Motor starting voltage dip calculator

Electrisim

Motor

Rated output P

Rated voltage

Line-to-line; the network voltage before the start.

Efficiency

Power factor at full load

Locked-rotor current I_LR/I_n

From the datasheet, typically 6–8 for standard motors.

Power factor while starting

Typically 0.2–0.4 at locked rotor.

Starter

Method

Direct on lineStar-deltaAutotransformerSoft starter

Autotransformer tap

Soft starter current limit

× I_n

Supply

Supply given as

TransformerFault level at the bus

Transformer rating

kVA

Short-circuit voltage u_k

Transformer X/R

Fault level at the supply bus S″_k

MVA

X/R at the supply bus

Cable and limit

Cable length

Parallel cables per phase

Resistance R

Ω/km

At operating temperature, e.g. 0.232 Ω/km for 95 mm² copper at 70 °C.

Reactance X

Ω/km

Allowed dip at the motor

Often 10–15 % at the motor terminals; check your project rules.

Voltage dip at the motor terminals

—

Voltage during the start: —

Check the inputs: positive values, power factors between 0 and 1, efficiency up to 100 %.

Voltage dip at the supply bus: —
Starting current, with the dip: —
Starting current at full voltage: —
Rated current: —
Starting power at full voltage: —
Starting torque, share of full-voltage locked-rotor torque: —
Supply impedance: —
Cable impedance: —

Steady-state estimate at the moment of starting, with the rest of the network unloaded. With star-delta, autotransformer and soft starters the voltage is on the supply side of the starter.

Free calculator by Electrisim · How it works