Fault Current Contribution of Motor is Signifacant in Short Circuit Calculations, and its calculations are defined in IEC 60900 and ANSI C37.010
During Short Circuit utility stable supply dips and Rotor continues to rotate due to inertia, at this time Rotating magnetic field in the rotor tries to support reduced voltage and there by becomes Power Source. Hence during this time reaction of Motor will be similar to Generator. At this time Motor will have Sub transient of 20 to 25%.
Motor Contribution for Short Circuit Fault :
- Asynchronous Induction Motor : 4 x Ifla-motor
- Induction Motor with Standard VFD : 0
- Induction Motor with AFE VFD : 2 x Ifla-motor
Prospective Short Circuit Current Trafo + Motor Contribution
| Transformer Parameters | 2500 KVA | 2000 KVA | 1600 KVA | 1250 KVA | 1000 KVA | 630 KVA | 500 KVA | 400 KVA | 315 KVA | 250 KVA |
|---|---|---|---|---|---|---|---|---|---|---|
| Line Voltage Volts | 433 | 433 | 433 | 433 | 433 | 433 | 433 | 433 | 433 | 433 |
| Impedance % z | 6.25 | 6.25 | 6.25 | 5 | 5 | 4.5 | 4.5 | 4.5 | 4.5 | 4.5 |
| Trafo FLA Amps | 3334 | 2667 | 2133 | 1667 | 1333 | 840 | 667 | 533 | 420 | 333 |
| Trafo Isc KA | 53.3 KA | 42.7 KA | 34.1 KA | 33.3 KA | 26.7 KA | 18.7 KA | 14.8 KA | 11.9 KA | 9.3 KA | 7.4 KA |
| Motor Contribution KA 25% Motor Load without VFD | 3.3 KA | 2.7 KA | 2.1 KA | 1.7 KA | 1.3 KA | 0.8 KA | 0.7 KA | 0.5 KA | 0.4 KA | 0.3 KA |
| Motor Contribution KA 25% Motor Load with Standard VFD | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |
| Motor Contribution KA 10% Motor Load with AFE VFD | 0.7 KA | 0.5 KA | 0.4 KA | 0.3 KA | 0.3 KA | 0.2 KA | 0.1 KA | 0.1 KA | 0.1 KA | 0.1 KA |
| Total Motor Contribution KA | 4.0 KA | 3.2 KA | 2.6 KA | 2.0 KA | 1.6 KA | 1.0 KA | 0.8 KA | 0.6 KA | 0.5 KA | 0.4 KA |
| Highest SC Fault KA | 57 KA | 46 KA | 37 KA | 35 KA | 28 KA | 20 KA | 16 KA | 12 KA | 10 KA | 8 KA |
Motor Contribution will be upto 4 cycles incase of Induction Asynchronous Motors. Whereas it will be upto 8 cycles for synchronous Motors.
Now a days it is now Standard Practice to use VFD for Motor, In this case VFD-Motor set will not contribute for Short Circuit Current.
However, when the VFD is with AFE – Active Front End, i.e Regenerative, it will contribute for short Circuit Current.
Prospective Short Circuit Current Generator + Motor Contribution
| Generator Parameters | 2500 KVA | 2000 KVA | 1500 KVA | 1000 KVA | 750 KVA | 500 KVA | 400 KVA | 250 KVA |
|---|---|---|---|---|---|---|---|---|
| Line Voltage Volts | 415 | 415 | 415 | 415 | 415 | 415 | 415 | 415 |
| Sub Transient X”d | 18 | 17 | 17 | 16 | 15 | 11 | 10 | 10 |
| Genset FLA Amps | 3478 | 2782 | 2087 | 1391 | 1043 | 696 | 556 | 348 |
| Genset Isc KA | 19.3 KA | 16.4 KA | 12.3 KA | 8.7 KA | 7.0 KA | 6.3 KA | 5.6 KA | 3.5 KA |
| Motor Contribution KA 25% Motor Load without VFD | 3.5 KA | 2.8 KA | 2.1 KA | 1.4 KA | 1.0 KA | 0.7 KA | 0.6 KA | 0.3 KA |
| Motor Contribution KA 25% Motor Load with Standard VFD | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |
| Motor Contribution KA 10% Motor Load with AFE VFD | 0.7 KA | 0.6 KA | 0.4 KA | 0.3 KA | 0.2 KA | 0.1 KA | 0.1 KA | 0.1 KA |
| Total Motor Contribution KA | 4.2 KA | 3.3 KA | 2.5 KA | 1.7 KA | 1.3 KA | 0.8 KA | 0.7 KA | 0.4 KA |
| Highest SC Fault KA | 23 KA | 20 KA | 15 KA | 10 KA | 8 KA | 7 KA | 6 KA | 4 KA |
Motor contribution is the Current feedback by Motor into System during short Circuit condition. Due to Rotor inertia, it keeps on running for some time, even if there is a deep in supply voltage, and now motor becomes source and feeds back in to network. This Amount of Current is dependent on the Motor Impedance. Duration of this condition is normally up to 4 to 5 Cycles.
To calculate highest vault of Short Circuit Fault at the main PCC Level, If we consider Infinite utility source to Transformer primary Plus 4 times Full load current of Motor as Motor Contribution to short Circuit fault, this will be highest possible Short Circuit Current that system will be able to feed for a Short Circuit Fault at any point in the electrical network.