Voltage reduction can save energy


To my knowledge such techniques are only applied to single-phase loads, but of course the voltage reduction devices are made up in 3-phase design because they can be economic only where greater numbers of these (smallish) loads are being operated. Then, as a rule, they are (more or less) evenly distributed across the 3 phases.

But it remains to be considered that different loads react in totally different manners to the application of this technique:

  • Incandescent lamps: Yes, there is a savings potential, although the drop of power is somewhat less than by the square of the voltage, as you would expect from an ohmic load, because the resistance varies with temperature. In a cold state an incandescent lamp has only ≈10% of the resistance it has at normal operation. But: The efficiency drops drastically with voltage! Although the lifetime expectancy increases by about the same factor, the application is not recommended because under normal conditions the efficiency of an incandescent lamp is already very poor.
  • Other ohmic loads: What could these be? Only heaters, such as cookers, ovens and so on. These are usually thermostatically controlled, so a reduction in input power by reducing the voltage will result in a shift of the on/off ratio of the thermostatic switch if it is of the plain and simple bi-metallic type. Electronic control will have the same result. The heat-up time becomes longer, and the average power intake across the whole heat-up and subsequent keep-warm period remains the same, i. e. only disadvantages.
  • Fluorescent lamps with magnetic ballasts: This is the only application where the voltage reduction is very efficient and highly recommendable. The losses in the ballast drop by the square of the current, while the lamp efficiency slightly increases with reduced current. At the same time the voltage drop across the lamp increases when the current decreases, so the lamp power drops under-proportionally while the ballast power (loss) decreases by the square of the current. This yields a triple effect in favour of better overall luminaire efficiency.
  • Fluorescent lamps with electronic ballasts: Here there is usually no visual or palpable effect because electronic ballasts are commonly made up in a way as to offset any input voltage variances. This is in principle an advantage but excludes them from any opportunity to influence their operating behaviour by varying the line voltage. Rather, since the output power is kept stable, the input current will increase by the same factor as the voltage is reduced. Load on the mains becomes heavier, losses rise, and the number of ballasts that can be fed from one circuit drops.
  • Compact fluorescent lamps: These are basically small fluorescent lamps with integrated electronic ballasts, but the ballasts are of a plainer and cheaper design. Mostly their electronic control is merely designed to come from a square to a linear voltage dependence of the input power, not right through to a stable input power.
  • Other electronic equipment (PCs etc): You could as well group electronic ballasts in here. Like them, a PC or any other electronic circuit requires a certain power to fulfil its function, and in order to supply the required power to the electronic circuitry the usually applied switch-mode power supplies will draw a higher current from the mains when the voltage is lower. In fact there are already many switch-mode power supplies around which are designed for universal use at all line voltages around the world (from 90 V to 264 V, including the permitted tolerance levels of ±10%). The current ratings usually reflect that the power has to remain constant. No point in voltage reduction, only disadvantages, apart from a slight mitigation of relative harmonic currents with lower voltage. But since TRMS current is higher, the absolute harmonic currents tend to be higher rather than lower.
  • Induction motors: Don’t do it! As with electronic devices, the power demand at the shaft is practically the same at lower or higher voltage, since the rotational speed is mainly determined by the frequency, hardly by the voltage. So current will increase. Remember that the starting torque is proportional to the square of the voltage. Normally the losses in an induction motor will increase with both over- or undervoltage. Now the motor and the driven system must be designed in a way so that, when the voltage is within the tolerance levels, the system works properly. Yet, when you decide to reduce the line voltage only within the tolerance range, it remains to be considered that the start-up current of an induction motor is about 7 times the rated current and that the voltage reduction gear introduces additional impedance into the circuit! So the voltage may very well drop far below the lower tolerance limit at the moment when the motor is switched on, the motor may stall, the (potentially vital) driven system will fail to start, and the current will persist in the start-up current range! Therefore not just no point in reduction, but even dangerous!


The voltage reduction technique is a very good means of making fluorescent lamps with magnetic ballasts more efficient. In fact at 207 V a 58 W lamp with a class B1 ballast is already more efficient than with an electronic class A3 ballast! Similar effects may eventually be expected in street lighting (high pressure discharge lamps).

With incandescent lamps a certain energy saving is possible if a substantial loss of light output is accepted. Lamp life can be substantially extended.

With nearly all other applications the effect is either futile or adverse, may even become dangerous! So the voltage reduction technique should only be applied where dedicated lines for the lighting are provided and the type of ballasts used is known.

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