How are
they Built?


The Brentford Voltage Regulating Transformer, or Regulator as commonly known, is a variable ratio transformer.

Variation in the transformer ratio is achieved by the movement of carbon roller contacts over a column of turns where a strip of insulation has been removed to allow the contacts to run on the conducting material of the transformer coil.

This arrangement allows continuous variation across the range of voltages present on the coil turns.

Application of the Brentford Voltage Regulator

OLYMPUS DIGITAL CAMERA Although the Regulator can be supplied as a stand-alone item it is most commonly used in combination with other fixed ratio transformers to handle larger capacity throughputs.

This method of construction allows the Regulator to remain a standard product, while achieving considerable variation in the range of output that can be produced. Such construction methods allow the same Regulator to be used to provide 24,000 Amps, at say 40 Volts, or alternatively 160 Volts at 6,000 Amps. Clearly this methodology allows a wide range of applications to be catered for.

Using this approach, units with capacities in the order of several MVA can easily be produced.

Construction of the Brentford Voltage Regulator

At the heart of the Regulator are the coils. While some small capacity regulators are uncompensated, all larger capacity power regulators are fully compensated. Compensation is a method where by the naturally high reactance of long thin coils, used in the construction of the Brentford Regulator is limited to a low value across the range of variation. Limiting the reactance tends to produce a linear variation in the output voltage of the regulator, across the range of operation.

Contact pick up tracks are machined onto the faces of the coils. Depending on the required output arrangement there may be tracks on both sides of the coil, or only on one.

The coils are then mounted on a core of grain oriented silicon steel laminations. The core and coils are mounted in a steel framework that allows roller contacts to be presented to the coil faces at a constant pressure across the range of operation.

To achieve variation in the output voltage the roller contacts are moved along the coil face. Roller contacts are housed within retaining assemblies that are in turn fixed to a carrier board. The carrier board’s position is maintained by a chain drive system that is in turn attached to the drive mechanism, either hand- or motor-driven.


The construction of the Brentford Regulator allows the same basic unit, with only minor changes, to be used either immersed in cooling fluid, or directly air-cooled.

For the larger power ratings of regulator, fluid immersion is used exclusively.

Where fluid immersion is employed secondary cooling can be provided by air natural, forced air, or water-cooling.


Due to the flexibility of the design several arrangements can be provided. These range through single phase, two phase, three phase, single end output (boost only), double end output (Brentford “Q” connection for double boost), to double output for buck boost arrangements where the output of the regulator starts at a maximum voltage, decreases to zero, and then again increases in voltage but with the polarity reversed.

Single, and double boost connections are typically used for applications where the minimum output voltage is close to zero, and the output increases to a maximum value. Double boost is used where the rating of the overall equipment is greater than can be supplied by the single boost connection, or where an economic advantage is gained by use of this method.

Double boost combines two outputs of opposite polarity from the regulator via a transformer using the Brentford “Q” connection. For the cost of the second set of roller contacts, and pick-up gear, the output current capacity of the regulator is doubled.

Buck boost connection is used where the minimum output voltage is above zero volts. Basically the connection comprises of two fixed ratio transformers and a double output voltage regulator.

One fixed ratio transformer (commonly referred to as the Main Transformer) has an output voltage approximating to the mid-point of the range required. The second fixed ratio transformer (commonly referred to as the Buck Boost Transformer) has an output voltage approximating to the difference between mid-point, and minimum / maximum required voltage, and there output windings are connected in series. The input winding of the buck boost transformer is connected across the two outputs of the regulator.

T&R_9-Oct-2009_184 (2) (800×521) With the roller contacts located at one end of the regulator a maximum voltage at a specific phase relationship to that of the fixed ratio transformers is produced.

The transformers are arranged so that in this condition the output voltage of the buck boost transformer is subtracted from the output voltage of the main transformer.

This provides the minimum output voltage of the arrangement.

As the roller contacts are moved to the centre of the regulator the output voltage of the regulator, and hence the buck boost transformer approximates to zero. The combined output of the arrangement is therefore the output voltage of the main transformer.

Continuing to move the roller contact in the same direction will increase the regulator output voltage. In addition the polarity of the voltage will have reversed so that the output voltage of the buck boost transformer is now added to that of the main transformer. By this means the maximum output voltage of the combination is achieved.


The Brentford Regulator has good short term overload capacities. For some applications we allow a level of de-rating within our design that is invisible to the client.

Almost any duty can be catered for. If you have specific requirements that you wish to discuss please contact our sales force.

General classifications for duty types are broken down as follows:

1. Light. Where the loading continuously vary in respect of voltage, and current requirements. There are breaks in loading during each day.

2. Medium. This is where production is continuous but the loading in respect of voltage, and current requirements vary on a regular basis.

3. Heavy. Where production is continuous with loading remaining constant for long periods in respect of voltage, and current requirements.


Motor control is required for the vast majority of applications. In addition to the motor control we also include an emergency hand-wheel for use if the automatic control is not available, and for maintenance purposes.

End of travel limit switches are provided with motorised systems, and if required, additional interlock / indication contacts can be provided at end of travel.

In addition, if required, an externally adjustable limit switch can be provided for customer setting.

We are also able to offer a potentiometer feedback that is proportional to regulator position, for inclusion into customers’ control schemes.

Automatic control schemes can be provided if required. Details of customer’s requirements would need to be provided at the quotation stage.

Contact Us

15-16 Woodbridge Meadows, Guildford, Surrey. GU1 1BJ

Phone: +44 (0)1483 568 281

Fax: +44 (0)1483 504 961

E-mail: sales@transformers.co.uk