Power Factor Controller Symbol
Definition: The Power Factor Controller symbol represents an automatic power-factor-correction (APFC) relay that measures the reactive power in an AC system and switches capacitor bank stages in or out to maintain a target power factor (cos φ) close to unity, depicted in industrial power diagrams as a labelled rectangular block with measurement inputs CT (current transformer) and VT (voltage transformer), a GND reference, and output relay contacts Out1 and Out2 that switch capacitor contactors.
Also known as: APFC relay, power factor correction relay, cos phi controller, reactive power controller, capacitor bank controller, PF relay.
What the Power Factor Controller symbol means
The Power Factor Controller symbol represents a measurement-and-switching instrument that monitors the displacement power factor of an AC electrical installation and automatically connects or disconnects banks of power-factor-correction (PFC) capacitors to compensate for inductive reactive power. A lagging power factor (caused by induction motors, transformers, and fluorescent lighting ballasts) increases apparent power demand and is penalised by utilities; the controller corrects this to reduce reactive power charges and improve voltage stability.
In a one-line or control diagram the power factor controller symbol occupies the measurement and control layer: its CT input samples line current, its VT input samples line voltage, and its output relays actuate contactors that switch capacitor stages. The symbol communicates that the panel includes automatic reactive power compensation, distinguishing it from a fixed capacitor or manual switching arrangement.
How to identify the Power Factor Controller symbol
The Power Factor Controller symbol is a rectangle labelled 'PFC CONTROLLER', 'APFC RELAY', or 'cos φ RELAY'. On the left edge, pins CT (current transformer secondary input), VT (voltage transformer or direct voltage input), and GND (reference) are labelled. On the right edge, output pins Out1 and Out2 represent switched relay contacts that actuate capacitor-bank contactors. The block may additionally show a display or setpoint annotation (e.g. 'target cos φ = 0.95'). The combination of CT/VT measurement inputs and multiple switching outputs is the defining visual pattern.
Function in a circuit
The power factor controller measures the phase angle (φ) between the supply voltage and line current by comparing the CT secondary signal with the VT (or direct line) voltage signal. It calculates the reactive power (kvar) and compares it to the target power factor setpoint. When the measured cos φ falls below the target (too much lagging reactive power), the controller closes an output relay (Out1 or Out2) to connect a capacitor stage via a capacitor contactor, injecting leading reactive current. When the power factor overshoots (leading), the controller opens the relay to disconnect that stage. Multiple outputs allow step-by-step capacitor bank switching for proportional compensation.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60617 does not define a dedicated power factor controller symbol; it is represented as a measuring relay block per IEC 60617-07 (measuring and protection relay symbols) or as a labelled functional block. IEC 61921 (Power capacitors — Low-voltage power factor correction banks) covers APFC system design. |
|---|---|
| ANSI/IEEE 315 | IEEE C37.2-2008 (Electrical Power System Device Function Numbers) assigns function number 55 (Power Factor Relay) to devices that control reactive power compensation. ANSI Y32.2 / IEEE 315 represents the controller as a labelled functional block; no unique glyph is standardised. |
| Key difference | Both IEC and ANSI/IEEE use labelled rectangular block conventions for the power factor controller. IEC uses device function descriptions; IEEE C37.2 assigns function number 55 for identification in protective relaying drawings. |
Terminals / pins
| Pin | Name |
|---|---|
| ct | CT |
| vt | VT |
| gnd | GND |
| out1 | Out1 |
| out2 | Out2 |
Typical values
Measurement range: cos φ 0.5–1.0 lagging/leading; input voltage: 100–480 V AC (direct or via VT); current input: 1 A or 5 A CT secondary; output relays: 4–14 stages (common sizes: 6, 8, 12 stages); switching time: 20–60 s per stage (to avoid capacitor inrush resonance); target power factor setpoint: typically 0.90–0.99; operating temperature: −10 to +55°C (panel mount).
Where the Power Factor Controller symbol is used
- Industrial factories with large induction motor loads requiring automatic reactive power compensation to avoid utility penalties
- Commercial buildings with HVAC systems, elevators, and fluorescent lighting generating lagging reactive power
- Power-distribution substations with automatic capacitor bank control for voltage regulation
- Data centres compensating for the reactive demand of UPS systems and switched-mode power supplies
- Renewable-energy sites where variable generation affects local power factor and voltage profile
- Motor control centres (MCCs) with integrated power factor correction panels controlled by an APFC relay
Example
In a 400 V industrial distribution panel, the one-line diagram shows a power factor controller block with its CT input connected to a 200/5 A current transformer on the main incomers L1 line, its VT input connected directly to the 400 V bus, and two output relay contacts (Out1, Out2) wired to the coils of two 25 kvar capacitor contactors. When the factory load causes cos φ to drop to 0.82, the controller closes Out1 to switch in the first 25 kvar stage; if cos φ remains below 0.95, it closes Out2 for the second stage — automatically correcting the power factor to the 0.97 setpoint.
Key facts
- The Power Factor Controller symbol represents an automatic reactive-power compensation relay that switches capacitor bank stages via output relays (Out1, Out2) based on CT and VT measurement inputs.
- IEEE C37.2-2008 assigns device function number 55 (Power Factor Relay) to power factor controllers in protective-relaying and control drawings.
- The controller measures phase angle between current (CT) and voltage (VT) to calculate reactive power in kvar, then switches capacitor stages to maintain a target cos φ close to unity.
- Typical target setpoint is 0.95–0.99 lagging; utilities in many countries impose reactive-energy tariff penalties for industrial consumers with cos φ below 0.9 or 0.85.
- Output switching is intentionally delayed (20–60 s per stage) to prevent simultaneous switching transients and capacitor bank inrush resonance.
- Common controllers have 4–14 switching stages; each stage connects or disconnects a fixed-kvar capacitor bank through a dedicated capacitor contactor.
- IEC 61921 governs low-voltage automatic power factor correction (APFC) bank design, including controller interface and protection requirements.
Frequently asked questions
What does the power factor controller symbol look like?
The power factor controller symbol is a labelled rectangle with CT, VT, and GND measurement pins on the left side (inputs from current and voltage transformers) and output relay contact pins Out1 and Out2 on the right side (switching capacitor contactors). The block is labelled 'APFC RELAY', 'PFC CONTROLLER', or 'cos φ RELAY'.
What does the power factor controller symbol mean on a panel diagram?
The symbol indicates that the panel includes automatic reactive power compensation. The controller measures the installation's power factor continuously and switches capacitor bank stages in or out to correct lagging reactive power and maintain a target cos φ close to unity, reducing reactive-energy tariff charges.
What is cos φ and why does the power factor controller maintain it?
Cos φ is the cosine of the phase angle between voltage and current waveforms — the displacement power factor. A value of 1.0 means voltage and current are in phase (purely resistive load, maximum efficiency). Inductive loads (motors, transformers) cause lagging current (cos φ < 1), increasing apparent power demand and utility charges. The controller adds leading reactive power from capacitors to bring cos φ back toward 1.0.
What standard defines the power factor controller symbol?
No single standard defines a unique glyph. IEEE C37.2-2008 assigns device function number 55 (Power Factor Relay) for labelling in protective-relaying diagrams. IEC 60617-07 covers measuring and protection relay symbols using labelled rectangular blocks. IEC 61921 governs APFC bank design requirements.
What are the CT and VT pins on the power factor controller symbol?
CT is the current transformer secondary input (1 A or 5 A) that measures line current magnitude and phase angle. VT is the voltage transformer input (or direct 100–480 V connection) that measures line voltage. Together they allow the controller to compute the reactive power (kvar) and power factor (cos φ) of the installation.
How many capacitor stages can a power factor controller switch?
Common power factor controllers switch 4 to 14 capacitor stages. Each output relay contact (Out1, Out2, etc.) connects to a dedicated capacitor contactor. More stages allow finer reactive power compensation steps, reducing hunting (oscillating between stages) and improving power factor regulation accuracy.
Why is there a time delay between capacitor stage switching in an APFC relay?
A time delay of 20–60 seconds between switching operations prevents simultaneous inrush transients that could cause resonance between capacitors and transformer inductance, damaging capacitors and causing voltage spikes. The delay also prevents rapid hunting (repeated switching in and out) when load reactive demand is near a stage boundary.
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