In short. When a fault drops the system voltage, spinning motors act briefly as generators and feed current into the fault. The contribution is a multiple of motor starting current, decays within a few cycles, and matters for breaker making and breaking duty on motor-heavy boards.
Why motors contribute
An induction motor’s rotor flux persists for a short time after the supply collapses. The motor’s inertia keeps it turning, so it generates a voltage and feeds current toward the fault. Synchronous motors contribute for longer, since they have excitation.
How much
As a rough guide, the initial contribution of an induction motor is close to its locked-rotor current, typically five to seven times full-load current. A group of motors can be lumped together as an equivalent motor with the same total rating. Contribution decays quickly, usually in one to a few cycles for induction motors, so it affects peak and initial breaking current more than later values.
| Motor type | Contribution |
|---|---|
| Induction motor | Initial approx. locked-rotor current, decays in a few cycles |
| Synchronous motor | Higher, lasts longer with excitation |
| Motor with VFD | Usually negligible, since the drive blocks reverse flow |
| Small motors on LV | Often lumped as a group |
When it can be ignored
Standards allow neglecting motor contribution when motors are small relative to the source, or when the motor current is less than about one percent of the initial symmetrical current without motors. On boards dominated by large motors, or with a generator source, include it.
Where it matters
- Breaker making and breaking capacity at motor control centres.
- Busbar mechanical stress from peak current.
- Arc-flash incident energy at motor-fed boards, since contribution may last through the clearing time of fast devices.
- Protection: motor contribution flows through feeder breakers in the reverse direction. Directional or time-delayed settings must tolerate it.
How to model
- List motors by board with ratings and running status in the case studied.
- Choose individual or lumped representation.
- Apply the standard’s treatment for contribution and decay.
- Run maximum cases with motors, minimum without. See maximum and minimum.
Illustrative example
An MCC board has 600 kW of running motors at 415 V. The group’s total full-load current is about 1,000 A, so the initial contribution might be around 5,000 to 7,000 A. Added to the source contribution, it may push the board past its breaker rating margin. The study identifies this and recommends a higher-rated breaker or a series reactor.
Motors feed faults
When voltage collapses during a fault, a running induction motor’s stored magnetic energy and rotor inertia let it act briefly as a generator, feeding current into the fault. The contribution decays within a few cycles, but it adds to the first-cycle peak current and breaker duty.
Motor contribution ≈ motor starting (locked-rotor) current, or 1 ÷ Xd” of the motor
Rules of thumb (confirm against the standard)
| Motor group | Typical assumption |
|---|---|
| Large HV motors | Modelled individually with subtransient reactance |
| LV motor groups | Grouped, contribution about 4 to 6 times the group’s rated current |
| Small motors | Often ignored below a set size, per the standard |
Example (illustrative)
A board with 400 kW of running motors at 415 V has about 700 A of rated motor current. At 5 times, motor contribution is about 3.5 kA. If the supply contributes 20 kA, the motors add about 17 percent to the first-cycle level.
Where it matters
- Chiller, pump and compressor plants with large motor load.
- Generator-only operation, where the generator is weak and motor contribution is a larger proportion.
Modelling guidance
- Include only motors that run at the same time as the study case.
- Use manufacturer reactance where available.
- State the grouping assumption in the report.
Common mistakes
- Neglecting motors on motor-dominated boards.
- Including motors in the minimum case.
- Treating VFD-fed motors as contributing.
What to send us
Send the motor schedule. Send your application details. See also load schedule.
Questions engineers ask
Do all motors feed a fault?
Only those running at the time of the fault, and not those isolated by drives.
How long does it last?
Usually a few cycles for induction motors, longer for synchronous motors.
Does it affect relay settings?
Yes, mainly instantaneous and short-time elements on feeders, which see reverse contribution.
Can I lump all motors together?
For many studies yes, if they are on the same board and similar in type.
Values are illustrative. Use motor data from the schedule.