Motor starting & VFD drives — starting current, busbar voltage dip, torque and drive harmonics

Two checks a factory or plant engineer cannot skip when a large motor is added. Block A checks motor starting: the locked-rotor impedance is injected into the real load-flow solution, so the busbar voltage dip at the instant of starting, the starting current multiple and the starting torque come from an actual network solution rather than a rule of thumb; the result is compared with the allowable dip of GB 12326 / engineering practice (15–20%). Block B checks the variable-frequency drive: the typical 6/12/18/24-pulse harmonic current spectrum gives the per-order harmonic currents and THDi, compared against IEEE 519 / GB-T 14549-1993 limits, and the PWM reflected-wave estimate gives the critical cable length, the drive end DC bus and the motor-terminal peak voltage against NEMA MG1. Enter nameplate data; every number carries its basis and standard number, and missing data is shown as a dash, never as zero.

Scope and limits: this page does not compute anything itself — it maps your inputs into engine requests and shows only the values the engine returns. Block A is a quasi-steady starting check: the starting impedance is a load in a steady-state load flow, so no electromagnetic transient is simulated and no rotor swing or starting time-current curve from motor constants is produced; the starting torque follows the engine estimate T proportional to the square of the starting current ratio. Block B is a table-driven average model using published typical and field-measured drive spectra, not a switching-level PWM simulation: the harmonic magnitudes vary with drive brand and model, so the drive manufacturer spectrum should be requested for a design submission. Where inputs are missing the engine returns unknown and this page shows a dash rather than an assumed value.