Q6Electrical Machine Design

The design of a wound rotor for an induction motor proceeds through the following necessary steps, following the stator design (which must be completed first since several rotor parameters are derived relative to the stator design).

Step 1 — Selection of rotor slot number: choose the number of rotor slots S2 carefully relative to the stator slot number S1 and pole number P, specifically avoiding combinations known to cause cogging (S1=S2) or synchronous crawling (S2/P, or related differences/sums, equal to small integers such as 1, 6, 7, etc., as illustrated in the crawling-harmonic problem discussed elsewhere in this paper), while also ensuring an integral or suitable fractional slots-per-pole-per-phase for good winding distribution.

Step 2 — Determination of rotor turns per phase: the rotor turns per phase are determined from the desired open-circuit slip-ring voltage (a design/specification choice, often limited to a safe value for slip-ring and starting-resistor insulation, typically not exceeding a few hundred to about 1000V for standard designs) using the transformer-like EMF ratio relationship between the stator (primary) and rotor (secondary, at standstill) windings:

Step 3 — Determination of rotor conductor cross-section: the rotor full-load current per phase is estimated (approximately related to the stator current by the inverse of the turns ratio, adjusted for the different phase relationships of a wound rotor referred back through the effective transformation ratio), and the conductor cross-sectional area is then obtained by dividing this current by the chosen current density for the rotor winding.

Step 4 — Determination of coil span and winding arrangement: the coil span (pitch) is chosen, often equal to or close to the rotor slot pitch corresponding to the pole pitch, and the winding layout (single or double layer) is determined based on the number of rotor slots and required winding distribution.

Step 5 — Design of slip rings and brush gear: the slip ring diameter and axial length are determined based on the rotor current (for adequate current-carrying capacity and heat dissipation) and the required insulation/clearance for the rotor voltage, and appropriate brush grade, size, and current density are selected to safely conduct the rotor current from the rotating slip rings to the external stationary starting-resistor/control circuit.

Step 6 — Verification of rotor leakage reactance and starting performance: the rotor leakage reactance is estimated from the slot and end-winding geometry, and the overall starting torque and current characteristics (with external rotor resistance inserted, as is standard practice for wound-rotor motors to improve starting torque and limit starting current) are verified against the required specification, iterating the design (adjusting rotor turns, slot dimensions, or conductor size) if necessary until satisfactory performance is achieved.