CP Automation: A practical approach to power quality and efficiency

Voltage optimisation (VO) is often promoted as a simple way to cut energy use and costs. While matching supply voltage to equipment needs is effective in principle, real-world savings are more complex. VO is just one element of a broader power quality strategy, and understanding its limits and engineering implications is key to achieving genuine efficiency. John Mitchell (JM), global sales and marketing director at CP Automation, discusses this topic with independent power management consultant Steve Young, MIET (SY).

JM: How would you summarise VO?

SY: The terminology around voltage reduction, such as “optimisation”, “stabilisation”, “regulation” and “reduction”, is often used interchangeably. However, ‘voltage management’ is the most accurate description of the technologies and methods involved. These range from transformer tap adjustments to more sophisticated on-load voltage regulation systems designed to maintain supply within an optimum window.

The fundamental principle is simple, many electrical loads are designed to operate most efficiently at or near their nominal voltage. Where the supply consistently exceeds this level, for example, where voltages are closer to the UK statutory upper limit of 253V (or around 440V for three-phase), carefully reducing voltage can lead to lower energy consumption, reduced stress on equipment and extended asset life.

However, the extent of those benefits depends heavily on the type of loads present, the site’s electrical topology and the broader power quality environment.

JM: This brings us neatly to load types. How important is the difference between voltage-dependent and voltage-independent loads?

SY: A key factor in evaluating the potential for voltage management is distinguishing between voltage-dependent and voltage-independent loads.

Voltage-dependent loads, such as incandescent lighting, discharge lighting with magnetic ballasts and some direct on-line induction motors, draw power in proportion to the supply voltage. Reducing voltage here can indeed deliver measurable energy savings, often accompanied by extended equipment life due to lower operating stress.

Meanwhile, voltage-independent loads, including IT and electronic equipment, high-frequency lighting with electronic ballasts, resistive heating with thermostatic control and motors driven by variable speed drives (VSDs) will draw roughly the same power regardless of supply voltage. Some loads may draw higher current at lower voltage to maintain power output, negating any real energy savings.

Understanding the proportion of each type of load is essential when predicting the real-world impact of a voltage management strategy. In many industrial and commercial sites, only a fraction of the total load is genuinely voltage-dependent.

JM: Does that mean there’s no one-size-fits-all solution?

SY: Exactly. Matching the supply voltage more closely to the optimum design level can have a positive impact on equipment performance and longevity. Overvoltage can saturate motor cores, which increases iron and copper losses, generating unnecessary heat and reduced lifespan.

In lighting systems, overvoltage shortens lamp life and increases energy consumption. For example, a 230V-rated linear lighting load operating at 240V, will consume nearly 9% more energy.

However, under-voltage must also be approached with caution. Operating induction motors below their design voltage increases slip and reduces torque, potentially leading to reduced process throughput, overheating or premature failure.

Similarly, reducing voltage to thermostatically controlled heating elements can cause them to run longer, resulting in no net energy saving and, in some cases, increased standing losses.

JM: How does VO apply to motor loads in particular?

SY: Three-phase induction motors, the workhorses of most industrial processes, present a complex picture. Efficiency is typically highest at around 75% load when supplied at design voltage.

Lightly loaded (<50%), non-VSD motors can see modest efficiency gains from voltage reduction, albeit, this will be a fraction of the energy reduction seen by voltage dependent lighting. Heavily loaded motors may experience performance issues if the voltage is lowered too far.

Motors with VSDs behave differently again and generally do not exhibit significant energy savings at a lower applied voltage. They will generally maintain power output regardless of supply voltage, however, reduced DC bus voltage and increased current draw can make them more susceptible to power dips and nuisance tripping. Any voltage management plan must therefore consider the motor population in detail, including control methods, duty cycles and load profiles.

JM: What about modern electronic loads?

SY: Generally, this kind of equipment can be classed as voltage independent. Switched-mode power supplies, common in modern hardware, IT and automation are designed to maintain constant power draw, though increased current can increase protective device loading and nuisance tripping.

JM: In your experience, how common is a genuine requirement for additional VO hardware

SY: Introducing additional hardware, such as series transformers, inevitably introduces no-load and load losses, which must be factored into any cost-benefit analysis.

In my experience, the most effective and viable way of adjusting the applied voltage is getting your HVSAP to adjust the off-load tap changer on your existing distribution transformer. If it is already on tap 1, a viability assessment should be undertaken to consider the benefits of upgrading the existing transformer for a low loss unit with an extended LV range. So, to answer your question, it would be very uncommon.

JM: How do you calculate the approximate overall gains that could be expected from a controlled reduction in the applied voltage?

SY: Understanding how different items of load bearing equipment react to incremental voltage changes is the first step. The next step is to ascertain the approximate amount of energy consumed by these load types. We use the concept of Effective Voltage Dependency (EVD) to calculate the net benefit.

EVD can be summarised as the ratio of actual power reduction when compared to the theoretical reduction predicted by the (V²/R) relationship. For example, an installation dominated by voltage-dependent lighting might approach an EVD closer to one, while a modern application with a high proportion of VSDs and electronic loads could see an EVD closer to zero.

JM: That sounds like a lot to consider. So, how should businesses approach VO?

SY: Ideally, Voltage Management shouldn’t be treated as a standalone solution. Instead, it should form part of a holistic power quality and energy efficiency strategy.

The process begins with a thorough site survey, load profiling and power quality analysis to understand how the applied voltage should be considered in conjunction with other key electrical characteristics. In many cases, addressing underlying issues such as harmonic distortion, poor power factor or transformer inefficiency can deliver greater gains than voltage reduction alone.

Where voltage management is viable, its deployment should be carefully considered, ensuring that equipment operates within design parameters, without compromising the reliability of any connected loads.

JM: What’s the main takeaway for when using VO?

SY: Voltage Management should be considered as part of an overarching strategy, with a full understanding of its limitations and associated risks. Incorrect application can increase the risk of under-voltage conditions.

By integrating voltage management into a broader power quality strategy, businesses can move beyond simplistic and often erroneous savings claims and achieve genuine, long-term improvements in efficiency, reliability and asset performance.

To find out more about improving industrial power quality in different applications, visit the CP Automation website.

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