Lecture Notes: Primary Resistor Reduced Voltage Starters

Introduction

• Topic: Primary Resistor Reduced Voltage Starters
• Instructor: Jim Pytel
• Objective: Understand the reduced voltage starting method using primary resistors.
• Goal: Reduce inrush current and modify acceleration starting torque characteristics.

Importance of Reduced Voltage Starting

• Direct application of full voltage to a motor at standstill causes high inrush current.
• Inrush can stress the electrical distribution network and incur financial penalties.
• Full voltage can damage mechanical components like belts, chains, and gears.

Types of Reduced Voltage Starters

• Methods Include:
  • Electromechanical primary resistors
  • Part winding
  • Wye-Start Delta-Run (Star-Delta Starter)
  • Solid state soft starters and motor drives
• Focus Today: Primary resistors

Primary Resistor Reduced Voltage Starter

• Mechanism:

  • Uses resistors in series with motor stator connections.
  • Reduces the voltage applied to the motor during start-up.
  • Initially reduces inrush current and starting torque.

• Key Concepts:

  • Ohm's Law: Voltage drop across resistors is proportional to current and resistance.
  • Kirchhoff’s Voltage Law: Total applied voltage is divided between resistors and the motor.
  • Closed Transition: Motor is never de-energized during start-to-run transition.

Operation

• Start-Up: Resistors in series limit current and reduce mechanical stress.
• Transition: At a certain speed, resistors are bypassed, allowing full voltage.
• Open vs. Closed Transition: Discussed differences; closed avoids de-energizing motor.

Limitations and Considerations

• Torque Reduction: Not suitable for all applications.
• Heat Dissipation: Resistors get hot; must be bypassed after start-up.
• Efficiency: Less efficient than motor drives but more cost-effective.

Control Schematics

• Ladder Logic Examples:
  • Operator-initiated start and run vs.
  • Automated transition with rotational speed switch.
  • Importance of removing subjective human judgment.

Advanced Configurations

• Multi-Stage Resistors: Stepwise limitation of inrush current.
• Rotational Speed Switches: Adjust transition timing based on system needs.

Graphs and Comparisons

• Comparison of inrush current and acceleration across different resistance values.
  • Full Voltage Starter: High inrush, fast acceleration.
  • 5 Ohm Resistor: Less inrush, longer acceleration.
  • 10 Ohm Resistor: Even less inrush, longer acceleration.

Conclusion

• Review: Reduced voltage starters reduce inrush current and modify starting torque.
• Primary Resistor Method: Uses series resistors; resistors bypassed once motor reaches speed.
• Further Learning: Explore use of timers in future lectures.
• Reminders: Regular review and practice enhance understanding.