Forstå elektrisk ubalanse
Elektrisk ubalanse — also called phase unbalance, phase imbalance, voltage unbalance eller current unbalance — is a condition in a three-phase system where the voltages or currents in the three phases are unequal in magnitude, or are not separated by exactly 120 electrical degrees. This asymmetry, whether it originates in the supply or inside the motor windings, produces unbalanced electromagnetic forces, excessive winding heating, negative-sequence currents, torque pulsations, and a characteristic vibrasjon at twice the linjefrekvens.
What makes electrical unbalance deceptive is the leverage involved: even a small voltage imbalance of 2–3% can drive a current imbalance six to ten times larger, quietly eroding motor efficiency, heat margin and insulation life. It is one of the most common — and most overlooked — problems in industrial facilities, arising from utility supply issues, poor in-plant distribution, or defects inside the motor itself. Because its vibration signature shares frequencies with several genuinely mechanical faults, it is also one of the most frequently misdiagnosed conditions a maintenance team encounters.
1. What Is Phase Imbalance? Voltage, Current and Phase-Angle Unbalance
“Phase imbalance” is the everyday shop-floor name for the same condition, and it shows up in three distinct but linked forms. Knowing which one you are measuring matters: voltage unbalance is the cause the supply imposes on the motor, while current unbalance is the amplified effekt the motor suffers in response.
Spenningsubalanse
Voltage unbalance is the inequality of the three line-to-line (or line-to-neutral) voltages. It is measured by reading the voltage between each phase pair — AB, BC and CA — and is expressed as a percentage using the NEMA definition: % voltage unbalance = (maximum deviation from the average ÷ the average) × 100. As a worked example, phases of 477 V, 480 V and 483 V average 480 V; the maximum deviation is 3 V, giving 0.625% unbalance. NEMA MG-1 regards anything below 1% as acceptable, while IEC practice tolerates up to about 2%. Voltage unbalance is the parameter to trend first, because it is the upstream driver of almost everything that follows.
Strømubalanse
Current unbalance is the inequality of the three phase currents (IA, IB, IC), measured with a clamp meter and calculated with the same maximum-deviation formula. The headline fact about current unbalance is its sensitivity: because the motor’s negative-sequence impedance is low, a modest voltage unbalance is magnified into a current unbalance roughly six to ten times larger. A barely noticeable 1% voltage unbalance can therefore appear as a 6–10% current unbalance — which is precisely why current is the more sensitive early-warning measurement, and why a rising current imbalance on an otherwise stable supply points to a developing fault inside the motor.
Fasevinkelubalanse
The third form is angular: the three phasors are no longer separated by exactly 120°, even if their magnitudes are equal. This is less common than magnitude unbalance and cannot be seen with a simple voltmeter — it requires a power-quality analyser that resolves the phasor relationships. Angular unbalance produces the same pulsating torque and extra heating as magnitude unbalance, and the two often occur together.
2. How Electrical Unbalance Produces Vibration in Motors
The link between an electrical asymmetry and a mechanical vibration runs through the air-gap magnetic field. In a balanced machine the rotating field is smooth and the radial magnetic forces sum to a steady, symmetric pull. Unbalance breaks that symmetry and introduces a negative-sequence component — a field rotating backwards relative to the main field — which beats against it and modulates the magnetic force.
The dominant result is a vibration at dobbel linjefrekvens: 100 Hz on a 50 Hz supply, or 120 Hz on a 60 Hz supply. This 2× line component is purely electromagnetic in origin — it is the pulsating attractive force across the luftspalte, not a mechanical force from the rotating mass. Its amplitude scales with the degree of unbalance, so a worsening supply or a developing winding fault shows up as a steadily rising 100/120 Hz peak in the spektrum.
A second, subtler signature appears at 1× løpehastighet, modulated by the slip-pole-pass frequency (the number of poles multiplied by the slip frequency). This pole-pass modulation creates sidebands around the running-speed peak and is the classic fingerprint of rotor-related electrical problems such as ødelagte rotorstenger. Reading these sidebands correctly is what lets an analyst separate a supply-side unbalance from a fault baked into the rotor.
3. Distinguishing Electrical from Mechanical Unbalance
Because the electromagnetic 2× line-frequency component sits very close to twice running speed on a two-pole motor, it is routinely confused with mechanical faults such as feiljustering or looseness, which also generate 2× shaft-speed energy. Telling them apart is the single most useful diagnostic skill for motor vibration, and there are two reliable tests.
The first is frequency precision. An electrical component is locked to the mains at nøyaktig 100 Hz or 120 Hz, whereas a mechanical 2× sits at twice the actual running speed — which, because of induction-motor slip, is always slightly below twice synchronous speed. With enough spectral resolution the peaks separate: a line-locked peak that does not move with load is electrical; a peak that tracks shaft speed is mechanical.
The second — and most decisive — is the power-off test. Watch the suspect peak in real time and cut power to the motor. A true electrical component vanishes instantly at switch-off, because the magnetic forcing disappears the moment the current stops, whereas a mechanical component decays gradually as the rotor coasts down. This instantaneous-disappearance test is the classic, unambiguous way to confirm an electrical origin, and it needs nothing more than a live spectrum display and the stop button.
4. Causes of Electrical Unbalance
The sources of unbalance fall naturally into three layers, moving from the grid inward to the machine.
Problemer med strømforsyning
At the supply, unbalance commonly comes from unbalanced distribution transformers, large single-phase loads connected to one phase of a three-phase service, unequal impedance among long transmission lines, or wider utility fault conditions. These produce a voltage unbalance that is present before the power even enters the building, and they are diagnosed by measuring at the service entrance.
Fasilitetsdistribusjon
Inside the plant, the usual culprits are a single high-resistance connection in one phase, a blown fuse that partially loses a phase, unequal cable lengths giving the conductors different impedances, or — at the extreme — single-phasing, the complete loss of one phase. A loose or corroded terminal is the most frequent and the most easily fixed of these, and it often presents as an unbalance that worsens under load as the joint heats.
Årsaker knyttet til motoren
When the supply is verified balanced but the current is not, the fault is inside the motor. Turn-to-turn shorts reduce the effective turns in one phase; manufacturing variation can leave the winding resistances slightly unequal; terminal connections degrade; and partial short or open circuits in a damaged winding create severe asymmetry — all overlapping with broader feil i statorviklingen. Air-gap eccentricity — a rotor not centred in the bore — is a related electromagnetic cause that produces its own unbalanced magnetic pull and frequently accompanies winding problems.
5. Effects on Motor Performance
Overoppheting
Overheating is the most serious consequence and the mechanism through which unbalance kills motors. The asymmetry sets up negative-sequence currents that dissipate extra heat, while one phase ends up carrying far more current than it was designed for. The temperature rise is disproportionate to the cause: a rule of thumb holds that a 3% voltage unbalance can produce an 18–25% rise in winding temperature. Since insulation life roughly halves for every 10 °C of additional temperature, the result is rapid insulation ageing and premature failure — a 3% voltage unbalance can cut motor life by as much as half.
Efficiency, Power Factor and Energy Cost
Unbalance lowers efficiency through circulating and negative-sequence currents that do no useful work, reduces the power factor, and raises overall energy consumption — a typical moderate unbalance costs 1–2% in efficiency. The extra draw is easy to underestimate over a year of continuous running; the Trefasemotoreffektkalkulator helps quantify the additional input power the unbalance is wasting.
Torque Pulsations and Vibration
Electrically, the negative-sequence field produces a pulsating torque at twice line frequency that drives torsjonsvibrasjon in the drive train and can excite torsional resonanser. Radially, the same forcing appears as the 100/120 Hz vibration described above, whose amplitude is proportional to the degree of unbalance and which is easily mistaken for stator faults or magnetic pull because they all live at the same electrical frequencies.
Reduced Service Life and Derating
Taken together, the thermal stress shortens insulation life and forces the motor to be operated below its nameplate rating. NEMA addresses this directly with a derating curve: above 1% voltage unbalance the motor’s usable capacity must be reduced, and at 5% unbalance the derating factor falls to roughly 0.75 — meaning a quarter of the motor’s rated output is sacrificed simply to keep it within thermal limits.
6. NEMA and IEC Limits for Voltage and Current Unbalance
Two standards frame the acceptable limits, and they use slightly different definitions, so it is worth being precise about which one a measurement follows.
NEMA MG-1 defines voltage unbalance as the maximum deviation from the average divided by the average (the formula used throughout this article) and recommends operating motors on supplies with no more than 1% voltage unbalance. Above that, NEMA requires the motor to be derated along its published curve; it explicitly advises mot running a motor where the voltage unbalance exceeds 5%.
IEC uses the symmetrical-component definition — the ratio of the negative-sequence voltage to the positive-sequence voltage — and generally tolerates up to about 2% in continuous operation. For the small unbalances seen in practice the two definitions give similar numbers, but for reporting and acceptance testing it matters which one is quoted.
For current, there is no single universal limit, but a widely used field guideline is to keep current unbalance below about 10%, investigate above that, and treat anything beyond it as a developing fault. Because of the six-to-ten-times amplification, holding voltage unbalance under the NEMA 1% target is the most effective way to keep current unbalance inside this band. The Motorens navneskiltstrømskalkulator gives the expected full-load current to compare each phase against.
7. Detection and Measurement
Voltage and Current Readings
Begin with the electrical measurements, taken with the motor running under its normal load. Read the three line-to-line voltages at the motor terminals — not the supply panel — so that the voltage drop along the feeders is captured, then compute the average and the percent deviation. Follow with a clamp-meter reading of each phase current, compare against the expected navneskiltets fullaststrøm, and calculate the current unbalance. Documenting and trending both values over time is what converts a one-off reading into an early-warning indicator.
Vibrasjonsanalyse
Vibration measurement confirms whether the electrical unbalance is actually reaching the structure and at what severity. Capture the spektrum at the motor frame and look for an elevated peak at exactly 100 Hz or 120 Hz, compare its amplitude against the machine’s baseline, and use the frequency-precision and power-off tests of Section 3 to separate it from a mechanical 2× caused by misalignment. A two-channel vibrasjonsanalysator with fine spectral resolution is the right tool, because separating a 100 Hz line peak from a 98–99 Hz mechanical peak demands resolution a simple overall-level meter cannot provide.
Termisk overvåking
Finally, measure winding or frame temperatures and look for a temperature imbalance between phases or an overall temperature higher than the load warrants. Because heat is the mechanism through which unbalance does its damage, a thermal anomaly often appears in step with — or even ahead of — the electrical symptoms.
8. Diagnosis with a Vibration Analyser
In the field, the electrical signature of unbalance is defined by its precise, mains-locked frequency, and resolving it cleanly is the work of a portable analyser. A two-channel instrument such as the Balanset-1A measures vibration at the motor frame and shows whether the dominant peak lands on the line-locked 100 Hz or 120 Hz — pointing to an electrical cause — or on 2× running speed, which would point to misalignment instead. The decisive confirmation remains the power-off test: with the live spectrum on screen, cut power and watch the suspect peak disappear instantly if it is electrical, or coast down with the rotor if it is mechanical. The Kalkulator for motorens elektriske defektfrekvens lists the exact line-related frequencies — 2× line, pole-pass sidebands and slip-related components — to search for, turning a confusing low-frequency spectrum into a checklist.
9. Correction, Prevention and Monitoring
Correcting Supply-Side Unbalance
When the unbalance is present at the service entrance, contact the utility; otherwise the fault is in the building. Check and tighten every connection in the distribution system, verify that fuses and breakers are intact, redistribute single-phase loads evenly across the three phases, and check the transformer tap settings. A surprising share of in-plant unbalance is nothing more than one loose or oxidised terminal carrying a higher resistance than its neighbours.
Correcting Motor-Side Issues
If the supply is verified balanced but the current is not, clean and tighten the motor terminal and cable connections first, then test for winding faults using insulation-resistance and current-signature analysis. A confirmed internal winding fault means rewinding or replacing the motor — there is no field repair for a turn-to-turn short.
Derating, Installation and Ongoing Monitoring
Where the unbalance cannot be eliminated, follow the NEMA derating curve and reduce the load to protect the windings, monitoring temperature closely. Prevent recurrence at installation by verifying voltage balance at the motor terminals before energising, sizing conductors to minimise voltage drop, and confirming the correct wye-versus-delta connection. In service, take periodic voltage and current readings, fold them into a wider tilstandsovervåking routine with trendanalyse, watch for blown fuses or tripped breakers, and run a power-quality survey wherever motor problems recur. Treating unbalance as a parameter to be trended — rather than a fault to be chased after failure — is what keeps it from quietly shortening the life of an entire motor population.
10. Ofte stilte spørsmål
What is the difference between voltage unbalance and current unbalance?
Voltage unbalance is the inequality of the three supply voltages and is usually the cause; current unbalance is the inequality of the three phase currents and is the amplified effect. Because the motor’s negative-sequence impedance is low, a small voltage unbalance produces a current unbalance six to ten times larger, which is why current is the more sensitive early-warning measurement.
At what frequency does electrical unbalance show up in vibration?
At twice the line frequency — 100 Hz on a 50 Hz supply or 120 Hz on a 60 Hz supply — because the negative-sequence field modulates the air-gap magnetic force at that rate. Rotor-related electrical faults add sidebands around 1× running speed at the slip-pole-pass frequency.
How do I tell electrical unbalance apart from mechanical unbalance or misalignment?
Use the power-off test: cut power to the running motor while watching the spectrum. A true electrical component vanishes instantly, while a mechanical one decays as the rotor coasts down. A line-locked peak at exactly 100/120 Hz that does not move with load is also a reliable electrical indicator.
What level of voltage unbalance is acceptable?
NEMA MG-1 recommends keeping voltage unbalance below 1% and requires derating above that, advising against operation beyond 5%. IEC, using a symmetrical-component definition, tolerates up to about 2%. Holding voltage unbalance under 1% is the most effective way to keep current unbalance within the commonly used 10% field limit.
Why does a small voltage unbalance cause so much heating?
The asymmetry creates negative-sequence currents that flow against the low negative-sequence impedance of the motor, dissipating extra heat, while one phase is overloaded. A 3% voltage unbalance can raise winding temperature by 18–25% and roughly halve insulation life.
Can a portable vibration analyser detect electrical unbalance?
Yes. A two-channel analyser such as the Balanset-1A resolves the 100/120 Hz line-locked peak, lets you run the power-off test, and reads the pole-pass sidebands that distinguish a supply-side unbalance from a rotor fault — all without a separate power-quality instrument.
