Calculators
Calculate soft starting
A soft starter lowers the voltage and ramps it up. Current follows linearly, torque quadratically, and that difference is what matters. This calculator therefore also answers the question backwards. How far may you reduce the voltage before the load stalls?
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
Please fill all fields with valid numbers.
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.
The grey values are an example. Simply enter your own.
Matching services
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.
Method and questions
Soft starting
Current and torque at reduced initial voltage, with the lowest voltage that still carries.
Discuss a projectWorked 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 and standards
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 with 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.
What is the advantage over star-delta?
The reduction is freely selectable rather than fixed at one third, and the transition is stepless, with 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 but does not control.
Why show the lowest initial voltage?
Because that is the actual setting. Set it 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.
Contact
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