Motor rotor assembly (laminated core, cage or magnets, and shaft) bought as a component

01

Set the balance grade and work out the permissible unbalance

Miki Pulley's technical note, quoting JIS B 0153 and the JIS B 0905-1992 table of recommended grades, defines the balance grade as the product of the specific unbalance and the angular velocity, with ω = 2πn/60 = n/9.55, and the permissible residual unbalance as U_per = e_per × m in g·mm. To set it you need the maximum service speed, the rotor mass, the bearing positions and the balancing planes; the lower the grade number, the finer the balance. [1]

In that table, G6.3 covers medium- and large-sized armatures for motors with a shaft centre height of at least 80 mm without special requirements, and small mass-produced armatures in vibration-insensitive or isolated applications. G2.5 covers medium- and large-sized armatures with special requirements and small armatures not meeting the G6.3 or G1 conditions; G1 covers small electronic machines with special requirements, and G0.4 armatures in precision grinding machines. [1]

Editorial calculation: for a G2.5 rotor at a maximum speed of 3,000 min⁻¹, e_per = 2.5 × 9.55 ÷ 3,000 ≈ 0.0080 mm, or about 8.0 g·mm per kg; a 10 kg rotor may then carry about 80 g·mm of residual unbalance in total, to be split between the two balancing planes. At G6.3 the same rotor would be allowed 6.3 × 9.55 ÷ 3,000 × 10 ≈ 201 g·mm. [1]

Give the supplier the motor type and frame, maximum service speed, required balance grade and balancing planes, rotor mass, lamination grade and stack length, cage material or magnet grade, shaft drawing with bearing seats and tolerances, runout limits, and the test reports you need (balance record, conductivity or magnet test).

02

Choose the cage conductor: die-cast aluminium, treated aluminium or copper

Zeybek, Kayış and Diler (Materials, 2025) state that 15–25% of induction motor losses occur in the rotor and that over 90% of induction motors use squirrel-cage rotors, which consist of laminations, conductor bars, end rings and a shaft. In their work the cage was made by high-pressure die-casting into the stacked laminated core. [2]

Adding 0.05 wt% boron through an AlB8 master alloy to 99.7% aluminium and heat-treating the rotors at 550 °C for 10 h raised the cage aluminium's conductivity from 60.62% IACS to 63.1% IACS, and the efficiency of the motor from 90.35% to 91.53%. The authors give 100% IACS as 58 MS/m at 20 °C under ASTM E1004. [2]

The same authors report that copper cages, though more conductive, are harder and costlier to die-cast because of copper's higher melting point, thermal conductivity, density and unit cost; thermal shock can short bars to the laminations and cause inter-bar currents, and molten copper can form cuprous oxide or pick up impurities from the die and laminations, lowering conductivity. [2] Editorial calculation: 60.62% × 58 MS/m ≈ 35.2 MS/m and 63.1% × 58 MS/m ≈ 36.6 MS/m. Editorial note: ask for the cage alloy, its measured conductivity in % IACS and any post-casting heat treatment.

03

Check rotor length, shaft length and stiffness

An open-hardware redesign of a 2 kW poultry-house fan motor (HardwareX, 2026) started from a rotor of Ø100 mm, 190 mm axial length and 44 slots. The redesign cut rotor length from 190 mm to 110 mm (about 42%) and shaft length from 354 mm to 304 mm (about 14%), keeping the materials, to raise stiffness and critical speed and lower vibration, while the stator outer diameter grew from 160 to 190 mm. [3]

The authors report rear-section vibration reduced by up to 5.5 mm/s and full-load input power falling from 3,420 W to 2,632 W for the whole redesign, which also rewound the stator and replaced the housing and endcaps; the published files include rotor, shaft and 6206 ball-bearing models. [3] Editorial note: those gains came from the whole motor redesign, not the rotor alone; when you change rotor length, recheck the balance grade and permissible unbalance for the new mass. [1]

04

Aluminium cage, copper cage, permanent-magnet and wound rotors

05

Laminated core, shaft, bars and end rings, magnets, windings and balance corrections

06

Core diameter, stack length, rotor mass, maximum speed and balance grade for a rotor assembly

Rotor mass and maximum service speed both feed the permissible unbalance U_per = e_per × m, so give them with the balance grade. [1]

07

Send the motor, balance and material data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Motor: [type; frame; poles; rated power and speed]
Rotor: [diameter; stack length; slots; skew]
Shaft: [drawing; bearing seats; runout]
Conductor or magnets: [alloy and % IACS; or magnet grade]
Balance: [grade; maximum speed; planes]
Documents: [balance record; conductivity or magnet test]

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