Electrical engineering concepts

  Unit‑1: Basics of Control System (50 Objective Questions) A control system that does not use feedback is called: A) Closed loop system B) Open loop system C) Linear system D) Nonlinear system Answer: B) Open loop system A control system that uses feedback is called: A) Open loop system B) Closed loop system C) Linear system D) Nonlinear system Answer: B) Closed loop system A system whose parameters change with time is: A) Time invariant system B) Time variant system C) Linear system D) Nonlinear system Answer: B) Time variant system A system whose parameters remain constant with time is: A) Time invariant system B) Time variant system C) Nonlinear system D) Open loop system Answer: A) Time invariant system A linear system obeys: A) Superposition principle B) Resonance principle C) Feedback principle D) Stability principle Answer: A) Superposition principle A nonlinear system does not obey: A) Superposition principle B) Feedback principle C) Stability principle D) Control principl...

  Unit‑1: Three‑Phase Induction Motor (50 Questions) The stationary part of a three‑phase induction motor is called: A) Rotor B) Stator C) Shaft D) Frame Answer: B) Stator The rotating part of a three‑phase induction motor is: A) Stator B) Rotor C) Armature D) Commutator Answer: B) Rotor A three‑phase induction motor works on the principle of: A) Mutual induction B) Electromagnetic induction C) Self induction D) Resonance Answer: B) Electromagnetic induction Slip is defined as: A) Ns/N B) (Ns – N)/Ns C) N/Ns D) N – Ns/N Answer: B) (Ns – N)/Ns Slip speed is: A) Ns B) N C) Ns – N D) N – Ns Answer: C) Ns – N The most commonly used rotor type is: A) Slip ring B) Squirrel cage C) Wound rotor D) DC rotor Answer: B) Squirrel cage Slip ring induction motors are preferred for: A) Low cost B) High starting torque C) Small size D) Domestic fans Answer: B) High starting torque The equivalent circuit of an induction motor is similar to: A) Transformer B) DC motor C) Alternator D) Rectifier Answ...

  Motor Selection for Different Applications (Three-Phase Induction Motor) The selection of a three-phase induction motor depends mainly on the torque–speed requirement of the load . Different industrial loads require different starting torque, running torque, and speed characteristics . 1. Factors Affecting Motor Selection While selecting a motor, the following factors should be considered: Starting torque requirement Speed control requirement Nature of load torque Efficiency and power factor Duty cycle of operation Environmental conditions 2. Types of Load Torque–Speed Characteristics (1) Constant Torque Load Characteristics Torque remains constant at all speeds . Power increases with speed. Examples Conveyors Hoists Cranes Positive displacement pumps Suitable Motor Slip-ring induction motor Squirrel cage induction motor (high starting torque) Curve Torque │ │────────────── │ │ └──────────────── Speed 3. V...

Maintenance Procedure and Rewinding of Three-Phase Induction Motor Three-phase induction motors are widely used in industries, pumps, compressors, conveyors, and machine tools . Proper maintenance increases efficiency, reliability, and service life of the motor. 1. Maintenance Procedure of Three-Phase Induction Motor Maintenance is generally divided into routine maintenance and preventive maintenance . (A) Routine Maintenance 1. Cleaning Remove dust, oil, and dirt from the motor body. Clean cooling ducts and ventilation openings to avoid overheating. 2. Checking Electrical Connections Tighten all terminal connections . Loose connections may cause sparking and voltage drop . 3. Bearing Inspection Check bearings for noise, vibration, or overheating . Apply proper lubrication regularly. 4. Temperature Check Ensure the motor operates within permissible temperature limits . 5. Insulation Check Measure insulation resistance using megger . Low insulation res...

  Speed Control of Three-Phase Induction Motor The speed of an induction motor is given by: N s = 120 f P N_s = \frac{120f}{P} ​ Where: N s N_s ​ = Synchronous speed (rpm) f f  = Supply frequency (Hz) P P  = Number of poles Thus, the speed can be controlled by changing: Supply voltage Frequency Number of poles Rotor resistance (for slip-ring motor) 1. Stator Voltage Control (Squirrel Cage Motor) Principle The speed of an induction motor is proportional to the square of stator voltage . By reducing the stator voltage , the torque decreases , causing speed to drop. Method Supply voltage is controlled using: Autotransformer AC voltage controller Thyristor controller Characteristics Simple method Used for small motors Suitable for fan and pump loads Applications Fans Blowers Small pumps 2. Pole Changing Method (Squirrel Cage Motor) Principle From the speed equation: N s = 120 f P N_s = \frac{120...

  No-Load Test and Blocked Rotor Test of Three-Phase Induction Motor, Losses and Efficiency These tests are performed to determine the parameters, losses, and efficiency of a three-phase induction motor without loading it fully. 1. No-Load Test (Open Circuit Test) Purpose The no-load test is conducted to determine: Iron (core) losses Friction and windage losses Magnetizing current No-load power factor Test Circuit 3-Phase Supply │ ┌─────────┐ │ Ammeter │ └─────────┘ │ ┌───────┐ │ Motor │ └───────┘ │ Wattmeter & Voltmeter Procedure Run the motor at rated voltage and frequency . Keep the motor without mechanical load . Record: Line voltage V 0 V_0 ​ No-load current I 0 I_0 ​ Input power W 0 W_0 ​ Power Measured P 0 = W 0 P_0 = W_0 ​ This input power mainly represents: Iron losses Friction and windage losses Small stator copper loss ...

  Maintenance of Different Types of Single-Phase Motors and Rewinding Single-phase motors are widely used in fans, pumps, refrigerators, washing machines, and small machines . Proper maintenance increases efficiency, reliability, and life of the motor . 1. Maintenance of Single-Phase Motors Maintenance is generally divided into routine maintenance and preventive maintenance . (A) Routine Maintenance These are simple checks done regularly. 1. Cleaning Remove dust, dirt, and oil from motor surface. Clean ventilation openings to prevent overheating. 2. Bearing Inspection Check bearings for noise and overheating . Apply proper lubrication if required. 3. Tightening of Connections Ensure terminal connections are tight . Loose connections may cause sparking and heating . 4. Insulation Check Check insulation resistance using megger . Prevents short circuit and leakage current . (B) Preventive Maintenance Performed periodically to avoid fail...