Special machines
Modern motion-control machines beyond the classical AC and DC types — steppers, BLDC, PMSM, SRM.
Lessons
Stepper motors — VR, PM, hybrid + drive modes
Discrete-angle motion per pulse. Variable-reluctance vs permanent-magnet vs hybrid (1.8° 200-step/rev). Full / half / micro-step drives. Pull-in vs pull-out torque-speed envelope and missed-step risk.
Brushless DC (BLDC) motors
Inverted DC: stator 3-φ windings + rotor PM. Trapezoidal back-EMF (vs PMSM sinusoidal), 6-step commutation via 3 Hall sensors, T = k_t·I torque law. Drones, EVs, HDDs, power tools.
Permanent-magnet synchronous motors (PMSM)
Sinusoidal back-EMF, SPM vs IPM saliency, dq-axis torque equation, field-oriented control with MTPA and field weakening. The dominant EV traction motor (Prius, Tesla Model 3) and industrial servo.
Switched-reluctance motor (SRM)
Pure reluctance torque, T = ½·i²·dL/dθ. Salient rotor of laminated steel only — no windings, no magnets. Asymmetric half-bridge per phase. Rising interest as a rare-earth-free EV traction alternative.
Linear induction motor (LIM)
Rotary IM unrolled into a flat plane. v_s = 2·τ·f. Slip + thrust analogs of rotary. End-effect efficiency penalty (60-80%). Applications: maglev (Transrapid, JR-Maglev), urban transit, EMALS, roller coasters.
Synchronous reluctance motor (SynRM)
Salient steel rotor + sinusoidal 3-φ supply. Pure reluctance torque T = (3/2)(P/2)(L_d − L_q)·i_d·i_q. NO magnets, NO windings on rotor. FOC same as PMSM. Modern rare-earth-free PMSM alternative (ABB, Siemens, Bosch).
Hysteresis synchronous motor
Smooth hard-magnetic-material rotor. Constant T from rest to sync (flat T-s). Locks at exact n_s. Very smooth and quiet. Classic uses: precision clocks, audio capstans, gyroscope rotors. Low η (5-30%) — niche today.
Universal motor + AC commutator motors
Series-wound DC motor that runs on AC (T = K·I² → unidirectional). High RPM, high power density, droopy curve — drives nearly every corded power tool. Plus repulsion, compensated series, and AC traction motor history.
Servo motors — DC and AC servo systems
Closed-loop position/velocity control. DC servos (PM + encoder + PI). AC servos = PMSM + high-res encoder + FOC drive. Cascaded current/velocity/position loops. CNC, robots, fab equipment.
Synchros and resolvers
Small electrical machines for angular position transmission. CG/CX/CT/CDX three-letter codes. Classic CX→CT servo loop. Resolver = 2-φ variant with V·sinθ + V·cosθ outputs. Brushless via rotary transformer. Aerospace + military legacy.
Schrage motor
3-φ AC commutator motor with variable speed (50-150% n_s) by brush shifting + simultaneous PF control. Rotor carries primary 3-φ winding + auxiliary commutator winding. Industrial workhorse 1920s-60s; Bimbhra / GATE EE / PSU exam syllabus.