Unbalance happens when a rotor's mass is not evenly spread around its axis. As speed rises, even a small unbalance creates vibration, noise and bearing wear.
Static and dynamic unbalance
Static unbalance can be seen with the rotor at rest. Dynamic unbalance only shows when the rotor spins, because it comes from mass offsets in two different planes. Longer rotors usually need two-plane balancing.
Balance quality grades
ISO 21940-11 defines balance quality grades such as G6.3 and G2.5. The grade and the rotor's maximum speed together set how much residual unbalance is allowed. Small electric motor rotors are commonly specified at G6.3 or tighter.
Correction
Mass is removed by drilling or milling, or added with weights or balancing compound. Automatic balancing machines measure and correct in one cycle.
See our rotor dynamic balancing machine.