FISEKON GmbH – Fischer Elektrokonstruktion

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20 — Calculators

Calculate soft starting

A soft starter lowers the voltage and ramps it up. Current follows linearly, torque quadratically — and that difference is the whole point. This calculator therefore also answers the question backwards: how far may you reduce before the load stalls?

Input

The starting voltage as a percentage of the system voltage. From there the device ramps up.

Relative to the rated motor torque.

Result

Starting current at this initial voltage
—A
Is the torque enough?
—
Starting torque at this initial voltage
—%
Lowest initial voltage for this loadBelow this the motor will not start.
—%
Starting current at that lowest voltage
—A

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
Initial voltage at the soft starter
50 %
Load torque at start
40 %

Result

Starting current at this initial voltage
193 A
Is the torque enough?
Yes. The torque is well above the load.
Starting torque at this initial voltage
55 %
Lowest initial voltage for this load
42.6 %
Starting current at that lowest voltage
164 A

Formula

  • I(U) = I_A · U / U_N
  • M(U) = M_A · (U / U_N)²
  • U_min / U_N = √(M_load / M_A)

Assumptions and standards

  • The calculation describes the instant of starting with the rotor at standstill.
  • A soft starter controls voltage, not frequency. The breakdown torque therefore falls with the square of the voltage — unlike a variable frequency drive, which maintains it.
  • The ramp time is not included. It determines the thermal loading of motor and device and belongs in their sizing.

Frequently asked

What is the advantage over star-delta?

The reduction is freely selectable rather than fixed at one third, and the transition is stepless — there is no current peak at changeover. In return the device costs more and produces losses in operation unless it is bypassed.

When is a soft starter not enough?

When high torque is needed at low speed, or when the speed has to be varied in operation. Only a variable frequency drive does either. A soft starter limits the start; it does not control.

Why show the lowest initial voltage?

Because that is the actual setting. Set too low and the motor will not start — it hums and draws current until the protection operates. The figure shows where the lower bound lies; a margin on top is advisable anyway.

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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33428 Marienfeld (Harsewinkel)
Gütersloh officeWagenfeldstr. 2
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