Star-Delta Starting Calculation in Electric Motors

The Star-Delta starter reduces inrush current in three-phase motors, minimizing mechanical stress during startup.

This guide offers detailed formulas, variable explanations, tables, and real examples for industrial motor applications.

Common Values Used in Star-Delta Starting Calculations

The following table contains typical motor specifications and their associated calculated values when using Star-Delta starting. These are based on common industrial scenarios with standard motor parameters.

Table 1: Common Motor Ratings and Star-Delta Starting Parameters

Note: The Star-Delta starting current is approximately one-third of the full-load current in Delta mode. Values are based on standard IEC 60034 assumptions.

Formulas for Star-Delta Starting Calculation

A correct design involves several interrelated electrical formulas. Below are the essential equations and variables for accurate calculation.

Full Load Current (FLC)

Common values:

Star Starting Current

• Istart= Starting current in Star mode

This formula assumes ideal conditions. In reality, motor type and torque characteristics can slightly change the ratio.

Starting Torque in Star

Line Current in Delta

But in Star connection:

Power Consumed During Start (Star)

Where:

Transition Time

There is no fixed formula, but per IEC 60947-4-1:

Typical transition time: 2–15 seconds depending on load inertia.

Motor manufacturers or field testing determine this time more precisely.

Practical Formula Summary Table

Real-World Example 1: 15 kW Motor at 400 V, 50 Hz – Water Pump Application

Motor Nameplate Data:

Step 1: Calculate Full Load Current (FLC)

Step 2: Calculate Star Starting Current

Step 3: Select Contactors (According to IEC 60947)

Contactors Required:

• Main Contactor (KM1) → must handle star current (approx. 8 A)
• Delta Contactor (KM2) → must handle full-load current (24 A)
• Star Contactor (KM3) → must handle phase current (24 ÷ √3 ≈ 13.86 A)

Standard used: IEC 60947-4-1 defines utilization category AC-3 for motor starters.

Recommended Contactors:

• KM1 (Main): 25 A AC-3
• KM2 (Delta): 25 A AC-3
• KM3 (Star): 16 A AC-3

Step 4: Set Overload Relay

Overload relays are selected based on full load current, with a margin of 5–10%.

• Overload range: 22 A – 26 A
• Setpoint: ≈ 24 A

Step 5: Verify Suitability

• Starting current is reduced to 7.87 A
• Motor torque reduced to ~33%
• Transition time: 6–8 seconds (measured during commissioning)

This setup is safe for low-inertia loads like centrifugal pumps, where reduced torque is acceptable.

Real-World Example 2: 45 kW Motor at 400 V – HVAC Fan System

Motor Nameplate Data:

Step 1: Calculate Full Load Current (FLC)

Step 2: Star Starting Current

Step 3: Select Contactors

Required contactors:

Recommended: Choose contactors with at least 10% headroom for thermal capacity.

Step 4: Select Overload Relay

• Range: 72 A – 85 A
• Trip Class: 10A or 10B (for HVAC fans)
• Ensure relay has phase-loss protection

Step 5: Evaluate Suitability

• Starting current limited to ~25 A
• Startup torque reduced, but sufficient due to fan’s low torque demand
• Smooth transition observed within 5 seconds

Ideal configuration for ventilation systems that operate in long duty cycles with soft starts.

Common Application Areas for Star-Delta Starting

Key Notes and Recommendations

• Use Star-Delta only when:
  • Motor is rated above 5.5 kW
  • Load allows reduced torque during start
  • Electrical network can’t handle direct-on-line (DOL) inrush
• Ensure:
  • Contactors are mechanically interlocked
  • Transition delay is appropriate to prevent current peaks
  • Correct thermal protection is applied
  • Bypass resistors are used when needed for smooth switching

External References for Deeper Study

• IEC 60947-4-1: Contactors and Motor Starters – IEC Webstore
• NEMA MG1: Motors and Generators – NEMA
• IEEE Std 112: Standard Test Procedure for Polyphase Induction Motors – IEEE Xplore
• ABB Technical Guide No. 1 – Motor Protection