Electric Motor MCQ
Induction Motor, DC Motor, Star-Delta — motor types, working aur troubleshooting. Gulf me motor knowledge must hai. 5 parts mein divide.
Electric Motor Kya Hota Hai?
Main Mohammad Arif hoon — 12+ saal se Gulf aur Mumbai ke sites pe electrical supervisor hoon. Netmagic Data Center se Sidra Hospital tak, har site pe motors ka kaam dekha hai. Isiliye confident hoke keh sakta hoon: Gulf electrician ke liye motor knowledge sabse zaroori hai. Site pe 80% kaam motors ka hi hota hai.
Electric motor ek aisi device hai jo electrical energy ko mechanical energy me convert karti hai. Simple lagta hai — par samajhna padta hai ki ye kaam kaise karta hai, kab fail karta hai, aur kaise theek hota hai. Yahi MCQ section 03 me 70+ sawaal hain — aur yahi Gulf interview me sabse zyada puchha jaata hai.
3-Phase Induction Motor — Sabse Important
Gulf sites pe 95% motors 3-phase squirrel cage induction type ke hote hain. HVAC, pumps, compressors, conveyor — sab jagah yahi milega. Kyunki ye simple hai, maintenance cheap hai, aur rugged hai. Koi brush nahi, koi slip ring nahi — bas stator aur rotor.
Slip — Motor Ka Heartbeat
Synchronous Speed: Ns = 120 × f / P
Slip % = (Ns - Nr) / Ns × 100
Example: 4-pole, 50Hz → Ns = 1500 RPM | Nr ≈ 1450 RPM | Slip = 3.3%
Normal slip hota hai 2–5%. Slip zyada matlab motor overloaded ya kuch mechanically jam hai. Slip zero matlab synchronous motor (induction nahi) — ye alag concept hai.
Star-Delta me starting current 1/3 hoti hai kyunki: Star connection me phase voltage = Line voltage / √3. Power = V²/R — voltage 1/√3 guna → power 1/3 guna → current 1/3 guna. Simple logic yaad rakh, formula rat mat.
| Motor Type | Supply | Speed | Starting Torque | Gulf Application |
|---|---|---|---|---|
| Squirrel Cage Induction | 3-Phase AC | Near-sync | Medium | HVAC, pumps, fans |
| Slip Ring Induction | 3-Phase AC | Near-sync | High | Cranes, hoists |
| Synchronous | 3-Phase AC | Exact sync | Low (aux start) | Compressors, PF correction |
| DC Shunt | DC | ~Constant | Medium | Machine tools |
| DC Series | DC | Variable | Very High | Traction, cranes |
| BLDC | DC (electronic) | Variable | Medium | Precision drives, automation |
Motor Troubleshooting — Site Pe Kaam Aayega
Motor site pe fail hota hai, supervisor puchhta hai kya problem hai — tab ye table yaad aana chahiye:
Motor Start Nahi Hota
- → Supply nahi aa rahi (fuse check karo)
- → Overload relay trip (reset karo)
- → Contactor fault (coil check karo)
- → Shaft jam (mechanical load check)
Motor Garam Ho Raha Hai
- → Overload — current check karo (clamp meter)
- → Single phasing — teenon phase check karo
- → Cooling fan kharab ya blocked
- → Low voltage — voltmeter se check
Motor Vibrate Kar Raha Hai
- → Bearing failure (most common)
- → Misalignment (laser align tool se check)
- → Unbalanced load ya loose foundation
Motor Reverse Chal Raha Hai
- → Koi 2 phases swap hain
- → Phase sequence meter se verify karo
- → Fix: koi bhi 2 terminals swap karo
Zaroori Formulas — Interview Me Speed Se Bolni Padti Hain
Synchronous Speed: Ns = 120 × f / P (RPM)
Slip: s% = (Ns - Nr) / Ns × 100
Full Load Current: I = P / (√3 × V × PF × η)
Star-Delta start I: I_start = I_DOL / 3
VFD Speed: N = 120 × f_out / P
Real Example — Netmagic DC4: 30kW motor, 415V, PF=0.85, eff=90% → FLC = 30000 / (1.732 × 415 × 0.85 × 0.90) = 54.6A. Cable sizing ke liye is value pe 1.25 factor lagao → 68A. Yahan 70A MCB aur 16mm² cable select hua tha.
Gulf Interview Tips — Motor Pe
- Pole pair se speed nikalna: “2-pole kitna RPM?” sirf 3000 bolna, calculation nahi — speed se supervisor impress nahi, confidence se hota hai.
- Starting method selection: “5.5 kW tak DOL, 7.5–75 kW Star-Delta, 75 kW+ ya variable speed ke liye VFD” — ye rule yaad rakh.
- Overload relay setting: “Motor ke nameplate FLC pe set karo” — ye ek line bahut kaam aata hai.
- Single phasing: “Motor hot ho raha tha — toh teenon phase check kiya, ek phase missing tha” — real site experience wala jawab supervisor ko pasand aata hai.
- Insulation test: “Megger se check kiya — 500V DC, minimum 1 MegaOhm hona chahiye” — ye standard yaad rakh.
Motor Calculator Bhi Try Karo
FLC, starting current, cable size, breaker sizing — sab ek jagah. Nameplate values daalo, sab instantly milega.