Synchronous Machines – Verified PYQ Authority Guide
Synchronous Machines is organised as a topic-level PYQ authority page. It brings together the strongest verified question clusters, exam/year coverage and concept-level practice so students can revise the topic without jumping through unrelated notes. Every factual learning cue below is drawn from the existing verified, published and approved English PYQ corpus or from its Knowledge Graph relationships; the hub does not invent unsupported technical claims.
PYQ evidence snapshot
5 verified PYQs are currently mapped to this Topic hub. The represented years include 2025, 2026. Exam coverage currently includes Assistant Engineer (Electrical), Class-2, Road and Building Department, Deputy Executive Engineer (Electrical), Class-2, Sports Authority of Gujarat / Assistant Engineer (Electrical), Class-2,. Subject context includes Electrical Engineering. These values come from live mappings and can expand automatically when new verified PYQs are added.
Most useful verified PYQs to solve first
Start with the actual questions rather than memorising a generic note. The links below are ranked from the mapped corpus using repeat history and editorial quality, while the complete explanation stays on the individual question page.
- A synchronous generator has 8 poles and rotates at 750 rpm. 1. Generated frequency is 50 Hz. 2. If speed increases to 1500 rpm, frequency becomes 100 Hz. 3. Frequency depends on…
Deputy Executive Engineer (Electrical), Class-2, Sports Authority of Gujarat / Assistant Engineer (Electrical), Class-2, · 2026 · Frequency-Speed-Pole Relation
A generator with more poles produces the same frequency at a lower mechanical speed; for fixed pole count, frequency is directly proportional to speed.… - Consider the following statements regarding Synchronous Motor: 1. A synchronous motor is not self-starting and requires auxiliary means for starting. 2. It can operate at unity, lagging, or leading power factor depending…
Deputy Executive Engineer (Electrical), Class-2, Sports Authority of Gujarat / Assistant Engineer (Electrical), Class-2, · 2026 · Excitation and Power Factor Control
A conventional synchronous motor is not inherently self-starting. By adjusting field excitation it can operate at lagging, unity or leading power factor; over-excited operation… - These days alternators are designed to have larger air gaps for __________.
Assistant Engineer (Electrical), Class-2, Road and Building Department · 2025 · Effect of Large Air Gap
GPSC's final key accepts 'All of the mentioned'. A larger air gap reduces synchronous reactance and armature-reaction sensitivity, which improves stability/parallel operation; appropriate air-gap… - A synchronous machine has field winding on stator and polyphase armature winding on rotor. At steady state, which of the following is not true for its air gap field?
Assistant Engineer (Electrical), Class-2, Road and Building Department · 2025 · Relative Speed of Air-Gap Field
The final GPSC answer is B. A field winding placed on the stator produces an air-gap field stationary with respect to the stator. A… - The active power delivered by an alternator will _______ when the prime mover input is increased for the alternator connected to infinite bus bar.
Assistant Engineer (Electrical), Class-2, Road and Building Department · 2025 · Real Power Control
For an alternator connected to an infinite bus, bus voltage and frequency are fixed. Increasing prime-mover mechanical input increases torque angle δ and therefore…
Formula & key-relationship bank from verified solutions
f=PN/120; N_s=120f/P.N_s=120f/P.P_max ≈ 3VE/X_s Larger air gap → magnetic reluctance ↑ → X_s generally ↓ ⇒ stability limit ↑Relative speed: N_field/stator = 0 N_rotor/stator = N_s N_field/rotor = 0 - N_s = -N_sFor a cylindrical rotor (neglecting resistance): P = (3VE/X_s) sinδ Prime-mover input ↑ ⇒ δ ↑ ⇒ P ↑ (within stability limit)
Use these as revision triggers and open the linked PYQ before applying a formula numerically; variable definitions and assumptions belong to the exact solved question.
Core ideas repeatedly reinforced by the solved corpus
- A generator with more poles produces the same frequency at a lower mechanical speed; for fixed pole count, frequency is directly proportional to speed. f=PN/120. With P=8 and N=750 rpm, f=50 Hz; at 1500 rpm, f=100 Hz. Key…
- A conventional synchronous motor is not inherently self-starting. By adjusting field excitation it can operate at lagging, unity or leading power factor; over-excited operation is used for reactive-power support. Key relation: N_s=120f/P. Exam focus: Excitation controls power factor;…
- GPSC's final key accepts 'All of the mentioned'. A larger air gap reduces synchronous reactance and armature-reaction sensitivity, which improves stability/parallel operation; appropriate air-gap design also helps achieve a smoother magnetic-field distribution.
- The final GPSC answer is B. A field winding placed on the stator produces an air-gap field stationary with respect to the stator. A rotor moving at synchronous speed sees that stationary field moving backward at Ns, not…
- For an alternator connected to an infinite bus, bus voltage and frequency are fixed. Increasing prime-mover mechanical input increases torque angle δ and therefore increases the active power delivered to the bus.
Exam tips already validated in SRO solutions
- Use rpm in the 120-factor formula.
- Excitation controls power factor; frequency and pole count set speed.
- Think in terms of synchronous reactance and power-angle limit.
- Write both field and rotor speeds in the same reference frame before judging relative direction.
- Keep infinite-bus voltage and frequency fixed in your reasoning.
Common mistakes to avoid
- Frequency is not independent of rotor speed in a synchronous generator.
- Load changes torque angle, not steady synchronous speed.
- Do not assume a larger air gap is universally 'better'; it increases required field ampere-turns.
- Adding speeds instead of subtracting them is what produces the incorrect 2Ns conclusion.
- Do not expect frequency to rise appreciably when a single generator is tied to an infinite bus.
Topic and concept coverage
Mapped topic labels include Synchronous Machines. The concept trail includes Effect of Large Air Gap, Excitation and Power Factor Control, Frequency-Speed-Pole Relation, Real Power Control, Relative Speed of Air-Gap Field. Use these labels as a revision map: move from the broad area to the narrow concept, solve a verified PYQ, inspect the detailed reasoning, and then attempt another question from the same cluster.
Knowledge Graph navigation
Continue with Electrical Machines, Alternator Design, Alternator on Infinite Bus, Losses and Efficiency, Rotating Magnetic Fields, Synchronous Generator, Electrical Engineering, Alternator Load Loss, Effect of Large Air Gap, Excitation and Power Factor Control, Frequency-Speed-Pole Relation. These are canonical SRO entity links based on the Knowledge Graph and shared question mappings, not keyword-stuffed tag pages.
How to revise this authority page efficiently
- Solve before reading: answer a mapped PYQ first.
- Read the exact explanation: verify the correct principle, formula, distractor logic and common mistake on that question page.
- Move one level in the graph: use the closest concept/sub-topic/topic link rather than opening unrelated content.
- Reattempt: solve another verified PYQ from this hub and check whether the same error repeats.
Quality scope: this page is automatically maintained from SRO’s verified mapped corpus. It enriches one canonical authority URL instead of generating multiple near-duplicate pages for keyword variants. Manual authority articles are never overwritten by the automated engine.