AC Motors: Single-Phase and Three-Phase
Overview
AC motors convert alternating current electrical energy into mechanical energy. They are broadly classified into single-phase and three-phase types. Single-phase motors are used in household appliances, while three-phase motors power industrial machinery. Key topics for ITI Electrician Semester 3 include motor types, working principles, starting methods, slip, synchronous speed, and efficiency. Understanding these concepts is essential for both theory papers and practical assessments under NCVT guidelines.
Typed Notes
Synchronous Speed and Slip
Synchronous speed (Ns) is the speed of the rotating magnetic field produced by the stator. Formula: Ns = 120f/P. For a 4-pole motor at 50 Hz: Ns = 120×50/4 = 1500 RPM. The rotor of an induction motor always runs slightly below synchronous speed; this difference is called slip. Slip (S) = (Ns − Nr)/Ns × 100%. At no load, slip is nearly zero (0.5–1%). At full load, slip is 2–5% for squirrel cage and up to 7% for wound rotor motors. Slip is essential for torque production.
Single-Phase AC Motors: Types and Starting Methods
Single-phase induction motors are not self-starting because a single-phase supply produces a pulsating, not rotating, magnetic field. Types include: (1) Split-phase motor – uses a high-resistance auxiliary winding; used in fans, washing machines. (2) Capacitor-start motor – uses a capacitor in series with auxiliary winding for high starting torque; used in compressors. (3) Capacitor-start capacitor-run motor – two capacitors for best performance; used in air conditioners. (4) Shaded pole motor – cheapest, lowest starting torque; used in small fans and record players. (5) Universal motor – runs on both AC and DC; used in mixers and drills.
Three-Phase Induction Motor: Construction and Types
Three-phase induction motors have two main parts: Stator (stationary part with three-phase windings) and Rotor (rotating part). Two rotor types: (1) Squirrel Cage Rotor – bars short-circuited by end rings, robust, low maintenance, most common in industry. (2) Slip Ring (Wound Rotor) – has external resistance connected via slip rings to improve starting torque; used in cranes and hoists. The motor works on the principle of electromagnetic induction (Faraday's law). The rotating magnetic field (RMF) produced by the stator induces current in the rotor, creating torque.
Starting Methods and Starters for Three-Phase Motors
Large induction motors draw 5–7 times full-load current at starting, which can damage windings and disturb supply. Starters reduce this. (1) DOL (Direct On-Line) Starter – connects motor directly to supply; used for motors up to 5 HP; simplest and cheapest. (2) Star-Delta Starter – motor starts in star (voltage reduced to 1/√3 ≈ 58%), then switches to delta for running; reduces starting current to 1/3; used for motors above 5 HP. (3) Auto-transformer Starter – uses tapped transformer to reduce voltage; costly but smooth starting. (4) Rotor Resistance Starter – used only for slip-ring motors; adds external resistance to rotor circuit.
Topics & Sub-topics
Har topic ke saath sub-topics aur example — chapter ka complete coverage.
013-Phase Induction Motor
- Rotating magnetic field (RMF)
- Slip: Ns = 120f/P, N < Ns
- Squirrel cage & slip-ring types
Squirrel cage motor sabse common — pump, fan, compressor sab isi par.
02Single-Phase Motors
- Not self-starting — need auxiliary
- Capacitor start / capacitor run
- Shaded pole for small fans
Fridge ka compressor capacitor-start motor — capacitor kharab to hum-hum karta hai, start nahi hota.
03Synchronous Motor
- Runs exactly at Ns
- Needs excitation
- PF improvement use
Bade plants mein synchronous motor over-excited karke PF 1 ki taraf le jaate hain.
04Motor Applications
- Pump, fan, compressor, lathe
- Torque-speed matching
- DOL vs soft start selection
Bore-well pump usually 3 HP squirrel cage — overload relay 110% rating ka.
Key Concepts
Revision ke liye ek nazar — exam se pehle yahi padhna hai.
- 01Synchronous speed formula: Ns = 120f/P, where f = frequency (Hz) and P = number of poles
- 02Slip in induction motor: S = (Ns - Nr)/Ns × 100%; at full load slip is 2–5% for squirrel cage motors
- 03Single-phase induction motors are NOT self-starting; they need auxiliary starting methods (capacitor, shaded pole, split-phase)
- 04Three-phase squirrel cage induction motor is the most widely used industrial motor due to its robust, simple, and maintenance-free construction
- 05DOL (Direct On-Line) starter is used for motors up to 5 HP; Star-Delta starter is used for motors above 5 HP to reduce starting current
Questions & Solutions
Pehle khud try karo — phir solution kholo. Yahi tariqa topper banata hai.
A 6-pole, 50 Hz three-phase induction motor runs at 950 RPM. Calculate the synchronous speed and percentage slip.
Why is a single-phase induction motor not self-starting? How does a capacitor-start motor overcome this?
State the difference between a squirrel cage and a slip-ring induction motor. Where is each used?
What is a Star-Delta starter? Why is it used for motors above 5 HP?
Name four types of single-phase motors and give one application of each.
AC motors are classified as single-phase (not self-starting, need auxiliary methods) and three-phase (self-starting, used in industry). Key formulas: Ns = 120f/P and Slip = (Ns−Nr)/Ns × 100%. Squirrel cage motors are most common; slip-ring motors give high starting torque. DOL starters suit small motors (up to 5 HP); Star-Delta starters suit larger motors. Understanding slip, starting methods, and motor types is critical for ITI Electrician Semester 3 NCVT exams.
Chapter-end quiz
4 MCQsOption tap karo — turant pata chalega sahi ya galat, explanation ke saath.
Q1.What is the synchronous speed of a 4-pole, 50 Hz induction motor?
Q2.Which single-phase motor is used in electric drills and mixers because it operates on both AC and DC?
Q3.A Star-Delta starter reduces the starting current to what fraction of DOL starting current?
Q4.Which type of rotor is used in a squirrel cage induction motor?
Mark karo — dashboard par progress dikhega.