Use of capacitor duty contactor for capacitor switching
Power Contactor: is an electrically controlled switch that is used for switching an electrical power circuit. It is typically controlled by a circuit that has a much lower power level than the switched circuit.
Capacitor Duty Contactor: these are special types of contactors used to switch on capacitors and capacitor banks. Ordinary contactors get damaged easily over repeated switching of the capacitors. So capacitor duty contactors are the best choice for switching on capacitors as these type of contactors limit the inrush current and tends to increase the capacitor lifespan.
In Low Voltage industrial installations, capacitors are mainly used for reactive energy correction (raising the power factor). When these capacitors are energized, overcurrents of high amplitude and high frequencies (3 to 15 kHz) occur during the transient period (1 to 2 ms).
The amplitude of these current peaks, also known as “inrush current peaks“, depends on the following factors:
- The network inductances.
- The transformer power and short-circuit voltage.
- The type of power factor correction.
Fixed power factor correction consists of inserting, in parallel on the network, a capacitor bank whose total power is provided by the assembly of capacitors of identical or different ratings. The bank is energized by a contactor that simultaneously supplies all the capacitors (a single step). The inrush current peak, in the case of fixed correction, can reach 30 times the nominal current of the capacitor bank.
An automatic power factor correction system, on the other hand, consists of several capacitor banks of identical or different ratings (several steps), energized separately according to the value of the power factor to be corrected. An electronic device automatically determines the power of the steps to be energized and activates the relevant contactors. The inrush current peak, in the case of automatic correction, depends on the power of the steps already on duty and can reach 100 times the nominal current of the step to be energized.
It is designed to use AC6b contactors for Capacitor switching as the inrush current will be high. The power contactor is of AC3 duty which is insufficient to withstand the capacitor inrush current.