Electronic Servo AC Voltage Stabilisers & Power Conditioners

The EM and EMS range offers the most cost effective and efficient way to ensure a stable, undistorted mains supply for the vast majority of applications.

Based on the proven electronic servo design, a double wound transformer has its secondary winding connected between the incoming AC supply and the load, with the primary winding voltage controlled by a motor-driven variable auto transformer. A solid-state servo amplifier continuously monitors the output voltage of the stabiliser, and any error voltage is amplified to operate the servomotor of the variable transformer, driving its brush-gear to a point where it is injecting a voltage that will correct the supply voltage by adding or subtracting from it.

Over six thousand models span ratings from 1kVA to 8,000kVA, single or three phase, incorporating full transient spike and noise suppression if required. Available with a wide selection of input voltage ranges from ±8% to ±40%

IP Classified and Oil Immersed Stabilisers

For outdoor use or any installation in humid or contaminated environments a range of IP rated air-cooled enclosures and EMO oil filled steel tanks are available.

IP rated enclosures can be designed to specific requirements to conform to IP54. The robust oil immersion of stabilisers (EMO) results in more efficient cooling and a reduction in size compared to air-cooled stabilisers. They are available in both single and three phase models. The smaller units up to 100kVA are suitable for pole mounting and are designed for use in overhead distribution networks. The three phase stabilisers normally require a neutral but can be supplied with a static balancing transformer for three-phase 3-wire systems. Output voltage accuracy ±2% or ±5% (as required). The accuracy is maintained over the specified input voltage range from zero to full load irrespective of power factor, frequency variations, and ambient temperature variations. For three-phase units the accuracy is maintained irrespective of input voltage balance and output load balance.

EM & EMS Ratings Chart EM & EMS Dimensions Chart EMO Ratings & Dimensions Chart


  • High speed of response
  • Low internal impedance capable of sustaining high surge currents.
  • Efficiency better than 98% with up to 99.5% on higher models.
  • EM/EMS Turbo (T) stabilisers incorporate the advanced Rotavolt technology and additional forced air cooling resulting in a considerable cost and weight reduction per kVA.
  • Soft-start ensures low output voltage at the moment of switching on.
  • Soft-stop dampens the force of impact when the variable transformer brush gear reaches the end stop.
  • Full transverse and common mode spike / interference suppression when required.
  • IP classified and oil immersed (EMO) versions for outdoor use or for use in hostile environments.
  • Wide range of optional extras including input / output circuit breaker, by-pass switches, distribution boards and any other features to customer's requirements can be built into the cabinet.
  • Stabilised Output Voltage Can be preset to any value within these ranges:-
H Models
200 to 254 V single phase
346 to 440 V three phase
Other voltages to order.
L models
100 to 127 V single phase
173 to 220 V three phase
  • Output voltage accuracy: ±0.5% accuracy maintained over the full range of input voltage variations from zero to full load irrespective of power factor, frequency variations and ambient temperature changes from -5 deg C to +45 deg C. Three phase stabilisers will also maintain the accuracy of each phase, both line to line and line to neutral, even if the input voltage load and power factor are totally unbalanced. If a lower accuracy of ±5% is adequate for your equipment needs, then the upper and lower input voltage swing maybe extended by 5% beyond the figures quoted for the higher accuracy.
  • Input voltage range is shown as a percentage of the pre-set output voltage. The EM models offer a choice of 11 ranges (R8 to R40) and 3 taps A, B and C. While the EMS models are available with a single tap with equal under and over voltage correction. The taps can be altered at ay time but not the range.
  • Supply frequency 47 to 65Hz
  • Power factor has no effect on the performance
  • Waveform distortion negligible with any type of load
  • Speed of response Defined as the time taken to restore a 10% voltage variation to within 2%, these will vary 0.15 sec on smaller units to 1 sec for larger models.
  • Quality Management System. Assessed and approved to ISO 9001:2000
  • CE Conformity Conforms to the requirements of council directive 89/336/EEC EMC (Valid to Oct 2009) & 2004/108 EEC emissions / immunity and directive 2006/95/EC, (Formally 73.23.EEC) Low voltage directive.
  • Spike Suppression. Metal oxide transient voltage suppression is fitted as standard. Additional protection can be provided to attenuate spikes and noise down to harmless levels
  • Lightning Arrestors. Can be provided to protect the stabiliser and the load from the harmful effects of induced lightning strikes to the incoming supply distribution network.
  • Three Phase models are normally supplied for "star" connection and require an incoming neutral. If no neutral is available a static balancer can be fitted to provide an artificial neutral.
  • Environment the quoted current ratings apply to ambient temperatures from -5 Deg C to +45 Deg C. Derate by 2% per each additional degree C up to 70 Deg C. Temperature stability better than 0.03% per degree C. Maximum altitude 1000m. Derate by 2.5% for each additional 500m, EM stabilisers are suitable for indoor tropical use.
  • Surge rating ten times the rated current up to 2 seconds. Three times up to 1 minute. Twice up to 5 minutes
  • Audible noise level less than 40dB at 1m distance EM & EMS models 50dB for EMT & EMST models
  • Environmental Management Policy. Assessed and approved to ISO 14001:2004
  • Construction, the larger stabilisers are enclosed in robust floor standing cubicles, with door access for easy installation and servicing. The smaller single phase models are suitable for floor or bench mounting, Enclosures to IP20 BS60529
  • Oil Immersed Stabilisers (EMO). Oil Immersed stabilisers are enclosed in a welded sheet steel tank; all covers are lipped to prevent any ingress of moisture and are retained by stainless steel bolts. Supplied complete with lifting lugs, and pole clamp (if applicable), all finished in Dark Admiralty Grey (RAL 7031). The enclosures are fitted with the following: Oil sight gauge, free air breather tube, porcelain bushings and earth terminal.
  • Insulating Oil, an uninhibited transformer oil is used, conforming to IEC 296('82) class 1 & 2 and BS148('84) class 1 & 2 specification.