Generates a starting set of motor protection settings — thermal overload, overcurrent, earth fault, negative sequence, locked rotor, under-current and voltage — from motor nameplate data.
Standard method — stated scope limits apply
Motor Nameplate
Starting
CT & System
Enter motor data and generate.
About This Calculation
An induction motor's biggest protection challenge is that its own normal starting current — typically 5–8 times full-load current — looks electrically identical to a stalled-rotor overload condition, except for duration. Motor protection has to correctly tell these apart: allow the high inrush during a legitimate start (thermal capacity curve), but trip quickly if the rotor genuinely stalls or the motor is overloaded for too long, since sustained high current at zero or low speed dumps heat into the rotor and stator far faster than the motor's steady-state thermal design accounts for.
Frequently Asked Questions
Why can’t a simple instantaneous overcurrent relay protect a motor?
It would trip on every normal start, since starting current is genuinely several times full-load current for a few seconds — the relay has to distinguish a normal, time-limited starting transient from a genuine sustained overload or stall, which is exactly what a thermal (I²t) or motor-specific thermal-model relay does instead of a simple instantaneous element.
What is locked-rotor (stalled) protection and why is it time-critical?
A stalled rotor draws locked-rotor current indefinitely with zero cooling airflow from shaft rotation — rotor bar and stator winding temperature rises far faster than during a normal start, since there’s no acceleration to bring current back down. Locked-rotor protection uses a much shorter time limit than the normal starting-current allowance specifically because of this.
Why does motor protection need negative-sequence (unbalance) protection?
A small voltage unbalance across the three phases produces a disproportionately large negative-sequence current in the motor, which creates a counter-rotating flux and induces significant extra heating in the rotor — a motor can overheat from unbalance alone even while total current looks acceptable on a simple overcurrent relay.
What is the difference between thermal overload and true RMS overload protection?
A basic bimetallic thermal overload responds to average current heating effect reasonably well but doesn’t distinguish between fundamental current and harmonic-rich current from VFD supply or unbalance. A true RMS or motor-thermal-model relay measures actual heating-equivalent current more precisely, which matters increasingly as VFD-fed and unbalanced-supply motors become common.