Key Parameters to Consider When Selecting Low-Voltage Capacitors

During the selection process of low-voltage capacitors, many users are unsure about how to choose the right equipment or which parameters need to be considered. When selecting low-voltage capacitors, the following key parameters should primarily be taken into account:

 

(1) Rated Voltage (Un): This refers to the voltage value (RMS) applied between the two poles of the capacitor during its design. For electronic capacitors, the unit of rated voltage is typically volts (V), while for power capacitors, it is commonly kilovolts (kV).

Rated voltage is a critical parameter for capacitors and is the first parameter that must be clearly defined. For AC capacitors, the price is not significantly related to voltage, whereas for DC capacitors, voltage has a substantial impact on cost. Rated voltage is directly related to the thickness and number of layers of the film.

 

(2) Rated Capacity (Qn): This indicates the reactive power (kvar) output by the capacitor at rated voltage and rated frequency. It is the pricing unit for power capacitors.

Common capacities for low-voltage capacitors include: 15 kvar, 20 kvar, 30 kvar, 40 kvar, etc.
Common capacities for high-voltage capacitors include: 100 kvar, 200 kvar, 300 kvar, 334 kvar, 400 kvar, 417 kvar, 500 kvar, etc.

 

(3) Rated Capacitance (Cn): This is the capacitance value adopted during the design of the capacitor, representing its ability to store charge (μF).

For AC capacitors, the rated capacitance is calculated based on the rated voltage and rated capacity. The capacitance tolerance is as follows:
① For low-voltage capacitors:

If Qn ≤ 100 kvar, the capacitance tolerance is -5% to +10%.

If Qn > 100 kvar, the capacitance tolerance is ±5%.
The maximum capacitance difference between different phases should not exceed 1.08.

(4) Rated Frequency: The frequency specified during the design of the capacitor.

(5) Rated Current (In): The AC current (RMS) specified during the design of the capacitor, measured in amperes (A).

(6) Loss: The active power consumed by the capacitor, measured in kilowatts (kW).

(7) Loss Tangent (tan δ): The ratio of the equivalent series resistance to the capacitive reactance of the capacitor under a specified sinusoidal AC voltage and frequency. It can also be understood as the ratio of loss to capacity: tan δ = P/Q, where P is active power and Q is capacity.

(8) Residual Voltage: The voltage remaining across the capacitor terminals after it has been disconnected for a certain period.