What is electric motor rotor balancing?

The causes fall into a few groups.

• manufacturing — material inhomogeneity, machining tolerances, a shifted core stack, fan asymmetry;
• repair-related — rewinding a rotor, replacing a commutator, cage repair, weld overlay, metal spraying, fan replacement, shaft repair;
• operational — contamination, deposits, a lost balance weight, a broken fan, thermal distortion, wear.

Even a new rotor can have imbalance, since a perfectly even mass distribution is practically unattainable in manufacturing — that is why rotors in critical machines are balanced during production. What remains after balancing is called residual imbalance, and it should stay below the allowable level for that rotor.

With static imbalance, the center of mass is offset from the axis of rotation — if such a rotor is placed freely on very light rests, its heavy side will tend to sink. Static balancing is used mainly for narrow parts where axial length is small relative to diameter — fans, pulleys, thin disks.

In a long rotor, the overall center of mass can lie on the axis while two unbalanced masses at opposite ends, 180° apart, still create a moment — the rotor starts to "rock" as it turns. That is moment imbalance, which is why most motor rotors need dynamic balancing: performed while the rotor spins on a dedicated balancing machine, correcting mass distribution in two planes.

In practice, most rotors have both a static and a moment component at once — this is called combined, or dynamic, imbalance.

The rotor is mounted on the machine’s supports; as it turns, sensors measure vibration, force, phase and speed. The system determines how much correction is needed, in which plane, and at what angle. Phase reveals the angular position of the unbalanced mass relative to a reference mark — without a phase signal, you can know imbalance exists but struggle to pinpoint where correction is needed.

Balancing often uses a trial weight of known mass placed at a specific position so the system can determine the rotor’s sensitivity, after which the software calculates the correction needed. Knowing the correction radius precisely matters — the same mass at a different radius creates a different imbalance, since imbalance is tied to the product of mass and radius.

The main methods are removing material (drilling, milling, grinding), adding mass (balance weights, screws, plates), or moving existing weights on designs with special adjustable elements.

Correction can only be made in technologically approved zones — drilling at random can weaken a part, damage the core or winding, or disrupt ventilation. Balance weights must be securely fastened: at high speed they too carry a large centrifugal force, and one coming loose can destroy the motor or wreck the bearings.

• after rewinding a wound rotor or repairing a squirrel-cage;
• after replacing a commutator or repairing an armature;
• after replacing or repairing a shaft;
• after replacing a fan or repairing the core;
• after weld overlay, metal spraying, or machining that changes mass distribution;
• after repairing poles or replacing magnets;
• after an accident, transport, or long storage, if a shifted weight or distortion is suspected.

Balancing is done after a repair is finished, not before — any intervention (a new winding, a weld, weld overlay, machining) changes mass distribution, so balancing performed beforehand would be pointless.

Balancing does not fix a bent shaft and does not correct misalignment — those defects need to be resolved separately, before or instead of balancing. A bent shaft often produces a strong component at running speed (1× RPM) that resembles imbalance, so checking shaft run-out before balancing matters.

A mechanically unstable rotor should never be balanced — if the core is loose, the fan is loose, or the stack is not properly clamped, mass distribution keeps changing and the resulting balance will not hold. Balancing also does not eliminate resonance: if a structure’s natural frequency sits close to running speed, even a small excitation can produce significant vibration.

A rigid rotor does not change shape significantly from elastic deformation within its operating speed range, so a balance achieved on a machine at low speed stays valid in service. A flexible rotor — typical of long shafts and high-speed assemblies — can deform significantly while spinning, pass through critical speeds, and change its mode shape, so it may need specialized high-speed balancing with several correction planes.

Critical speed is the range where the excitation frequency approaches the rotor system’s natural frequency; in that zone, vibration can rise sharply and the rotor can deflect heavily.

On a balancing machine, the rotor is removed and balanced separately. Field balancing is done without disassembly, as part of the assembled unit, when the rotor is large, disassembly is impractical, or the imbalance appeared only after installation — vibration of the assembly is measured and trial weights are fitted right at the installation site.

An induction motor rotor’s balance is checked after cage repair, bar or end-ring replacement, or fan or shaft work. Balancing a wound rotor matters especially after rewinding — the new winding, bands, wedges and impregnating material can shift the mass of different zones slightly.

Balancing a DC machine armature covers the shaft, core, winding, commutator, bands and fan as a complete assembly — even if a new commutator has the same nominal mass, the distribution of copper, solder joints and the fit can differ, so the assembled unit needs balancing, not just the commutator alone.

Imbalance places a cyclic radial load on the bearings, speeding up raceway fatigue, wear, heating and grease breakdown — so correct balancing directly affects bearing life. Heavy rotor vibration can also change the actual air gap during rotation, and in severe cases cause the rotor to touch the stator.

In DC machines, armature vibration can worsen brush-to-commutator contact, causing brush bounce, sparking, uneven wear and damage to the working film — so good armature balancing directly affects commutation.

After correction, the rotor is run again on the balancing machine, and residual imbalance, phase and reading stability are determined — the actual result can differ from the calculated one due to inaccuracies in mass, angle or radius, so balancing is nearly always an iterative process.

Different rotor types call for different accuracy grades — there is no point balancing a large low-speed rotor to the same requirements as a high-speed spindle. Overly strict requirements can raise cost substantially without a real benefit to the machine; the goal is a residual imbalance low enough for safe, stable operation.

• balancing a bent shaft or a loose core;
• ignoring a bearing defect before balancing;
• correcting in a non-approved zone or too close to a critical area;
• a poorly secured balance weight;
• balancing an incomplete rotor, or in a non-final configuration;
• replacing the fan after balancing without re-checking;
• trying to fix misalignment with balancing, or masking resonance;
• no verification run and no report.

• Why does an electric motor vibrate heavily?
• How do I measure shaft run-out?
• How do I test the rotor?
• How do I test the bearings?
• What is vibration diagnostics?
• How do I align an electric motor?
• How do I test a motor after repair?

EPR (Elektropromremont) diagnoses and dynamically balances rotating assemblies of electric machines after repair and restoration.

Depending on the design, rotors of induction and synchronous motors, wound rotors, DC machine armatures, traction rotors, fans and other rotating assemblies can be balanced.

Before balancing, the following may be performed:

• shaft inspection and run-out measurement;
• checking fits and inspecting the core;
• checking the fan and the assembly’s mechanical stability;
• dynamic balancing in two planes with a verification run;
• a report covering initial and residual imbalance.