Calculators
Calculate starting current and voltage dip
The starting current is on the nameplate and quickly worked out. The interesting question is the second one. What does that current do to the voltage in the network? If the lights flicker and the controller reboots whenever the big pump starts, the answer is here.
Result
- Starting current
- —A
- Voltage dip at startRelative to the system voltage at the connection point.
- —%
- Starting apparent power
- —kVA
- Voltage remaining during start
- —V
- Remaining starting torqueTorque falls with the square of the voltage. That is why a dip can prevent the start altogether.
- —%
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
Starting current
What the motor draws when switched on and how far the voltage dips.
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
- System voltage
- 400 V
- Network impedance at the connection point
- 0.02 Ω
- Power factor at start
- 0.3
Result
- Starting current
- 385 A
- Voltage dip at start
- 3.18 %
- Starting apparent power
- 267 kVA
- Voltage remaining during start
- 387 V
- Remaining starting torque
- 93.7 %
Formula and standards
Formula
- I_A = I_N · (I_A / I_N)
- ΔU/U ≈ √3 · I_A · Z_network · sin φ_A / U
- S_A = √3 · U · I_A
- M_A(U) / M_A(U_N) = (U / U_N)²
Assumptions and standards
- The starting current is strongly inductive. What drives the voltage dip is therefore mainly the reactive component, and that is what is used.
- The network impedance is entered here. It covers everything from the supply up to the motor connection point.
- The calculation describes the instant after switching on, with the rotor at standstill. As speed rises the current falls.
- An estimate. The actual current curve against speed depends on the motor type and is not linear.
What voltage dip is still acceptable?
It depends on what else is on the same network. Lighting shows a few percent as visible flicker. Contactor coils typically drop out below seventy to eighty percent of nominal voltage, and then the installation switches itself off at every start. Network operators set their own limits at the point of supply.
Why is starting torque so sensitive?
Because it goes with the square of the voltage. Ten percent less voltage means nineteen percent less torque. If the voltage dips badly, the torque may no longer overcome the load torque. The motor stalls and keeps drawing starting current until the protection operates.
What helps against too large a dip?
You can reduce the impedance with a larger cross-section, a shorter feeder or a stronger transformer. Or you limit the starting current with star-delta, a soft starter or a variable frequency drive. Which route is right is decided by the load torque that still has to be overcome.
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