How it works
Switched Reluctance Motor (SRM): rotor has no windings or magnets — salient poles of soft iron. Stator poles are switched sequentially by a converter; the rotor aligns to minimise magnetic reluctance. Torque equation: T = ½·I²·(dL/dθ). Advantages: simple rugged construction, wide speed range, fault tolerant. Disadvantages: high torque ripple, acoustic noise. Brushless DC (BLDC): rotor carries surface-mounted NdFeB magnets; stator has three-phase winding excited by an inverter with Hall-sensor position feedback. Back-EMF is trapezoidal; commutation occurs every 60° electrical. Hysteresis motor: rotor made of semi-hard magnetic material (Alnico); runs at synchronous speed; constant torque during both acceleration and running — used in tape drives and clocks. Linear Induction Motor (LIM): unrolled version of a rotary induction motor; produces linear thrust instead of torque; primary is the stator, secondary is a conducting reaction rail.
Key points to remember
Switched reluctance motors have the highest fault tolerance of all the special machines because each phase is electrically and magnetically independent — loss of one phase reduces torque but does not stop the motor. BLDC motors offer the highest power density and efficiency among small motor types, up to 95% efficient. Hysteresis motor torque is proportional to the volume of the hysteresis loop of the rotor material, unlike induction motor torque which depends on slip. LIM applications include maglev trains (Inductrack), people movers, and HVAC damper actuators — thrust F = BIL analogous to F = BIL for a conductor. Cogging torque in BLDC motors (interaction between PMs and stator slots) is reduced by skewing slots or magnets, typically by one slot pitch.
Exam tip
The examiner always asks you to compare SRM and BLDC motors on torque production mechanism, rotor construction, and applications — structure your answer with a three-row table and include the key phrase that SRM produces torque by reluctance variation while BLDC produces it by Lorentz force on current-carrying conductors.