Soft Starter Method for Starting Electric Motors

Introduction

• Various methods for starting an electric motor include:
  • Direct On Line (DOL)
  • Star-Delta
  • Auto Transformer
  • Primary Resistor
  • Solid-state power electronic devices like VFD or Soft Starter
• Focus of the lesson: Soft Starter
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Importance of Soft Starters

• Electric motors require large electricity amounts during acceleration to nominal speed.
• Soft Starter:
  • Limits surge of current (inrush current) and torque.
  • Ensures safer, smoother start-up.
  • Protects motor and extends lifespan.
  • Reduces heating from frequent start/stops.
  • Minimizes mechanical stress on motor and shaft.
  • Reduces electrodynamic stresses on power cables.
  • Reduces electrical supply system demands, cutting costs.

Applications of Soft Starters

Industrial Applications with High Inertia Loads

• Example: Air Scrubber or Dust Collector
  • Contains large fans requiring time to move.
  • Fans pull air into filters for dust collection and return clean air.

Water Supply Applications

• Example: Pumps in processes
  • Slow startups prevent dangerous pressure surges.

Conveyor Systems

• Moving heavy loads can be optimized.

Wiring and Control

• Placement: Inside control panel
• Power Supply:
  • 3-phase power in from top.
  • 3-phase power out to motor from bottom.
• Control Methods:
  • Direct start/stop wiring
    • No PLC needed.
    • Less expensive.
  • Ethernet Control
    • PLC required.
    • Allows feedback, adjustable control, and monitoring.

Internal Workings

• Main Component: Triac
  • Limits applied voltage to motor.
  • Structure: Two back-to-back Thyristors (SCRs).
  • Function: Internal pulse to gate allows current flow, ramping up current slowly to motor.
• Soft Starter vs. DOL:
  • Voltage: Gradual increase in soft starter vs. sudden inrush with DOL.
  • Current Regulation: More controlled with soft starter.
  • Torque: Gradual increase with soft starter vs. sudden with DOL.

Advantages of Soft Starters

• Mechanical Effects:
  • Prevents sudden impact and excessive wear.
  • Reduces strain on components (belts, pulleys, couplings, bearings).
  • Minimizes maintenance, unscheduled shutdowns, and extends component life.
• Electrical Effects:
  • Avoids heavy current surges.
  • Prevents burnout of contacts and motors.

Conclusion

• Ideal for industrial applications with high inertia load requiring large inrush current.
• Importance of wiring, control, and internal mechanisms (Triac).
• Prevention of undesirable mechanical and electrical effects.

Final Notes

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