How to Select a Contactor?
As equipment to disconnect load power, selection of contactor should meet requirements of controlled equipment. Except that the rated operational voltage is the same with the rated operational voltage of controlled equipment, load power, usage category, control mode, operation frequency, working life, installation method, installation dimension and economy of controlled equipment are the basis of selection. How to choose a right contactor? The principles are as follows:
- Voltage class of alternating current contactor should be the same with load, and selected contactor should adapt to load.
- Calculating current of load should conform to capacity class of contactor. That is to say, calculating current is less than or equal to eh rated operational current of contactor. Making current of contactor is bigger than the starting current of load. When breaking current is bigger than workload, breaking needs current. Calculating current of load should consider practical working environment and condition. For load with a long starting time, peak current of half an hour cannot surpass appointed heating current.
- Short-term dynamic-thermal stabilization test. Three-phase short circuit current of line should not exceed allowable dynamic-thermal stable current of contactor. When short circuit current is shut off by use of contactor, it should also test breaking capability of contactor.
- Rated voltage and current of wiring attracted by contactor, quantity of auxiliary contactor and current capacity should meet requirements of wiring of connection circuit. It is necessary to consider length of control circuit of contactor. Generally speaking, with the recommended voltage value, contactor can work with a voltage which is 85~110% of the rated voltage. If the line is too long, since the voltage has a sharp drop, contactor coil can not reflect the order of switching on; and since electrical capacity of line is too big, it can possibly not act on the order of trip.
- Test allowable operating frequency of contactor according to operating times. If operating frequency surpasses the stipulated value, the rated current should be double.
- Parameters of short circuit protection element should be used together with parameters of contactor. Matching of contactor and breaker should be decided according to overload coefficients of breaker and coefficients of short circuit protection. Appointed heating current of contactor should be smaller than overload current of breaker, and connecting and disconnecting current of contactor should be smaller than short circuit protection current of breaker. Only in this way can breaker protect contactor. In practice, proportion of appointed heating current and rated working current of contactor under one voltage class is among 1~1.38. While inverse time limit overload coefficients of breaker has a big quantity and breakers of different types are different, thus matching of the two is very hard to have a standard and cannot form a matching table, thus it needs practical calculation.
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