(a) these are cheaper in cost, simple and rugged in construction and require little maintenance
(b) they are capable of operation under a wide range of power factors both leading and lagging
(c) they have maximum efficiency and maximum starting torque
(d) they have very good speed regulation
Answer: (a) these are cheaper in cost, simple and rugged in construction and require little maintenance
Explanation:
CONCEPT: Polyphase Induction Motor
A polyphase induction motor is an AC motor that works on electromagnetic induction and uses a rotating magnetic field produced by multi-phase AC supply (usually three-phase) to produce torque.
It is often called a “rotating transformer” because:
- Stator → Primary
- Rotor → Secondary (short-circuited and rotating)
MAIN REASON FOR WIDESPREAD USE ⭐
Polyphase induction motors are most widely used because of:
✔ Low cost
✔ Simple construction
✔ Rugged design
✔ Low maintenance
✔ High reliability
Especially in squirrel-cage induction motors, there are:
- No brushes
- No commutator
- No slip rings
Hence, fewer wearing parts → longer life → less maintenance.
DETAILED REASONS FOR POPULARITY ⭐
1. Simple Construction
- Construction is very simple.
- Stator
- Rotor
- No complex mechanical parts like commutators.
- Easy to manufacture and assemble.
Result:
👉 Lower manufacturing cost.
2. Rugged and Strong Design
- Rotor (squirrel cage type) is made of solid aluminum/copper bars.
- No delicate parts.
Can withstand:
- Mechanical shocks
- Overloads
- Harsh environments
Result:
👉 Very durable and reliable.
3. Low Maintenance
- No brushes → No brush wear.
- No commutator → No sparking.
- No frequent servicing required.
Compared to DC motors:
- DC motors need regular brush replacement.
- Induction motors do not.
Result:
👉 Low maintenance cost.
4. Low Cost
- Simple design reduces manufacturing cost.
- No expensive components.
- Installation cost is also low.
Result:
👉 Economical for industries.
5. High Reliability
- Fewer moving parts.
- No frequent breakdowns.
- Long operating life.
Result:
👉 Suitable for continuous operation.
6. Self-Starting Nature ⭐
- Polyphase supply produces rotating magnetic field.
- Phases are displaced by 120° (in 3-phase).
- This creates continuous RMF.
- RMF induces rotor current.
- Torque is produced automatically.
Hence:
👉 No external starter required.
7. Good Efficiency
- Efficiency is reasonably high.
- Copper and iron losses are minimized.
- Modern motors use silicon steel laminations.
Result:
👉 Good energy utilization.
8. Reasonably Good Power Factor
- At full load, power factor is fairly good (≈ 0.8 to 0.9).
- Suitable for industrial systems.
WHY OTHER OPTIONS ARE INCORRECT ❌
(b) Operate at Leading and Lagging Power Factor ❌
- Induction motors always operate at lagging power factor.
- They cannot operate at leading power factor.
Only synchronous motors can operate at:
✔ Leading
✔ Lagging
✔ Unity power factor
Hence, this option is wrong.
(c) Maximum Efficiency and Maximum Starting Torque ❌
- Induction motors have good efficiency.
- But not always maximum.
- Starting torque of DC series motor is much higher.
So, this is not the main reason for popularity.
(d) Very Good Speed Regulation ❌
- Speed of induction motor depends on load.
- Slip increases with load.
- Speed decreases slightly.
DC shunt motor has better speed regulation.
Hence, this option is incorrect.
SELF-STARTING EXPLANATION ⭐ (Important for Exams)
In a 3-phase induction motor:
- Three windings are placed 120° apart.
- Three-phase supply has 120° phase difference.
- This creates rotating magnetic field.
- RMF rotates at synchronous speed.
- Rotor current is induced.
- Torque is produced.
Therefore:
👉 Polyphase induction motor is self-starting.
IMPORTANT CHARACTERISTICS ⭐
| Feature | Induction Motor |
|---|---|
| Starting | Self-starting |
| Cost | Low |
| Maintenance | Very Low |
| Construction | Simple |
| Reliability | High |
| Power Factor | Lagging |
| Speed Control | Limited |
APPLICATIONS ⭐
Due to these advantages, they are used in:
✔ Pumps
✔ Fans
✔ Compressors
✔ Conveyors
✔ Elevators
✔ Machine tools
✔ Mills
✔ Textile industries
FINAL CONCLUSION ⭐
Polyphase induction motors are most widely used AC motors because they have simple and rugged construction, low cost, high reliability, and require very little maintenance, especially in squirrel-cage type motors.
KEY ONE-LINE EXAM STATEMENT ⭐
Polyphase induction motors are most widely used because of their simple construction, low cost, ruggedness, and minimum maintenance.