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19 — Calculators

Calculate star-delta starting

Star-delta starting cuts the starting current to a third. That is the good news. The bad news sits right beside it: it also cuts the starting torque to a third. Design by current alone and you build a plant that hangs in star.

Input

From the nameplate; applies to direct-on-line starting in delta.

From the nameplate, likewise for direct-on-line.

Relative to the rated motor torque. A centrifugal pump needs little, a loaded conveyor a lot.

Result

Starting current in star
—A
Will the motor start in star?
—
Starting current direct on line
—A
Starting torque in starRelative to the rated torque.
—%
Saving compared with direct on line
—%

A guide, not a design to standard. We give no warranty for the correctness of the results, the applicable standards and case-by-case verification govern.

Worked example

The default case the calculator starts with, worked through once. Change the values above and it recalculates immediately.

Inputs

Rated motor current
55 A
Starting current ratio I_A / I_N
7
Starting torque ratio M_A / M_N
2.2
Load torque at start
40 %

Result

Starting current in star
128 A
Will the motor start in star?
Yes. The starting torque in star is well above the load torque.
Starting current direct on line
385 A
Starting torque in star
73.3 %
Saving compared with direct on line
66.7 %

Formula

  • I_A(delta) = I_N · (I_A / I_N)
  • I_A(star) = I_A(delta) / 3
  • M_A(star) = M_A(delta) / 3
  • Starting condition: M_A(star) > M_load

Assumptions and standards

  • In star connection each winding sees the voltage divided by root three. Current and torque therefore fall to a third of the direct-on-line values.
  • The nameplate figures apply to direct-on-line starting in delta.
  • Changing over from star to delta produces a brief current peak that can reach the direct-on-line starting current. It is not shown here and belongs in the design of the protection.

Frequently asked

When is star-delta unsuitable?

Whenever high torque is needed from the start: loaded conveyors, piston compressors, agitators in viscous media. It suits loads whose torque rises with speed — fans and centrifugal pumps.

What happens at the changeover?

The motor is briefly disconnected, the field decays, and reconnecting in delta produces a current peak. At an unfavourable instant it reaches the direct-on-line starting current — half the benefit is gone. Change over only once the speed is close to its final value.

Why exactly a third?

The winding voltage falls by root three, and with it the winding current. In addition, the line current in star equals the winding current, whereas in delta it is root three larger. Together that gives one third. Torque goes with the square of the voltage, so it also falls to a third.

Calculating is the easy part.

A formula gives you a number. Designing a plant also demands installation method, grouping, discrimination, the standards in force and a look at the installed base. That is what we take on.

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