FISEKON GmbH – Fischer Elektrokonstruktion

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18 — 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.

Input

Given on the nameplate. For squirrel-cage motors it is usually between 5 and 8.

From the network impedance calculator. It determines how far the voltage dips at start.

At start the motor is strongly inductive. Values between 0.2 and 0.4 are usual.

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 — which is why a dip can prevent the start altogether.
—%

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
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

  • 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.

Frequently asked

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 — 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?

Either reduce the impedance — larger cross-section, shorter feeder, stronger transformer — or limit the starting current: star-delta, soft starter or variable frequency drive. Which route is right is decided by the load torque that still has to be overcome.

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)
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