torque conditions. In synchronous machines, the magnetic field must be provided by means other than induction, such as from separately excited windings or permanent magnets. A fractional-horsepower motor either has a rating below about 1 horsepower (0.746 kW), or is manufactured with a standard-frame size smaller than a standard 1 HP motor. Many household and industrial motors are in the fractional-horsepower class. Notes: Rotation is independent of the frequency of the AC voltage. Rotation is equal to synchronous speed (motor-stator-field speed). In SCIM, fixed-speed operation rotation is equal to synchronous speed, less slip speed. In non-slip energy-recovery systems, WRIM is usually used for motor-starting but can be used to vary load speed. Variable-speed operation. Whereas induction- and synchronous-motor drives are typically with either six-step or sinusoidal-waveform output, BLDC-motor drives are usually with trapezoidal-current waveform; the behavior of both sinusoidal and trapezoidal PM machines is, however, identical in terms of their fundamental aspects. In variable-speed operation, WRIM is used in slip-energy recovery and double-fed induction-machine applications. A cage winding is a shorted-circuited squirrel-cage rotor, a wound winding is connected externally through slip rings. Mostly single-phase with some three-phase. Abbreviations: BLAC – Brushless AC BLDC – Brushless DC BLDM – Brushless DC motor EC – Electronic commutator PM – Permanent magnet IPMSM – Interior permanent-magnet synchronous motor PMSM – Permanent magnet synchronous motor SPMSM – Surface permanent magnet synchronous motor SCIM – Squirrel-cage induction motor SRM – Switched reluctance motor SyRM – Synchronous reluctance motor VFD – Variable-frequency drive WRIM – Wound-rotor induction motor WRSM – Wound-rotor synchronous motor LRA – Locked-rotor amps: The current you can expect under starting conditions when you apply full voltage. It occurs instantly during start-up. RLA – Rated-load amps: The maximum current a motor should draw under any operating conditions. Often mistakenly called running-load amps, which leads people to believe, incorrectly, that the motor should always pull these amps. FLA – Full-load amps: Changed in 1976 to "RLA – rated-load amps". Self-commutated motor Brushed DC motor Most DC motors are small permanent magnet (PM) types. They contain a brushed internal mechanical commutation to reverse motor windings' current in synchronism with rotation. Electrically excited DC motor A commutated DC motor has a set of rotating windings wound on an armature mounted on a rotating shaft. The shaft also carries the commutator. Thus, every brushed DC motor has AC flowing through its windings. Current flows through one or