Induction motors as a short-circuit current source — back-feed under IEC 60909

The [guide on short-circuit current calculation using the IEC 60909 impedance method](/guides/nen-1010/kortsluitstroomberekening-iec-60909-impedantiemethode) covers the general calculation method for the initial symmetrical short-circuit current. This article covers a contribution that is easily overlooked: running induction motors that briefly act as a current source themselves at the moment a fault occurs.

Why a motor briefly feeds current back

A running induction motor contains a magnetic field maintained by the rotor current, generated by the slip difference between the rotating stator field and the mechanical rotor speed. When a short circuit occurs near the motor and the system voltage suddenly collapses or drops sharply, the rotor — due to its mechanical inertia — keeps turning for a moment at nearly the same speed, while the magnetic field in the rotor is still present. The motor briefly behaves like a generator at that moment: it feeds a decaying current into the fault, supplied by the magnetic energy stored in the rotor.

Why this contribution decays quickly

Unlike the contribution of a synchronous generator (see the guide on subtransient and transient reactance of generators), which is sustained by an active excitation system and therefore persists much longer, an induction motor has no external excitation source — the rotor field simply dies out once the stored magnetic energy is exhausted. The contribution of an induction motor is therefore characteristically short-lived: typically already strongly damped within a few system cycles after fault inception, with a time constant considerably shorter than that of a synchronous generator.

When IEC 60909-0 requires this contribution to be included

IEC 60909-0 specifies that the combined contribution of induction motors must be included in the short-circuit current calculation once their combined contribution to the initial symmetrical short-circuit current exceeds a significance threshold set by the standard, relative to the short-circuit current without the motors. In an industrial installation with a large amount of connected motor power — for example a compressor room, a pump station, or a large climate installation in greenhouse horticulture — the combined contribution of all running motors can easily exceed that threshold, even if no individual motor appears significant on its own.

How the motor contribution is modelled

For the calculation, an induction motor (or an equivalent group of motors) is modelled using a subtransient reactance, derived from the ratio between the starting current (locked-rotor current, see also the guide on measuring starting current) and the motor's rated current. The higher this ratio, the lower the effective reactance and the greater the motor's contribution to the peak value of the short-circuit current immediately after fault inception.

Practical relevance for circuit breakers

The motor contribution is particularly relevant for the making capacity (Icm) of a circuit breaker close to a group of large motors: the first peak current after fault inception is higher due to the combined contribution of the network and the motors together. For the breaking capacity at a slightly later moment (for example after the intrinsic time delay of a relay), the motor contribution has usually already largely decayed and is therefore less determinant — unlike the network's own contribution, which remains intact throughout the fault. This distinction between the effect on the peak value (Ip) and on a later breaking current is similar to, but not the same as, the distinction made for converter-fed sources, where the contribution is instead limited by the converter's control system rather than by a physical decay process.

Practical relevance

When carrying out a short-circuit current study for an installation with substantial motor power (for example a production hall with several large drives, or an agricultural business with an extensive climate installation), it should be checked whether the combined motor contribution exceeds the IEC 60909-0 significance threshold, and if so, this must be included both in determining the required making capacity of circuit breakers close to the motors and in assessing the selectivity of the associated protection devices.

Common mistakes

Treating motors purely as loads in a short-circuit current study, without recognising that they briefly supply current themselves during a fault.
Ignoring the motor contribution because no individual motor appears large, while the combined contribution of many smaller motors can still exceed the IEC 60909-0 significance threshold.
Treating the motor contribution with the same time constant as the contribution of a synchronous generator — the motor contribution decays considerably faster and is therefore mainly relevant to the peak value, not to a later breaking current.
Dimensioning the making capacity of a circuit breaker close to a large motor group without including the motor contribution — this can result in a breaker facing a higher peak current during a real fault than it was dimensioned for.