Questions
10–12 questions in major PSU papers
Difficulty
Medium-Hard
Importance
High yield — core subject for all electrical PSUs
Overview
Electrical Machines is the bedrock of electrical engineering, focusing on electromechanical energy conversion and power transformation. In PSU exams, this topic is critical as it carries significant weightage, requiring a deep understanding of machine characteristics, test procedures, and performance parameters.
DC Machines
DC machines function on the principle of electromagnetic induction and Lorentz force. Mastery of the shunt, series, and compound characteristics is vital for predicting performance under varying load conditions.
- Back EMF: Eb = (P * Z * N * phi) / (60 * A)
- Torque equation: T = (P * Z * phi * Ia) / (2 * pi * A)
- Speed control: Flux control (field), Armature control (resistance), and Voltage control (Ward-Leonard)
- Interpoles are used to minimize commutation sparking
- DC series motors have high starting torque and should never be started on no-load
Transformers
Transformers are static devices that change voltage levels using mutual induction. Exam focus is primarily on determining equivalent circuit parameters through open and short circuit tests, along with efficiency calculations.
- EMF Equation: E = 4.44 * f * N * Phi_max
- OC Test determines constant losses (core/iron loss)
- SC Test determines variable losses (copper loss)
- Efficiency calculation: Output / (Output + Iron loss + Variable loss)
- Maximum efficiency occurs when Iron loss = Copper loss
- All-day efficiency = (Output energy in kWh) / (Input energy in kWh)
Induction Motors
Three-phase induction motors are the workhorses of industry due to their robustness. You must understand the slip-torque relationship and the impact of rotor resistance on the starting current and torque.
- Slip equation: s = (Ns - Nr) / Ns
- Synchronous speed: Ns = 120f / P
- Rotor frequency: fr = s * fs
- Starting torque is proportional to rotor resistance
- Maximum torque is independent of rotor resistance
Synchronous & Special Machines
Synchronous machines operate at constant speed, while special machines like stepper motors and BLDC are increasingly common in automated systems. Focus on V-curves, regulation methods, and stepping angle calculations.
- Synchronous speed: Ns = 120f / P
- Voltage regulation: ZPF, EMF, and MMF methods
- V-curves show relationship between Field Current and Armature Current
- Stepper motor step angle = (360 / (Phases * Poles))
- BLDC motors use electronic commutation instead of brushes
Formula Sheet
Eb = (P*Z*N*Phi) / (60*A)
T = (P*Z*Phi*Ia) / (2*pi*A)
E = 4.44 * f * N * Phi_m
s = (Ns - Nr) / Ns
Ns = 120 * f / P
fr = s * fs
Max Torque Condition: R2 = s * X2
Step Angle = (360 / (Ns * Nr))
Exam Tip
Always convert your machine parameters to per-unit values before performing system-level calculations to save time and prevent calculation errors.
Common Mistakes
- Confusing the purpose of OC and SC tests, specifically which loss is measured in each.
- Neglecting the slip factor when calculating rotor frequency and torque in induction machines.
- Forgetting that maximum torque in an induction motor is independent of rotor resistance, despite starting torque being dependent on it.
More Revision Notes
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