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Star-Delta Starter Calculator

Calculate reduced starting current

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Result
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How It Works

Star (Y) Start:
• Winding voltage = VL/√3
• Current ≈ 1/3 of DOL
• Torque ≈ 1/3 of DOL

Delta (Δ) Run:
• Full voltage applied
• Full current & torque
• Normal operation

Comparison

ParameterStarDelta
Current33%100%
Torque33%100%
Voltage/winding58%100%

Related

Motor CurrentTransformerkVA to Amps

Understanding Star-Delta Motor Starting

Star-delta (also called wye-delta or Y-Δ) starting is a reduced voltage starting method for three-phase induction motors. It reduces starting current to approximately 1/3 of direct-on-line (DOL) starting current, minimizing voltage dip and stress on the electrical system.

How Star-Delta Works

Starting Phase (Star/Y): Motor windings are initially connected in star configuration. Each winding receives line voltage ÷ √3 (about 58% of full voltage). This reduces starting current to ~33% of DOL current.

Running Phase (Delta/Δ): After the motor accelerates (typically 5-10 seconds), contactors switch windings to delta configuration. Each winding now receives full line voltage for normal operation.

Advantages

  • Starting current reduced to ~33% of DOL
  • Reduces voltage dip on supply
  • Less stress on motor windings
  • Simple and economical compared to VFDs
  • No power electronics required

Limitations

  • Starting torque also reduced to ~33%
  • Not suitable for loaded starts
  • Current spike during star-to-delta transition
  • Requires motor with 6 accessible terminals
  • Motor must be delta-rated at supply voltage

Frequently Asked Questions

In star connection, each winding receives line voltage ÷ √3 (58% of delta voltage). Since current is proportional to voltage, and line current in star is the same as phase current (vs. √3× in delta), the net effect is starting current of ~33% compared to DOL.

Starting torque is approximately 33% of DOL torque. Torque is proportional to voltage squared, so reducing voltage to 58% (1/√3) gives torque of (0.58)² ≈ 0.33 or 33%. This is the main limitation of star-delta starting.

Use star-delta for motors that start unloaded or lightly loaded, where reduced starting torque is acceptable. Good applications: centrifugal fans, centrifugal pumps (with open discharge valve), compressors. Avoid for: conveyors, crushers, loaded pumps.

Only 3-phase motors designed for delta connection at line voltage, with all 6 winding terminals accessible. For 415V supply, the motor must be rated 415V delta (or 415V/240V dual voltage). Motors rated 415V star cannot use star-delta starting.

There's a brief current spike (transient) when switching from star to delta. The motor, now running at reduced speed, suddenly receives full voltage. This spike can be 1-2× FLA. Closed transition starters minimize this by briefly connecting resistors during changeover.

Typically 5-15 seconds, depending on motor size and load inertia. The motor should reach 75-85% of full speed before switching to delta. Too short = high transition current. Too long = motor overheating. Timing is usually adjustable on the starter.

Star-delta: Cheapest, fixed 33% current/torque reduction. Soft starter: Adjustable current limit, smooth acceleration, ~3× cost. VFD: Full torque at any speed, energy savings, speed control, ~5× cost. Choose based on application needs and budget.

A motor rated 415V delta is designed for 415V across each winding in delta. In star, each winding sees 240V (415/√3), which is safe. But a motor rated 415V star is designed for 240V per winding - connecting it in delta at 415V would apply 415V per winding, damaging it.