Voltage imbalance

Voltage imbalance: The invisible load on your motors and installation

In an ideal electrical installation, the three phases are perfectly balanced: the voltages are equal in amplitude and the phase angles are exactly 120 degrees to each other. Practice, however, is unruly. Voltage imbalance (or voltage asymmetry) is a common Power Quality phenomenon that often goes unnoticed until components fail.

Especially in industrial environments and data centres, imbalance causes unexplained wear and tear. A small voltage imbalance can lead to a disproportionate power imbalance in running machines, resulting in overheating and efficiency loss. Where engineers often first think of mechanical faults or overloading, the cause regularly lies in voltage quality. HyTEPS analyses the source of the imbalance and advises on the right mitigating measures to ensure your operational reliability.

For whom is this relevant?

This information is crucial for professionals responsible for the continuity and safety of heavy electrical installations:

• Installation managers (IV): Who have to deal with unexplained tripping of protection devices or hot cables.
• Maintenance Managers: who see electric motors failing faster than specifications predict.
• Engineers in industry: who work with many direct-online motors or variable speed drives.
• Facility Managers in Data Centres: Where a high density of single-phase servers can cause skewed loads.

Why is imbalance a risk to your operations?

Voltage imbalance is often underestimated because the installation usually keeps running. However, efficiency and safety decrease drastically. The consequences can be divided into three categories:

1. Thermal overload of motors and transformers

This is the most critical consequence. A small voltage imbalance of just 2% can lead to a current imbalance of as much as 15% to 20% in the windings of an asynchronous motor.

• Consequence: Engine temperatures rise exponentially.
• Impact: For every 10°C rise in temperature, the life of winding insulation halves. A motor running continuously at 3% imbalance will fail prematurely.
• Derating: To prevent this, you need to 'derate' the motor. This means that you may only load a 100kW motor with 85kW. So you are not using your capital efficiently.

2. Overloading of the neutral conductor (Neutral)

In a perfectly balanced grid, the vectorial sum of currents is zero; no current flows through the neutral conductor. In case of imbalance (caused by uneven load), an equalising current passes through the neutral.

• Risk: The neutral conductor is often not fused and sometimes has a smaller diameter than the phase wires. At high unbalance currents, the neutral conductor can overheat, causing a fire hazard.

3. Damage to power electronics

Equipment such as variable frequency drives (VFDs) and inverters have rectifier bridges at the input. With voltage unbalance, the diodes are loaded unevenly. One phase may have to supply most of the current, overloading these diodes and causing the drive to fail or break down.

What causes it?

The cause may be external (grid operator) or internal (your own installation).

1. Uneven distribution of single-phase loads (Most common): Many modern buildings and industries have heavy single-phase consumers (lighting, PCs, servers, small motors). If these are all connected to phase L1, the voltage at L1 drops, while L2 and L3 remain relatively high. This causes voltage imbalance at the main distributor.
2. Grid faults: A broken capacitor bank, a bad contact in a transformer or a fuse that is out in one phase leads directly to serious imbalance.
3. Asymmetrical impedance: Sometimes the cabling itself is the cause, for example in the case of non-twisted cables over long distances or busbar systems with uneven geometry.

Common errors of imbalance

Only measure average current: Many panel meters show an average. At 100A, 100A and 160A, the average of 120A may seem acceptable, but phase 3 is heavily overloaded. Always look at the values per phase.

Confusion with harmonics: Zero current is often directly attributed to harmonic contamination (3rd harmonic). However, imbalance is an equally, if not greater, cause of zero currents. A Power Quality analysis makes the distinction.

Ignoring derating: Running a motor 'on the edge' while there is imbalance is asking for trouble. The NEMA derating curve should be strictly followed.

Looking for cause outside the door: "It's down to the grid operator." This is often incorrect. Although the incoming voltage has an influence, the majority of imbalance problems are caused by the internal distribution of the installation.

Confuse voltage vs current: A small voltage imbalance (source) causes a large current imbalance (load). Don't just focus on the voltage; the current imbalance is what does the damage.

When is specialist help needed?

In complex situations, simply "switching groups" is not possible or sufficient. Engage HyTEPS' engineers when:

• You are dealing with failure of critical processes and the cause is unclear.
• You are planning a major expansion (e.g. heat pumps or EV chargers) and want to be sure that your current transformer can handle it in terms of balance.
• You doubt whether your measurements are interpreted correctly. HyTEPS uses high-end measurement equipment that also records transients and waveforms (Continuous Waveform Recording), allowing us to look deeper than standard meters.
• You are looking for a solution that addresses Power Quality in its entirety (i.e. both imbalance, harmonics and reactive power).