Variable transformers

METREL variable transformers are available as:

  • Single or poly phase types;
  • Autotransformer or insulated variable transformer;
  • Manual or motor driven variable transformer;
  • Open / panel mount or enclosed construction;
  • Air-cooled, optional oil-cooled variable transformer.

METREL variable transformers provide continuously adjustable voltage from zero to 100% or 113% of the line voltage. Their operation is simple and efficient. Cooper wire is wound on a toroidal core by using high precision winding machines. Sliding trace of the winding is properly smoothed to provide low resistance and long wearing track for the carbon brush. Some models are silver plated, providing lower output impedance. The core is made of strip-wound oriented silicon steel for low electrical losses and high magnetic densities. The coil is insulated from the core by means of a special insulation support that also prevents movement of coil turns. Variable transformers are wound in a manner to ensure that voltage between the two turns is small enough to avoid harmful sparking or excessive heating of shorted turns.

METREL variable transformers provide an output voltage waveform that is a precise reproduction of the applied input voltage waveform. Slider is mounted on shaft but electrically insulated from it. With a brush holder, it serves also as a heat sink. Only standard METREL variable transformers are listed in this catalogue.

Technical regulation 

Three general regulations serve as a base for function, quality and safety of METREL variable transformers: European Low voltage directive 2006/95/EC (2014/35), International standard IEC 61558-1:2005+A1:2009 and IEC 61158-2-14:2012.



METREL variable transformers are applied to various products or applications including the following:

  • Power supplies;
  • Laboratory and test equipment;
  • Speed control devices;
  • Computer peripheral equipment;
  • Welding controls;
  • Variable speed devices for large machinery;
  • Electroplating and anodising;
  • High-voltage electronic tube circuits;
  • Spare supply regulators in broadcast transmitters;
  • Voltage stabilizers;
  • High voltage test set;
  • Meter test bench;
  • Hydro generation plants;
  • Battery chargers;
  • DC motor controls;
  • Plastic blow moulding machines;
  • Control of furnace transformers;
  • Lightening regulation;
  • Laboratory stirrers;
  • AC, DC brush motors;
  • High current motorized voltage regulators;
  • High current plastic plating operations.



High reliability

METREL variable transformers are practically maintenance free. Long-life operation is assured with: 

  • Precision winding;Surface forming of winding; 
  • Tight tolerances in preparing sliding track; 
  • Deep varnishing and baking also for fixing winding structure; 
  • A spring-loaded solid carbon brush. 

Power factor

Power factor of load has very little effect on the operation of a METREL variable transformer in the range from 0.5 lagging to 0.5 leading. Like any transformer, the METREL variable transformer reflects the load power factor to the line with very little change. Only for very light loads, possibly under 10%, will the lagging power factor of the METREL variable transformer become significant due to magnetizing current.

High efficiency

METREL variable transformers have low electrical losses under all load conditions. Efficiency is 98.5 percent at maximum output voltage selected. This efficiency remains high, even at greatly reduced load voltage. 

Ambient temperature

METREL variable transformers are designed for continuous operation in ambient of 0 °C to 40 °C, at full rated load. When operated above 40 °C, the output power must be rated in accordance with diagram 1. For example, when operating model HSH 230/4 in a 60 °C ambient, rated output current would be: 70 % x 4 A = 2.8 A.


All METREL variable transformers are designed for operation at 50/400 Hz. They may be operated at higher frequencies, without derating, however regulation becomes poorer. Units listed for 230 V operations may be applied on 115 V at 25 Hz, however rated currents remain the same.

Linear output voltage

METREL variable transformers have the advantage of providing output voltage that varies linearly in proportion to the angle of rotation of the output voltage selector. Because of the large number of increments of output voltage selection with the slider, the output voltage is practically steeples.

Installation guidelines

For safety and reliable operation of METREL variable transformers the following requirements need to be fulfilled:

  • good venting;
  • appropriate wiring;
  • over current protection;
  • avoiding corrosive, high humidity and dust places or protection against these environmental conditions;
  • preventing short circuits on axis;appropriate design and construction of equipment with built-in variable transformers.

Venting of power devices reduces their heating and thus rated performances can be applied. Power lines must have high enough cross-section, be fixed and secured with good contact to prevent overheating and additional voltage drops. Primary overcurrent breaking device must be properly selected and use of load protection fuses is recommended. Fuses / residual circuit breakers (RCD) prevent excessive heating due to overload and prevent fire generation as a result of overheating.Regardless if the variable transformers are designed for harsh environment, it is best for reliable operation and long lifetime to keep them in non-aggressive environment. The axis is on one side connected to metal base. If the other side of axis is electrically connected to the same base (through the housing), this will present a short circuit coil of the transformer with increasing power consumption, overheating, and even generating high leakage currents and stray magnetic fields. It is important that the brushes are not leaving in one spot for extended periods to avoid gradually increase contact resistance and eventual overheating and damage of the variable transformer unit.



Parallel connections

METREL has a solution for paralleling two single-phase transformers. Output current can be doubled by using the balancing choke and mechanical paralleling of sliders on common shaft.

Serial connections

Serial connection is intended for application of variable transformers in installations with higher input voltage than rated. Two variable transformers of the same type are connected in series and enable operation with double voltage of rated for one.  

Dual voltage tap slides

This possibility enables generating variable differential voltage with the same or opposite phase related to input voltage. Typical applications are boosting regulators.

Shaft modifications

The shaft provided with each model accommodates the METREL transformer’s voltage selector knob when mounted on panels not exceeding the thickness shown in dimension data. Modification to the shaft, either in length or end diameter, is available for both manual and motor-operated units.

Product groups

  • Open variable transformer types (subassemblies for panel mount or other built-in equipment) HSG; HST; HTG; HSM; HTM with Accessories (Buttons, Scales, Motor drives);
  • Desk top variable transformer types (HSN, HTN);
  • Power supplies (MA 4804, MA 4852, MA 4853).