Q7Electrical Machine Design

The design of a wound rotor for an induction motor, following completion of the stator design, proceeds through these necessary steps.

Step 1 — Rotor slot number selection: choose the number of rotor slots S2 relative to the stator slot number S1 and pole number P, specifically avoiding unfavorable combinations known to produce cogging (magnetic locking at standstill, occurring notably when S1=S2) or synchronous crawling (parasitic locking at a sub-synchronous speed due to harmonic fields, as demonstrated quantitatively in the worked crawling-harmonic problem elsewhere in this paper, occurring when S2/P or related slot-number ratios equal small integers).

Step 2 — Rotor turns per phase from target slip-ring voltage: the number of rotor turns per phase is chosen based on the desired open-circuit slip-ring voltage (a value limited for safety/insulation reasons, and also affecting the sizing of any external starting resistance and its control switchgear), using the transformer-like turns-ratio relationship between stator and rotor windings (as demonstrated quantitatively in the worked stator/rotor turns-ratio problem elsewhere in this paper).

Step 3 — Rotor conductor cross-section: the rotor full-load current per phase is estimated from the stator current referred through the effective turns ratio, and the conductor cross-sectional area is obtained by dividing this current by the chosen rotor winding current density.

Step 4 — Coil span and winding layout: the rotor coil span/pitch and winding arrangement (single or double layer) are chosen based on the selected number of rotor slots, generally aiming for a similar winding distribution quality (in terms of winding factor and harmonic suppression) as achieved in the stator design.

Step 5 — Slip ring and brush gear design: the slip ring dimensions (diameter, axial length) are sized for adequate current-carrying capacity (based on rotor current) and sufficient insulation clearance (based on rotor voltage), and appropriate brush material, size, and current density are selected to reliably conduct rotor current between the rotating slip rings and the external stationary rotor circuit (starting resistors, control gear).

Step 6 — Verification against performance requirements: the resulting rotor leakage reactance, starting torque/current characteristics (with external rotor resistance as applicable), and thermal performance are checked against the required specifications, with the design iterated (adjusting rotor turns, slot dimensions, or conductor sizing as needed) until satisfactory overall motor performance is confirmed.