Motor - Short Circuit Tab

Provides an easy way to enter ANSI Standard impedances and interrupting duty multipliers. Code numbers are chosen according to the motor types, sizes and modeling method. Regardless of the code chosen, ANSI Standard interrupting value multipliers are used.

The codes available in this list change depending on what you have selected in the Type field of the Specifications tab. Using the ANSI Code field is the recommended method to enter motor impedances to assure that the proper interrupting duty impedance multiplier is used for ANSI Standard calculations^1,2,3,4.

See ANSI Code Impedances for more information.

If the Adjustable Frequency Drive check box is selected in the Specifications tab, the following choices are available for the ANSI Code field:

• Non-Regenerative: Motor does not contribute short circuit currents to upstream faults.
• Regenerative: Motor contributes to upstream faults. The impedance multipliers are used as per Conrad St. Pierre’s A Practical Guide to Short –Circuit Calculations.

Grouped motors with an adjustable frequency drive are non-regenerative motors and do not have contribution regardless of the motor type.

Reference:
¹ AC High Voltage Circuit Breakers Rated on a Symmetrical Current Basis, ANSI/IEEE Std. C37.010-1979
² Calculation of Fault Current for Application of AC High Voltage Circuit Breakers Rated on a Total Current Basis, ANSI/IEEE St. C37.5-1979
³ Low Voltage Power Circuit Breakers Used in Enclosures, ANSI/IEEE Std. C37.13-1981
⁴ Calculation of Short Circuit Currents with Contributions From Induction Motors, Walter C. Huening Jr., IEEE/IAS Mar/Apr 1982

When this option is selected, IEC short circuit calculations are used. See IEC Impedance and X/R Calculations.

If the motor has an adjustable frequency drive, you can also select the option for a regenerative or non-regenerative drive.

Grouped motors with an adjustable frequency drive are non-regenerative motors and do not have contribution regardless of the motor type.

You can either type in the reactance to resistance ratio, or click Calculate to have EasyPower fill in this field.

Subtransient reactance in percent on the motor Hp base. Normally this is 16.7% for induction motors.

If you have the locked rotor multiple (LRM) which is the ratio of the lock rotor current to the rated motor current, then you can calculate the impedance in percent using 100 / (LRM).

Saturated zero sequence reactance in percent on the motor’s MVA base. Zero sequence values are used in ground fault calculations.

Click to have EasyPower fill the X/R and X" or Xlr fields. You can specify the average HP for grouped motors in Tools > Options > Equipment.

Grounding

Grounding impedances only apply to wye grounded connections. The units are R+jX in ohms. If you only know the ground amperes of the circuit, enter the amp class and use the Calculate button to calculate the grounding impedance.

Neutral ground resistance in ohms. Grounding resistors are usually given in amperes. The impedance is found from the following equation:

R = Vln / I

Only wye grounded motors (YG) are modeled with grounds.

Neutral ground reactance in ohms.

Click to have EasyPower fill in the R and jX fields (if the Amp Class is specified) or to fill in the Amp Class field (if R and jX are specified).

This is the current in amps through the ground impedance at the rated voltage. You can enter data in this field directly in amps or calculate it based on the voltage and ground impedance R +jX using Calculate.

Provides an easy way to adjust the total motor horsepower used in determining short circuit currents. By changing the percent connected, the actual Hp (total connected value) entered in the Hp field can remain static. This reduces modeling errors and eliminates multiple databases for different contingencies.

For example, an MCC has a group of induction motors (all over 50 HP) that add up to a total load of 600 HP. However, 300 HP is considered backup and is not on-line. In order to keep proper records of the MCC HP, 600 would be entered in the HP field. Since only 300 HP is spinning at any one time and can contribute short circuit current, the connected field is set to 50%.