---
title: "Power factor: a capacitor bank fixes one of two problems"
url: "https://3dfs.com/technology/power-factor"
description: "A capacitor bank corrects the timing of the current but not its shape. We correct both at the panel, with no capacitor step to wait for."
updated: "2026-10-15"
---

# Power factor: a capacitor bank fixes one of two problems

> A capacitor bank corrects the timing of the current but not its shape. We correct both at the panel, with no capacitor step to wait for.

A capacitor bank corrects the timing of the current, but on a modern panel its shape is the larger problem. We correct timing and shape at the panel, with no capacitor step to wait for.

Power factor is the share of the current a site pays for that turns into work. For a century the fix was a capacitor bank.

## What power factor is

The panel corrects timing and shape together. Power factor has two parts, timing and shape. The meter multiplies them. A bank corrects the timing and cannot touch the shape [3](https://3dfs.com/technology/power-factor#src-3).

- Transformer capacity. Motor plants at low power factor. A motor plant at 0.75 power factor draws 133 kVA to run 100 kW, and the transformer carries the other 33 kVA [1](https://3dfs.com/technology/power-factor#src-1).

- Drives and supplies. Panels of rectifier loads. A drive whose current arrives exactly in step, carrying 31% distortion, still runs at 0.95 power factor [4](https://3dfs.com/technology/power-factor#src-4).

- Capacitor banks. Sites that already corrected. In a 2024 test a bank took the timing to 1.00. The true power factor at the meter read 0.85 on motors and 0.82 on fluorescent lamps [5](https://3dfs.com/technology/power-factor#src-5).

- At the panel. Correction beside the loads. The controller supplies the reactive and harmonic current from its own terminals, so timing and shape both move toward 1.0 [1](https://3dfs.com/technology/power-factor#src-1).

Power factor is active power divided by apparent power [2](https://3dfs.com/technology/power-factor#src-2). Under a distorted current it has two parts.

Displacement power factor (DPF) is the timing: the angle between voltage and current. A capacitor bank corrects this part. The distortion factor is the shape: 1 / sqrt(1 + THDi²), where THDi is the total harmonic distortion in the current. A bank cannot touch this part. When the voltage is close to a sine wave, the two multiply [3](https://3dfs.com/technology/power-factor#src-3):

PF = DPF × 1 / sqrt(1 + THDi²)

At 60% distortion, a site can have exact timing and a true power factor of 0.86.

The curves are computed for a sinusoidal voltage [2](https://3dfs.com/technology/power-factor#src-2),[3](https://3dfs.com/technology/power-factor#src-3). The points are a measured capacitor-bank case [5](https://3dfs.com/technology/power-factor#src-5) and the textbook six-pulse rectifier [4](https://3dfs.com/technology/power-factor#src-4).

## What it costs

Reactive current is paid for three times. The utility bills it as a power factor penalty or as kVA demand. Each charge has its own window, often 15 minutes for demand and a month for kVARh. The transformer pays for it as heat and lost capacity: a 500 kVA transformer at 0.75 power factor is a 375 kW transformer. The feeders pay for it as I²R loss on current that does no work.

An illustrative 60 Hz pair, not a capture. The readouts come from the refrigerated container bench test, row 4d [6](https://3dfs.com/technology/power-factor#src-6): power factor 0.82 with correction off and 1.00 with correction on.

The same reactive current is paid for three times: on the utility bill, in the transformer’s lost capacity and as heat in the feeders.

Multiply the kVA demand charge by the gap between the kVA billed and the kW used. Then add the cost of the shape problem.

A 2024 test ran an automatic capacitor controller on motors and fluorescent lamps. It reached a displacement power factor of 1.00 while the current distortion read 63% and 70%. The true power factor at the meter was 0.85 and 0.82 [5](https://3dfs.com/technology/power-factor#src-5). We found no published national total for what power factor penalties cost U.S. facilities.

## How the industry has dealt with it

The industry’s answer is the capacitor bank, fixed or switched. A contactor-switched bank is the slowest answer. It waits a default 10 seconds between steps and blocks a discharged step for 50 seconds [7](https://3dfs.com/technology/power-factor#src-7). A thyristor-switched bank is faster and closes within three cycles [8](https://3dfs.com/technology/power-factor#src-8).

Neither bank touches the shape. Both add capacitance, which resonates with the source reactance at the order sqrt(S_sc / Q). S_sc is the short-circuit power and Q the bank rating. Treating timing with one analog device and shape with another creates a second problem for each one it solves.

In one leading maker’s worked example, a 350 kvar bank on a 630 kVA transformer resonates at the 5.5th harmonic, next to the 5th a drive produces [9](https://3dfs.com/technology/power-factor#src-9). The Army’s facilities manual lists the results: blown fuses, failed capacitors and tripped relays [10](https://3dfs.com/technology/power-factor#src-10).

The bank was designed for induction motors. The loads are now rectifiers: drives, LED drivers, chargers and servers, whose timing is often close to exact while their shape is not [4](https://3dfs.com/technology/power-factor#src-4),[11](https://3dfs.com/technology/power-factor#src-11). Active harmonic filters and static VAR generators supply both from power electronics, one maker’s within two cycles for harmonics and a quarter cycle for power factor [12](https://3dfs.com/technology/power-factor#src-12).

An illustrative trace of the shape problem, not a capture. The three harmonics drawn are 20%, 14% and 9% of the fundamental. Together they give a distortion factor of 0.97 before any timing error [2](https://3dfs.com/technology/power-factor#src-2). The corrected trace removes them.

## How Software-Defined Electricity handles it

The controller connects to the panel bus through its own breaker, beside the loads and never in the power path [1](https://3dfs.com/technology/power-factor#src-1). It supplies the reactive and harmonic current the loads draw, through the 23rd order, from its own terminals, so the source sees the loads’ active current [1](https://3dfs.com/technology/power-factor#src-1).

It responds in under 5 microseconds, correcting the current waveform digitally and continuously, with no step to wait for [1](https://3dfs.com/technology/power-factor#src-1). It adds no capacitance at the fundamental. The bank resonance above does not arise. The correction range runs from plus or minus 90 degrees to unity, capacitive or inductive. The rating is 60 A per phase for the VectorQ5 and 50 A for the VectorQ2 [1](https://3dfs.com/technology/power-factor#src-1).

Four measured tests show the result. Deloitte facilitated a test at a 208 V panel, measured on a power quality analyzer. A lightly loaded 1.5 hp motor read 0.15 power factor with correction off and 0.99 with one unit on. Reactive power fell from 0.9 kVAR to 0. The cart’s current fell from 2.2, 2.5 and 2.5 A to 0.5, 0.4 and 0.5 A [13](https://3dfs.com/technology/power-factor#src-13).

An independent audit measured a 120/208 V panel on a power quality analyzer. It logged a power factor of 0.85 with the unit off and 0.955 with it on [14](https://3dfs.com/technology/power-factor#src-14).

On two server racks, a field assessment recorded power factor rising from 0.931 to 0.999 and from 0.831 to 0.995. Reactive current at the fundamental fell from 7.27 A to 0.18 A [15](https://3dfs.com/technology/power-factor#src-15). The container test, row 4d, recorded apparent power falling from 8,339 VA to 7,229 VA [6](https://3dfs.com/technology/power-factor#src-6).

## Where it matters

The penalty lands on motor-heavy sites. An [industrial plant](https://3dfs.com/solutions/industrial-manufacturing) runs drives and presses. A [water plant](https://3dfs.com/solutions/water-wastewater) runs pumps at part load. A [ship](https://3dfs.com/solutions/maritime) pays for reactive current in generator fuel and capacity.

Two Opportunity Pillars meet at the meter. At [Consumption](https://3dfs.com/technology/consumption), drives and supplies make the reactive and distorted current. At [Delivery](https://3dfs.com/technology/delivery), the utility bills it.

## What to measure

Read true power factor at the service entrance, not displacement power factor, with a Class A meter that reports both. If the two differ, the problem is the shape. A bank will not fix it. Read kVA demand in the same window the utility bills. Read the feeder current before and after.

The change is upstream of the panel, at the transformer and the meter. The branch circuits carry the same reactive, distorted current as before. A stepped bank misses loads whose reactive swings finish inside ten seconds: welders, presses and crane hoists [7](https://3dfs.com/technology/power-factor#src-7).

Start by reading true and displacement power factor at one panel.

Sources

- 3DFS, VectorQ Technical Overview (current revision), Technical Specifications table, pp. 9, 12 and 14. First-party.

- MIT OCW 6.622, Lecture 4 (Perreault), pp. 2-4. [https://ocw.mit.edu/courses/6-622-power-electronics-spring-2023/mit6_622_s23_lec04.pdf](https://ocw.mit.edu/courses/6-622-power-electronics-spring-2023/mit6_622_s23_lec04.pdf)

- A leading maker’s Electrical Installation Guide, Harmonic distortion indicators, Power factor. [https://www.electrical-installation.org/enwiki/Harmonic_distortion_indicators_-_Power_factor](https://www.electrical-installation.org/enwiki/Harmonic_distortion_indicators_-_Power_factor)

- Heydt, Electric Power Quality, Tables 10.4 and 10.8.

- Diniș and Popa, Energies 2024, 17(7), 1647, Table 25, true PF computed from the table. [https://doi.org/10.3390/en17071647](https://doi.org/10.3390/en17071647)

- 3DFS, refrigerated container bench test, rows 4d and 3b. First-party.

- A leading maker’s capacitor bank controller user guide, parameters 305 and 401, pp. 20 and 23.

- A leading maker’s thyristor-switched capacitor bank product page.

- A leading maker’s Electrical Installation Guide, Risk of resonance due to power-system harmonics. [https://www.electrical-installation.org/enwiki/Risk_of_resonance_due_to_power-system_harmonics](https://www.electrical-installation.org/enwiki/Risk_of_resonance_due_to_power-system_harmonics)

- U.S. Army, TM 5-689 (2006), paragraphs 5-4 a(2) and 5-4 b.

- Michalec et al., Energies 2021, 14(12), 3665, Table 3. [https://doi.org/10.3390/en14123665](https://doi.org/10.3390/en14123665)

- Two leading makers’ active filter catalogs, p. 4-45 and p. 10 (2020-12-18).

- Deloitte and Touche LLP, SDE Technology Verification Testing, p. 14. Facilitated by Deloitte with 3DFS.

- Intelligent Building Solutions, Report for Power Quality Correction by the 3DFS VectorQ2 unit, pp. 12 and 14. Independent audit.

- 3DFS, electrical network analysis of server racks at a data center, overview sheets. First-party.

Version 5.1