Induction machines
The world's workhorse motor — 90% of installed motor base. Stator's rotating field, induced rotor currents, slip-driven torque.
Lessons
How an induction motor turns
Stator's rotating field → induced rotor EMFs → bar currents → F = i × B → torque. Slip is what makes it work.
Slip and rotor frequency
s, f_r = s·f, E_2s = s·E_2 — three numbers that move together across every induction-machine operating region.
Induction motor equivalent circuit
Treat the rotor as a transformer secondary; the R_2/s trick reveals mechanical power as a fictitious resistor.
Torque-slip characteristic
The T-s curve, breakdown torque at s_m = R₂/X₂, and why T_max is independent of R₂ (rotor-resistance starting).
No-load + blocked-rotor tests
Two bench tests — the direct parallel of transformer OC/SC — that pin down every equivalent-circuit parameter.
Starting methods compared
DOL, star-delta, autotransformer, rotor resistance, soft starter, VFD — six ways to tame the inrush.
V/f speed control (VFD principle)
Keep V/f constant → keep flux constant → full torque at any speed. The trick that turned the IM into a variable-speed drive.
Circle diagram construction
One geometric figure built from NL + BR tests gives stator current, PF, torque, power, and efficiency at every operating point.
Induction generator (s < 0)
Drive the rotor above n_s and the same machine generates power. Grid-connected, SEIG, and the DFIG for variable-speed wind.
Double-cage and deep-bar induction motors
Breaking the single-cage R_2 trade-off: outer (high R, low X) and inner (low R, high X) cages give high starting torque AND high run efficiency. Deep-bar via skin effect. NEMA Designs A/B/C/D.
Cogging & crawling in induction motors
Slot-harmonic parasitic phenomena. Cogging = locks at start when S=R. Crawling = stuck at n_s/7 from 7th space-harmonic torque. Universal cure: skew rotor bars one stator slot pitch.
Single-phase induction motors
Double-revolving-field theory (1-φ field = forward + backward). Starter families: split-phase, capacitor-start, PSC, cap-start-cap-run, shaded-pole. ECMs increasingly replacing them in residential / small commercial.
Rotor-resistance speed control (slip-ring IM)
Wound-rotor IM with external R_ext via slip rings. Shifts T-s curve right: s_max ∝ R_2, T_max unchanged. Slip-power penalty = s·P_input. Modern variants: Kramer / Scherbius / DFIG wind turbines.