Rotating Machines and Electromagnetic Devices
Core concepts
Expect questions on synchronous machines, induction motors, DC machines, and transformers. For induction motors, synchronous speed is n_s = 120f/p and slip is s = (n_s − n_r)/n_s; rotor frequency is sf. Torque is proportional to V², so a 10 % voltage dip cuts torque by roughly 19 %. Synchronous machines regulate voltage and supply reactive support: over-excitation delivers VARs and appears capacitive to the system. Transformers belong here too - turns ratio a = N₁/N₂ = V₁/V₂, with current scaling inversely, and per-unit impedance governing short-circuit current. Losses are copper (I²R, load-dependent) and core (hysteresis plus eddy current, nearly constant); maximum efficiency occurs when copper loss equals core loss.
Traps to expect
Slip is often confused with rotor speed, and pole count is mistaken for pole pairs (use 120f/p, not 60f/p). In transformers, candidates reverse the current ratio and forget that a wye-delta bank shifts phase by 30°. Remember that a no-load or open-circuit test yields core loss, while a short-circuit or blocked-rotor test yields copper loss.
Worked example
A 4-pole, 60 Hz induction motor runs at 1746 rpm. Synchronous speed = 120 × 60 / 4 = 1800 rpm; slip = (1800 − 1746)/1800 = 0.03, or 3 %. Rotor frequency = 0.03 × 60 = 1.8 Hz. At 3 % slip the machine operates in its normal region, well below breakdown torque.