Electrical Machines

Electrical Machines – Verified PYQ Authority Guide

Electrical Machines is a sub-subject authority hub. It is designed to expose the important topic/concept clusters represented by verified SRO PYQs and guide students from broad revision into the exact solved questions that support each area. 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

33 verified PYQs are currently mapped to this Sub Subject 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.

1. 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.…
2. A DC motor has flux = constant and armature current = 20 A. 1. Torque is directly proportional to armature current under constant flux condition. 2. If current doubles, torque also doubles.…
   Deputy Executive Engineer (Electrical), Class-2, Sports Authority of Gujarat / Assistant Engineer (Electrical), Class-2, · 2026 · DC Motor Torque and Armature Current
   The electromagnetic torque of a DC motor is proportional to the product of air-gap flux and armature current. If flux is held constant, torque…
3. An induction motor operates at a slip of 5% with a supply frequency of 50 Hz. 1. Rotor frequency is approximately 2.5 Hz. 2. Rotor current frequency becomes zero at synchronous speed.…
   Deputy Executive Engineer (Electrical), Class-2, Sports Authority of Gujarat / Assistant Engineer (Electrical), Class-2, · 2026 · Rotor Frequency versus Slip
   Rotor-current frequency is proportional to slip: at standstill it equals supply frequency, and at synchronous speed it becomes zero. Rotor frequency fr=s f=0.05×50=2.5 Hz.…
4. An induction motor develops maximum torque at slip = 0.2. 1. If rotor resistance is doubled, slip at maximum torque increases. 2. Maximum torque value remains unchanged with rotor resistance variation. 3.…
   Deputy Executive Engineer (Electrical), Class-2, Sports Authority of Gujarat / Assistant Engineer (Electrical), Class-2, · 2026 · Effect of Rotor Resistance on Maximum Torque
   The torque-slip curve changes shape when rotor resistance is changed. The given initial s_max=0.2 is not needed to prove the qualitative relationships. Doubling R2…
5. A transformer has core loss = 500 W and full-load copper loss = 500 W. 1. Maximum efficiency occurs at full load condition. 2. At half load, copper loss becomes 125 W.…
   Deputy Executive Engineer (Electrical), Class-2, Sports Authority of Gujarat / Assistant Engineer (Electrical), Class-2, · 2026 · Load for Maximum Efficiency
   At x p.u. load, transformer output is proportional to x while copper loss is proportional to x² and iron loss remains approximately constant. The…
6. Consider the following statements regarding Commutation in DC Machines: 1. Commutation is the process by which alternating current in armature conductors is converted into direct current at the terminals. 2. Poor commutation…
   Deputy Executive Engineer (Electrical), Class-2, Sports Authority of Gujarat / Assistant Engineer (Electrical), Class-2, · 2026 · Sparkless Commutation in DC Machines
   During the short commutation interval, a coil is effectively short-circuited by the brush and its current must reverse from +I to −I. Interpoles are…
7. 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…
8. Consider the following statements regarding Transformer Efficiency: 1. The maximum efficiency of a transformer occurs when copper losses are equal to core losses. 2. Transformer efficiency remains constant irrespective of the load…
   Deputy Executive Engineer (Electrical), Class-2, Sports Authority of Gujarat / Assistant Engineer (Electrical), Class-2, · 2026 · Maximum and All-Day Efficiency
   Transformer core loss is approximately constant at fixed voltage/frequency, while copper loss is I²R and therefore varies with the square of load. The competition…

Formula & key-relationship bank from verified solutions

• f=PN/120; N_s=120f/P.
• T=kΦI_a.
• s=(N_s−N)/N_s; f_r=s f_s.
• Approximate condition: s_max ∝ R2/Xeq. At s=1, starting torque rises as rotor resistance is increased toward the optimum R2≈Xeq.
• Pcu(x)=x²Pcu,FL. Here Pcu(0.5)=0.25×500=125 W. Maximum η: x²Pcu,FL=Pi → x=1.
• Reactance voltage is associated with L·di/dt during current reversal; ideal commutation completes the reversal within the brush short-circuit time.

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…
• The electromagnetic torque of a DC motor is proportional to the product of air-gap flux and armature current. If flux is held constant, torque varies directly with armature current, so doubling current ideally doubles torque. Key relation: T=kΦI_a.…
• Rotor-current frequency is proportional to slip: at standstill it equals supply frequency, and at synchronous speed it becomes zero. Rotor frequency fr=s f=0.05×50=2.5 Hz. At synchronous speed s=0, fr=0; at standstill s=1, fr=f. Key relation: s=(N_s−N)/N_s; f_r=s f_s.…
• The torque-slip curve changes shape when rotor resistance is changed. The given initial s_max=0.2 is not needed to prove the qualitative relationships. Doubling R2 doubles the approximate slip at maximum torque until model limits; Tmax remains unchanged. Key…
• At x p.u. load, transformer output is proportional to x while copper loss is proportional to x² and iron loss remains approximately constant. The given losses make the maximum-efficiency load x=sqrt(500/500)=1 p.u. Every statement follows numerically, so B…

Exam tips already validated in SRO solutions

• Use rpm in the 120-factor formula.
• Constant flux converts the general torque equation into T∝I_a.
• Standstill: s=1; synchronous speed: s=0.
• Rotor resistance shifts the torque-slip curve horizontally, not vertically at Tmax.
• If core loss equals full-load copper loss, maximum efficiency is at full load—an instant result.

Common mistakes to avoid

• Frequency is not independent of rotor speed in a synchronous generator.
• Do not generalize constant-flux behavior into 'torque is independent of flux'.
• Rotor frequency is not independent of slip.
• Saying maximum torque occurs at synchronous speed; torque is zero at s=0.
• Scaling copper loss linearly with load instead of with load squared.

Topic and concept coverage

Mapped topic labels include Transformers, DC Machines, Synchronous Machines, Induction Motors, Magnetic Circuits, DC Motors, Electromechanical Energy Conversion. The concept trail includes Air-Gap Line, Coefficient of Coupling, Compensating Winding, DC Motor Back EMF and Converted Power, DC Motor Torque and Armature Current, Effect of Large Air Gap, Effect of Rotor Resistance on Maximum Torque, Energy Stored in Magnetic 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 Engineering, AC Machines, DC Machines, DC Motors, Electromechanical Energy Conversion, Induction Motors, Transformers, Synchronous Machines, Magnetic Circuits. 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

1. Solve before reading: answer a mapped PYQ first.
2. Read the exact explanation: verify the correct principle, formula, distractor logic and common mistake on that question page.
3. Move one level in the graph: use the closest concept/sub-topic/topic link rather than opening unrelated content.
4. 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.