Transmission & Grid calculator
Induction Motor Starting Current & Torque Calculator
Calculate induction-motor RMS winding current, slip, air-gap power and torque from a stator-referred per-phase equivalent circuit.
GridBasis: Stator-referred per-phase induction motor circuit
Results
- Enter values to calculate
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How to use this calculator
At startup an induction motor has slip s = 1. Its rotor branch draws substantial current, while the resulting torque must overcome load torque and accelerate the coupled inertia. This calculator evaluates the steady equivalent circuit at a chosen speed, including the magnetizing branch.
The result is RMS phase current, not the first asymmetrical peak. Starting protection must allow the actual acceleration time while respecting hot/cold thermal withstand and locked-rotor limits. A current multiplier alone does not establish an appropriate relay setting.
Formula
ns = 120 f / poles; s = (ns−n)/ns; Z1 = R1+jX1; Z2 = R2′/s+jX2′; Zm = jXm
Zparallel = Zm Z2/(Zm+Z2); I1 = Vphase/(Z1+Zparallel)
Vgap = Vphase − I1 Z1; I2′ = Vgap/Z2; Pag = 3 |I2′|² R2′/s
Te = Pag/ωsync,mechanical; ωsync,mechanical = 2πns/60; Pdeveloped = (1−s) Pag
Use RMS phase voltage and stator-referred rotor parameters. Wye: Vphase = VLL/√3 and Iline = Iphase. Delta: Vphase = VLL and Iline = √3 Iphase for a balanced winding set.
Worked example
Use the sample 480 V wye supply as 277.128 V per winding, 60 Hz, four poles and zero rotor speed. Synchronous speed is 1800 rpm and slip is 100%. Change speed toward 1800 rpm to explore the motoring region. Nameplate and manufacturer test data are needed to replace the illustrative impedances.
The sample circuit gives approximately 265.856 A RMS phase current and 158.009 N·m electromagnetic torque.
Assumptions and standards
- Balanced sinusoidal steady equivalent circuit; constant parameters; no core-loss resistance, saturation or deep-bar frequency corrections.
- Mechanical losses are excluded: developed torque and power differ from shaft output.
- Starting duration and relay thermal damage are not calculated.
Calculation model checked: November 4, 2026
Frequently asked questions
Why is motor starting current high?
At standstill slip is one, so the rotor branch has low effective impedance compared with normal running. The exact current depends on the motor equivalent circuit and supply impedance.
Can I enter line-to-line voltage?
Only for a delta winding, where winding phase voltage equals line-to-line voltage. For wye, divide line-to-line voltage by √3.
Can I use these results for final design?
Use them for preliminary design and checking. Final designs must be verified by a qualified engineer against the code edition and standards adopted where the work is installed.
Is my data sent anywhere?
No. Every calculation runs in your browser. Nothing you type is uploaded or stored on our servers.
References and further reading
Power system calculations: formulas and model guide