Purpose

Select a motor starting method from load torque, acceleration, supply disturbance, thermal duty and control needs.

Learning objectives

• Compare DOL, star-delta, soft starter and VSD
• Assess voltage dip
• Check acceleration torque
• Account for starts per hour

A starter must accelerate the actual load without unacceptable voltage disturbance or motor heating while providing required control and protection. Reducing current often also reduces available torque.

Core theory

DOL applies full line voltage and generally provides the motor's highest line-start torque with high inrush. Suitability depends on supply strength, motor, load and network limits.

Star-delta reduces winding voltage during star starting to 1/√3 of delta-running winding voltage; ideal line current and torque are about one-third of DOL values, so it is unsuitable for loads needing high starting torque.

Soft starters control voltage and current ramp but do not provide continuous speed control. VSDs control voltage and frequency and can manage speed/torque, but add EMC, harmonic, motor-insulation, cooling, braking and safe-torque-off considerations.

Assumptions: DOL starting torque is approximately rated torque for screening.

1. Need: Load requires about 0.70 rated torque.
2. Available: Star starting screening torque is about 0.33 rated torque.
3. Compare: 0.33 is below 0.70 before acceleration margin and losses.

No; star-delta is unlikely to accelerate this load and another engineered method is needed.

Reasonableness check: Available starting torque must exceed load torque, not merely equal a current limit.

Knowledge check

Answer: Star starting voltage and torque are substantially reduced. Ideal torque is roughly proportional to voltage squared.

Practical exercise

Rank four fictional loads by starting-torque and acceleration-time needs, then shortlist methods with stated uncertainties.

Summary

• Starting is a system study
• Lower current can mean lower torque
• Stops and isolation are different