stable current allowed by the contactor. When the contactor is used to disconnect the short-circuit current, the breaking capacity of the contactor should also be checked.
Rated voltage and current of the contactor suction coil, number of auxiliary contacts and current capacity shall meet the wiring requirements of the control loop.The length of the line connected to the contactor control circuit, the recommended operating voltage, should be able to operate at 85 ~ 110% of the rated voltage.If the line is too long, the voltage drop is too large, the contactor coil may not reflect the closing instruction;Because the line capacitance is too large, it may not work on tripping instructions.
Check the operating frequency allowed by the contactor according to the number of operations.If the operating frequency exceeds the specified value, the rated current should be doubled.
The parameters of short-circuit protection elements should be selected together with the parameters of contactor.When selecting, please refer to the sample manual, which generally gives the matching table of contactor and fuse.
The matching of contactor and circuit breaker shall be determined according to the overload factor and short circuit protection current factor of circuit breaker.The contactor's agreed heating current shall be less than the overload current of the circuit breaker, and the on and off current of the contactor shall be less than the short-circuit protection current of the circuit breaker, so that the circuit breaker can protect the contactor.In practice, the contactor agrees that the ratio of heating current and rated working current under a voltage grade is between 1 and 1.38, while the inverse time limit overload coefficient parameters of circuit breakers are more, and different types of circuit breakers are different. Therefore, it is difficult to have a standard for the matching between the two, and the matching table cannot be formed, which requires actual calculation.
The installation distance of contactor and other components should conform to relevant national standards and specifications, and maintenance and wiring distance should be considered.