Structurally, induction motors are primarily categorized into two types:
Simple construction with rotor made of conductive aluminum or copper bars forming a closed loop, resembling a "squirrel cage."
Low maintenance costs, widely used in fans, pumps, and other light-load starting applications.
Rotor windings are connected to external resistors via slip rings, allowing adjustable starting characteristics.
Suitable for heavy-load starting equipment such as cranes and crushers.
The speed of an induction motor is always slightly lower than the synchronous speed, a difference referred to as slip (typically around 3%). Its characteristics include:
• Load Sensitivity: Slip is minimal at no-load (close to synchronous speed) and increases linearly with load.
• Extreme Conditions: When the load exceeds the maximum torque (breakdown torque), the motor exhibits:
✓ Rapid speed drop
✓ Surge in stator current (up to 5-7 times the rated current)
✓ Excessive winding temperature rise (risk of insulation failure)
• High Starting Current: Direct starting current can reach 4-7 times the rated current.
→ Single cage design: Highest inrush current
→ Deep-bar/double cage design: Reduces starting current by 20-30% via skin effect.
• Classic Solutions:
✓ Star-Delta Starting: Reduces current to 1/3 of full-voltage starting, but torque also drops by 67%.
✓ Applications: Centrifugal pumps, air compressors, and other light-load starting devices.
• Adjustable Rotor Resistance: Achieved through external resistors:
✓ Limits starting current to 2-3 times the rated current.
✓ Boosts starting torque to 200-250% of the rated value.
• Typical Applications:
✓ Heavy-load equipment like ball mills and hoists.
✓ Industrial scenarios requiring frequent starts/stops.
1. Popularization of Variable Frequency Drive (VFD) Technology
Enables soft starting via VVVF control, eliminating starting current surges in squirrel cage motors.
Improves operational efficiency by 5-15% compared to wound rotor motors with resistor-based speed control.
2. Market Evolution
Wound rotor motors' market share has declined from 40% in the 1980s to less than 15% today.
VFD-equipped squirrel cage motors dominate (over 70% of the industrial motor market).
|
Motor Type |
Ideal Applications |
Techno-Economic Analysis |
|
Standard Squirrel Cage |
Continuous operation, light-load starts |
Low procurement cost, IE3 efficiency |
|
Wound Rotor |
Heavy-load starts, speed control |
High maintenance, being phased out |
|
VFD Squirrel Cage |
Precise speed control, energy-saving retrofits |
Optimal overall efficiency |
Induction motors remain central to industrial applications due to their simple structure and high reliability. With advancements in power electronics, squirrel cage motors have been revitalized through VFD integration, while wound rotor motors are gradually fading out. Looking ahead, the adoption of IE5 ultra-premium efficiency standards will drive induction motors toward higher power density and smarter functionalities.
