Voltage Regulation & Power Quality
What voltage fluctuation does to equipment
How undervoltage, overvoltage, dips and phase imbalance affect motors, electronics, lighting and heating loads, and what to measure before choosing a correction method.
Electrical equipment is designed for a nominal supply voltage and a tolerance around it. Inside that tolerance it performs as rated. Outside it, the effects range from slightly reduced output to overheating, nuisance tripping and shortened life. The effect depends on the type of load, how far the voltage moves and for how long.
This article describes the common forms of voltage variation, what each does to typical loads, and how to record the supply before deciding on correction.
Forms of voltage variation
- Sustained undervoltage: the voltage stays below nominal for minutes or hours, often at the end of a long feeder at peak demand.
- Sustained overvoltage: the voltage stays above nominal, often at night when local demand is low.
- Dips and swells: short reductions or increases lasting from a fraction of a cycle to a few seconds, often caused by large motors starting or faults elsewhere on the network.
- Phase imbalance: the three phase voltages differ from each other, usually because single-phase loads are unevenly shared.
- Transients: very short spikes from switching or lightning. These are a separate problem from voltage regulation and need surge protection.
Motors and compressors
An induction motor's torque varies roughly with the square of the voltage. At 90% voltage, available torque falls to about 81% of its rated value. A motor driving a fixed load then draws more current to keep turning, its windings run hotter, and its insulation ages faster. A compressor may fail to start against pressure and trip on overload.
Overvoltage is not harmless either. The motor's magnetising current rises, the iron runs closer to saturation and losses increase. Sustained overvoltage also heats the motor.
Phase imbalance is particularly damaging to three-phase motors. A small voltage imbalance produces a much larger current imbalance, and the extra heating concentrates in one winding. Many motor standards call for derating when voltage unbalance exceeds about 1%.
Electronics and controls
Switched-mode power supplies in computers, controllers and drives accept a wide input range and regulate their own output, so moderate sustained variation is often tolerated. The weaker points are elsewhere:
- Undervoltage protection in drives and controllers trips the process during a deep dip.
- Contactors and relays may drop out when their coil voltage falls, stopping equipment even though the electronics would have ridden through.
- Sustained overvoltage stresses input capacitors and surge-protection components.
Lighting
Lamp output and life are sensitive to voltage. Older lamp types change brightness noticeably with supply voltage. LED luminaires depend on their driver: a well-specified driver holds the LED current steady across its input range, but a driver at the edge of that range may flicker or shut down.
Heating and resistive loads
For a resistive heater, power varies with the square of the voltage. At 90% voltage, output falls to about 81%, so process heating takes longer. At 110%, output rises to about 121% and elements run hotter than designed.
| Parameter | Value | Condition |
|---|---|---|
| Induction motor torque | About −19% at 90% voltage | Torque varies roughly with voltage squared |
| Resistive heater output | About −19% at 90%; about +21% at 110% | Power varies with voltage squared |
| Motor current at fixed load | Rises at undervoltage | Extra heating in the windings |
| Electronic equipment | Often tolerated within its input range | Trips at deep dips or protection limits |
Measure before choosing a correction
The right response depends on which form of variation is present. A stabilizer corrects sustained under- and overvoltage and, with independent phase control, phase imbalance. It does not remove harmonics, suppress transients by itself or bridge an interruption. Before selecting equipment, record:
- Voltage on each phase over at least a week, including peak and off-peak hours.
- The minimum and maximum, and how quickly the voltage moves between them.
- Dips associated with large loads starting on site or nearby.
- The equipment that is tripping, failing or underperforming, and when.
A recording instrument that logs each phase gives a far better basis than a spot reading. With that data, the choice between servo correction, static correction or a different measure altogether can be made on evidence.
Practical signs of a supply problem
Repeated motor overload trips at the same time of day, compressors that struggle to start, lighting that changes brightness and controllers that reset during the evening peak are all signs worth recording. They point to a supply problem only when the voltage record confirms it; the same symptoms can come from undersized cabling or a faulty connection.
Where to go next
- Servo and static voltage stabilizers: where each fits
- Servo Voltage Stabilizers
- Voltage Stabilizers & Voltage Controllers
- Discuss your requirement: share the voltage record and the affected equipment.
Let’s find the right equipment.
Tell us what you’re working on. We’ll start with your application, load and installation needs.
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