Mostly, synchronous motors are of

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salient pole type machines Explanation: Synchronous motors designed for low and medium speeds are predominantly of the salient pole type. These motors feature a rotor with projecting poles, which is structurally optimized for the multi-pole configurations required to maintain a synchronous speed $N_s = \frac{120f}{P}$ at lower frequencies. Working Principle: The salient pole rotor consists of individual pole pieces bolted onto a spider, creating a non-uniform air gap. This geometry is ideal for low-speed operation where a high number of poles ($P$) is necessary. In contrast, smooth cylindrical rotors are utilized in high-speed applications like turbo-alternators to minimize centrifugal stresses and windage losses. Key Formulas: $N_s = \frac{120f}{P}$ — relationship between synchronous speed, frequency, and number of poles Key Points: Salient pole machines have a large diameter and short axial length.; The projecting pole construction provides robust support for field windings.; High number of poles leads to lower speed requirements for the same frequency.; Cylindrical rotors are preferred for 2-pole or 4-pole high-speed turbo-generators. Advantages: Better cooling due to projecting poles; Suitable for high-pole count, low-speed operation; Economical construction for medium to large power ratings Disadvantages: Higher windage loss at very high speeds; Not suitable for high-speed turbo-alternators due to centrifugal force limitations Applications: Hydro-electric power plants; Industrial low-speed synchronous motors; Synchronous condensers Additional Info: Salient pole rotors are characterized by a non-uniform air gap, which results in saliency and non-zero reluctance variation.. Option D (cylindrical) is incorrect because it describes high-speed rotors, typically used in thermal power plants. Answer: C is correct — Mostly, synchronous motors, especially those used in industrial applications, are of the salient pole type to accommodate a higher number of poles. Exam Tip: Always remember: salient poles = low speed, many poles; cylindrical = high speed, few poles (2 or 4).

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