Side-by-side comparison
| Parameter | Salient Pole | Non-Salient Synchronous Machine |
|---|---|---|
| Rotor construction | Projecting poles (salient) with concentrated field winding | Cylindrical drum with distributed field winding in slots |
| Air gap | Non-uniform — small under poles, large between poles | Uniform all around the circumference |
| Number of poles (50 Hz) | 4 to 40 poles — low speed 150 to 750 rpm | 2 or 4 poles — high speed 1500 or 3000 rpm |
| Reactance model | Two-reaction theory: X_d (direct axis) and X_q (quadrature axis); X_d > X_q | Single synchronous reactance X_s = X_d = X_q |
| Power equation | P = (VE/X_d)sinδ + V²(X_d - X_q)sin2δ / (2X_d X_q) — includes reluctance torque term | P = (VE/X_s)sinδ — single term only |
| Reluctance torque | Present — machine can produce torque even at zero excitation | Absent — torque is zero without excitation |
| Maximum torque angle | Less than 90° — pull-out occurs before δ = 90° due to reluctance term | Exactly at δ = 90° |
| Typical application | Hydro alternators, low-speed diesel generators | Thermal (steam/gas turbine) alternators, turbo-generators |
| Real machine example | Bhakra Dam 108 MW hydro alternator, 16 poles, 375 rpm | NTPC Singrauli 200 MW turbo-alternator, 2 poles, 3000 rpm |
| Stability limit | Higher — reluctance torque adds a stabilising term | Lower — only excitation torque opposes load |
Key differences
The key physical distinction is the air gap. A salient pole machine has a small air gap under the pole face (high permeance, giving large d-axis reactance X_d, typically 1.0–1.2 pu) and a large gap between poles (low permeance, small q-axis reactance X_q, typically 0.6–0.8 pu). This asymmetry creates the reluctance torque term V²(X_d - X_q)sin2δ / (2X_d X_q), which peaks at δ = 45° and shifts pull-out below 90°. Cylindrical machines have X_d = X_q = X_s, so one equation handles everything. For a 3000 rpm turbo-alternator, cylindrical construction is mechanically necessary at that speed — centrifugal force would tear off salient poles.
When to use Salient Pole
Use salient pole machine analysis (two-reaction theory with X_d and X_q) when solving problems involving hydro alternators or low-speed diesel generators — these always have non-uniform air gaps and a meaningful reluctance torque contribution.
When to use Non-Salient Synchronous Machine
Use cylindrical rotor (single X_s) analysis for all thermal power plant alternator problems — a NTPC 200 MW, 2-pole, 3000 rpm turbo-generator is the standard textbook example, and phasor diagrams simplify because X_d = X_q.
Recommendation
For electrical machines exams, write both torque equations clearly and identify which machine type the problem specifies. If the problem mentions hydro or low-speed, apply two-reaction theory with separate X_d and X_q. Thermal turbine problems almost always use the simpler single X_s model.
Exam tip: Examiners ask you to draw the phasor diagram for a salient pole synchronous generator at lagging power factor and to separate the voltage drop into X_d and X_q components — practice this phasor construction carefully as it is a 10-mark question staple.
Interview tip: Core company interviewers (NTPC, NHPC, PGCIL) ask you to explain why pull-out angle is less than 90° for a salient machine — answer with the sin2δ term and state that it peaks at 45°, pulling the combined torque peak below 90°.