Qwestrum Engineering360 · Electrical & Electronics · Electrical Machines – II
How does a three-phase induction motor work?
Rotating stator field induces current in rotor bars — rotor tries to catch the field but always runs slightly slower (slip). Workhorse motor in Indian industry.
Exam tip: keep SI units consistent end-to-end, write the governing relation symbolically before substituting, and sanity-check magnitude and sign.
Quick revision
Skim these first — then read the full notes below.
- Slip s = (Ns − N)/Ns — typically 2–5% at load
- Squirrel cage vs wound rotor — starting methods differ
- Star-delta starter common in India for large motors
- Torque-speed curve: starting, max, full-load points
Exams & GATE
- 1GATE: slip calculations, rotor frequency, power flow across air gap.
- 2Draw equivalent circuit for marks.
Interview prep
Maintenance interviews: "Why star-delta start?" — reduce starting current. "Cause of single phasing?" — fuse, contactor fault, thermal overload.
📖 Standard books (India)
Electrical Machines — Nagrath & Kothari
Read: Ch. 7–9
Transformers, DC machines, and induction motors
Understand the concept
From pump houses to conveyor belts in Indian factories, induction motors dominate. Nagrath & Kothari (or Ashfaq Hussain for simpler first pass) is standard.
Schematic diagram for study — aligned with standard B.Tech / GATE syllabus.
Three-phase induction motor. N_s = 120f/P rpm; slip s = (N_s − N)/N_s. Torque–slip curve is a GATE favourite.Explore related topics
See real electrical & electronics careers
After exams and interviews, see how engineers actually built careers — milestones and decisions from people in the field.