Selecting voltage regulation equipment

What this guide covers

This guide explains the decisions that lead to a voltage-regulation proposal and why each piece of information matters. It helps you prepare a useful enquiry. It is not a sizing tool and does not replace an engineering review: a rating and configuration are proposed only after the inputs have been checked against the actual site.

Start with the load

The rating of a voltage stabilizer or voltage controller follows from the load it will carry, not from the capacity of the incoming supply. List the connected load, then estimate the load that actually runs at the same time. Where loads are listed in kW, the power factor is needed to convert them to kVA. If the power factor is not known, the review states the value it assumes so that the assumption can be checked later.

Load behaviour matters as much as the total. Motors, compressors and lifts draw a starting current several times their running current for a short period; for direct-on-line motors this is typically five to seven times. The regulator must hold its output through those starts without tripping. Loads with electronic power conversion, such as variable-speed drives, UPS systems and LED lighting, should be listed separately because they affect the current waveform and the neutral.

Define the supply condition

The second input is the range over which the incoming voltage actually varies. A single reading is not enough. Record the lowest and highest values over a representative period, including peak-load hours and light-load hours at night. The typical input windows on the class pages, such as 340–480 V AC three phase, describe what each class is generally built around; whether a particular site falls inside that window is a question for the measured data.

Note whether the variation is slow (daily or seasonal) or fast (large loads switching nearby), and whether the three phases move together or separately. Phases that vary independently point towards correction on each phase rather than on the three phases together.

Choose the class

Servo voltage stabilizers sense the output continuously and use a servo-driven variable transformer to follow changes in the incoming voltage, typically holding the output within ±1% of the set voltage inside the stated input window. Single-phase units are typically up to 25 kVA; three-phase units cover the rest of the range. The voltage stabilizers and voltage controllers class covers two non-servo products: tap-changing stabilizers, typically 1–100 kVA, which switch transformer taps to keep the output within a stated band, typically ±2–5%, and suit dedicated loads and smaller supplies; and voltage controllers, which set an adjustable output voltage for heaters, test benches and process equipment rather than regulating a building supply. Servo controllers are open control boards for integration into a host stabilizer; they are selected against the host design, its motor and its sensing arrangement rather than against a site load.

Decide what the equipment regulates

A mainline unit regulates the whole supply after the incoming switchgear. A dedicated unit regulates one load or one group of loads. Mainline regulation is simpler to operate but sizes the equipment for everything on the supply, including loads that do not need it. Dedicated regulation keeps ratings smaller but needs a clear record of which circuits are regulated. The review compares both where the load list allows it.

Phase arrangement and nominal output

Nominal outputs are typically 230 V AC single phase and 400 or 415 V AC three phase, with the set point agreed for each unit. Servo voltage stabilizers can control the three phases independently or together; independent control suits unbalanced supplies and loads. A three-phase site with mostly single-phase loads can be unbalanced, and the imbalance should be measured rather than assumed. Where a generator feeds the same load, its voltage and frequency behaviour should be included in the supply data.

Installation conditions

Servo voltage stabilizers are typically floor standing indoors, and are air cooled or oil cooled; oil cooling is usual for higher ratings and for hot or dusty rooms. Record the room size, ventilation route, ambient temperature in summer, access for maintenance on all serviceable sides, cable entry, and any limits on noise. These conditions can change the enclosure, the cooling arrangement or the location, and they are cheaper to resolve before the order than after delivery.

Protection and maintenance access

Ask how the proposed configuration protects the load and the equipment, and how the unit is isolated for maintenance. A manual bypass switch is typical on servo voltage stabilizers. The protections and bypass arrangement included are listed in the product specification and the quotation for the agreed configuration; they are not assumed from the class name.

Next step

Gather the inputs listed with this guide, using the voltage-regulation enquiry checklist if helpful, and send them through the enquiry form. Missing items do not stop an enquiry: the review records what is missing and what has been assumed, and asks for the facts that would change the proposal.