The variable transformer, or to give it its more formal description, the continuously adjustable autotransformer, is one of the most useful devices ever produced for the smooth, distortionless adjustment of AC voltage and power, with innumerable uses in testing and in control of voltage dependent parameters – speed, lighting, heat, etc.

Originally introduced commercially as the “Variac”™ in 1933 by the former General Radio Company of Cambridge, Massachusetts, the patent covered the design of the carbon brushgear essential to smoothly vary the voltage.

The earliest “Variacs”™ employed a toroidal core comprising a stack of ring laminations, with a single layer winding of insulated copper wire, bared to form what is essentially a commutator, across which a carbon contact is moved to select the output voltage.

The first models were rated at some 500 VA (200 series) , and were followed a year or two later by larger units rated at about 2 kVA (100 series), and subsequently 5 kVA (50 series).

Initially “Variacs”™ were imported from the USA by Claude Lyons Limited as the agents for the former General Radio Company, but these were originally designed for 60 Hz supply and there were problems with magnetising current when used on 50 Hz, while it was only possible to connect the input across the whole winding (“line voltage connection”) giving variation from zero to supply voltage, and making it impossible to use the invaluable “over-voltage connection” with the supply to a tap on the winding and permitting output above input supply voltage.

Therefore from 1936 on, “Variacs”™ were manufactured in England, using larger cores permitting full operation on 50 Hz supply and with over-voltage.

The main developments in subsequent years have been the replacement in the 1950s of the stacked core by a spiral strip wound core permitting operation at higher flux density and providing higher output capability and lower no-load losses, and the introduction of a hard precious-metal coating to the contact surface, improving performance under surge conditions and extending product lifetime. These were designated the V series. In the late 1960s the V series was replaced by the Regulac R series.

With the acquisition of the product ranges of Berco Controls Limited (initially as Claude Lyons Controls Limited) in 1976 and of The Zenith Electric Company Limited in 1990, the opportunity was taken to rationalise the range under the UK Registered Trademarks Regavolt® and Varatran®, the latter designating our range of epoxy-encapsulated variable transformers.

The table below shows only the basic historic 115-120V, 230-240V and 440V 50/60 Hz models and will serve as a guide to the various series and their current production equivalents. There have been, and continue to be, many variants including ganged models for higher power and/or three-phase operation, high frequency units, and units for lower and higher supply voltage – click here for details.

Operating Voltage	Approximate trademark date and model number
	1930s – 1940s “Variac”™	1940s – 1960s “Variac”™	1940s – 1970s Regavolt®(Berco)	1960s – 1970s Regulac®	1970s – Present Regavolt®
230-240V 115-120V	200B	X1,X2	31A,31B,31C	RX1,RX1L	301  301L
230-240V 115-120V 440V	200-CUH, CMH 200-CU, CM	V3H – V6H (M) V3 – V6(M)	41 – 45A, C 41B, 42B 41 -45Z	RB1 – 4 (M) RB1L – 3L (M) RB2H – 4H (M)	401 – 404 (M) 402L, 403L (M) 402H, 404H (M)
230-240V 115-120V 440V	100-R, 100-L* 100-Q, 100-K*	100-R, 100-L* 100Q, 100-K* Continued production	71,72A, C,71B 71Z, 72Z	RK6 – 13 (M),RK6L, RK6L-M RK6H – 13H (M)	706 – 715 (M), 706L, 706L-M 706H – 715H (M)
230-240V		V30H, V30H-M			No direct replacement –use 715 or 1225
230-240V 115-120V 440V	50B, 50B-M 50A, 50A-M	50B, 50B-M 50A, 50A-M Continued production	121A, C	RQ25, RQ25-M RQ25L, RQ25L-M RQ25H, RQ25H-M	1225(M) 1225L (M) 1225H (M)

*100L, 100K were zero to line voltage only, 100R, 100Q provided over-voltage
Models in bold can be purchased via our online shop

We are very proud that many variable transformers supplied by this company have continued in service after even 50 or 60 years. With care and attention to track cleaning and brush wear they have an almost indefinite life. Refurbishment of an old unit in good condition is often possible but if the winding has been burned out due to misuse or overload it is more practical to supply a replacement from our current product range. The table will serve as a basic guide to the various historical series but we will always be able to offer a suitable replacement to any of our models, and indeed to those of other manufacture, and please contact us in case of need.