Rotating Machinery and Motor Controls
Speed, slip, and torque
Synchronous speed = 120 × f / P. A 4-pole machine at 60 Hz runs at 1800 rpm synchronous and, at 2% slip, about 1764 rpm at full load. NEMA Design B is the general-purpose default: moderate starting torque, low slip. Design letters tell you torque and inrush characteristics, not efficiency.
Protection and conductor sizing
NEC Article 430 separates three concerns: branch-circuit conductors (125% of table FLA), overload protection (typically 125% of nameplate current), and short-circuit/ground-fault protection (Table 430.52 percentages). A classic trap: using nameplate current where the code requires table FLA, or applying the 125% factor to the wrong element.
Worked example
50 hp, 460 V, 3-phase motor. Table 430.250 gives FLA = 65 A. Branch conductors = 125% × 65 = 81.3 A, so select a conductor with an ampacity of at least 81.3 A after adjustment - #4 Cu at 75 °C (85 A) works. Overload sizing instead uses the nameplate FLA off the motor, which may differ from 65 A.
Controls and starting
Know across-the-line, reduced-voltage autotransformer, wye-delta, soft starter, and VFD methods, and why each reduces starting current. VFDs introduce harmonics and common-mode voltage; long motor leads require load reactors or filters. Contactor versus starter: a contactor makes and breaks load; an overload relay provides running thermal protection; the combination is a starter. NEMA and IEC sizing conventions differ - IEC uses utilization categories.
Traps
Assuming reversing rotation is done by swapping any two of three leads (true only for three-phase), and confusing service factor with a continuous overload allowance for all conditions.