Power Capacitors Selection Guide
Engineering guide to selecting single-phase and three-phase cylindrical and box-type power capacitors (kVAr, 400 V / 450 V / 525 V) for PFC banks.
What are power capacitors? Power capacitors supply reactive power (kVAr) locally in power factor correction (PFC) banks to offset inductive loads such as induction motors, transformers, and welding systems, maintaining target $\cos \varphi$ and avoiding utility reactive power penalties.
This guide covers the selection criteria for products in the Power Capacitors category.
Subcategories and applications
- Three-phase power capacitors: Cylindrical aluminum-can three-phase capacitors from 0.5 kVAr to 50 kVAr for standard and detuned PFC steps.
- Three-phase box type power capacitors: Heavy-duty prismatic steel-case capacitors designed for high ambient temperatures and industrial duty.
- Single-phase power capacitors: Phase-to-neutral (230 V / 250 V) capacitors for unbalanced three-phase networks where each phase is compensated independently.
- Contactor switching modules: Early-make damping resistor blocks that limit capacitor inrush current peaks during step switching.
Voltage rating and application comparison
| Capacitor Type | Rated Voltage | Typical Power Range | Recommended Network Condition |
|---|---|---|---|
| Single-Phase | 230 V – 250 V AC | 0.25 – 10 kVAr | Phase-by-phase compensation in unbalanced loads |
| Three-Phase Standard | 400 V – 415 V AC | 0.5 – 50 kVAr | Clean networks with low harmonic distortion |
| Three-Phase Heavy Duty | 440 V – 450 V AC | 1 – 50 kVAr | Networks with voltage fluctuations or 7% detuned reactors |
| Three-Phase Detuned | 480 V – 525 V AC | 5 – 50 kVAr | Steps connected in series with 5.67%, 7% or 14% harmonic reactors |
Frequently asked questions
Why specify 450 V or 525 V capacitors on a 400 V network?
When connected in series with a harmonic filter (detuning) reactor, the fundamental voltage across the capacitor terminals rises above 400 V. Higher voltage ratings (440 V, 450 V, or 525 V) prevent dielectric breakdown and significantly extend service life under harmonic stress.
How does rated voltage affect effective kVAr output at 400 V?
Effective reactive power scales with the square of the operating-to-rated voltage ratio: $Q_{\text{eff}} = Q_{\text{rated}} \times (U_{\text{network}} / U_{\text{rated}})^2$. Always size detuned steps using their effective kVAr at 400 V.
Why must capacitors be switched with capacitor contactors or damping modules?
Energizing a discharged capacitor produces inrush currents up to 100× nominal current. Pre-charge damping resistor modules or dedicated capacitor contactors suppress this peak and prevent contact welding.