Motor contributions

[hhsting]

I have project its high rise building where the electrical designer provided motor contributions estimates with shirt circuit study for 240V single phase hvac motors. All combined hvac motors of all the units makes difference. Their is also elevator motors 50hp, 30hp at 208V three phase.

The electrical designer used 4x full load amps to estimate the motor contribution fault currents.

I am wondering where it says in any code or standard full load of 4x is okay and conservative approach? I search internet and found the following:
ANSI standard C37.010 offers guidance when calculating motor contribution for a group of LV motors if detailed motor data is not available. Assuming a motor contribution of four times rated full load current is acceptable. The standard arrived at this value by assuming that the motor contribution of 3,6 times rated current came 75% from induction motors and 4,8 times rated current from 25% synchronous motors…..

However I cannot find this in standard C37.010
or is it in any other code or standards?

 

[RRaghunath]

Agree. 4x seems too low for LV motors (it may be OK for HV motors).
The high / premium efficiency motors are the norm these days for LV motors and these motors have high starting current in the range of 8-10x and even more in some cases.
So, the norm has to be different for LV motors.

 

[dArsonval]

A page from a dated Cutler-Hammer Consulting Application Guide

 

[che12345]

Dear Mr. hhsting
This information may not be applicable in your location.
However, the IEC standard (IEC 60909-0 Clause 8.3 and... ) are the opinion of the international community. Treat it as another source of information pertaining to the subject on motor contribution.
Che Kuan Yau (Singapore)

 

[dpc]

It's a reasonable assumption for small motors if it includes motor feeder impedance. Typical value assumed for induction motors in ANSI is 16.7% but this doesn't include motor feeder. You can refer to the old IEEE Red Book and Buff Books for supporting documentation. Also keep in mind that any motors fed via adjustable frequency drives generally will not contribute fault current (non-regenerative drives) or contribute at a reduced level (regenerative drives).

Also, different assumptions might be used depending on the purpose of the SC calculation.

 

[bacon4life]

Were 240 V to 600 V synchronous motors common? I though synchronous motors usually only came in medium voltage applications and clocks.

 

[Gr8blu]

IEEE 3002.3:2018 is the current "amalgamation" of information related to Short Circuit Studies and Analysis previously available as a scattered set of inputs within the original IEEE color books. The potential short-circuit contribution from synchronous machines is generally higher than asynchronous due to the presence of a separately-powered rotor - which would ultimately result in a slower decay of amplitude in the event of a fault condition. It references all three of the following standards within its discussion in Sections 7.3 (Synchronous) and 7.4 (Induction).

IEEE C37.010:1999 assumptions and calculation methods work sufficiently well when considering large (>37 kW) machines, regardless of speed. They are more accurate when constrained to medium voltage symmetrical fault applications. In effect, the calculation methods uses a modifier that is applied to the subtransient (X'') reactance of the machine (along with other multipliers) to more accurately represent the momentary amplitudes found in the time period where circuit protection (e.g. high-speed breakers) operate. The net result of these modifiers is to bring the "effective" contribution down to something close to 4x rated nameplate current. This is usually lower than the actual maximum contribution for a fault at the machine terminals, but is adequate when considering the additional impedance of other network components that probably lie between the machine and the fault location.

IEEE C37.13:1990 and IEEE C37.41:2000 deal more specifically with low voltage applications (e.g. low voltage circuit breakers and both low- and medium-voltage fuses). Again, these use an "average estimate" of likely machine impedances as part of the calculation, based on the likely "mix" of equipment. Historically, the mix has been 75% induction to 25% synchronous, leading to a roughly (4x total nameplate current) approximation for short circuit purposes. This method is also suitable for mixes that have a larger induction percentage, because these loads tend to be larger in rating as well - thereby raising the "induction" portion of the total current even as the synchronous portion is reduced.

For a clear description of how the X'' values and corresponding multipliers were arrived at, see the discussion in both C37.10 and C37.13. (This data is also summarized in Tables 7 and 8 of 3002.3:2018).

And to bacon4life: synchronous machines come in all shapes, sizes, and ratings - just like asynchronous (induction) and direct current designs. Likewise, not all machines were intended to operate at either 50 or 60 Hz, either - there are some very different power grids out there; both 25 and 400 Hz systems are fairly common (depending on the industrial application).

 

[Parchie]

@hhsting,
The easiest way to know the motor contribution is just that = motor FLA/motor per unit impedance or the inverse of the locked rotor amp per unit!
You said the designer used 4 X, that means the per unit impedance of the motor is 1/4 or 0.25 (25%)! If the motor in question has an LRA per unit of 5.59, then the per unit impedance of your motor is 1/5.59 =0.179 (17.9%)!

 

[che12345]

Dear Mr. hhsting (Electrical)
"... high rise building where the electrical designer provided motor contributions estimates with shirt circuit study for 240V single phase hvac motors.... Their is also elevator motors 50hp, 30hp at 208V three phase... The electrical designer used 4x full load amps to estimate the motor contribution fault currents...."
1. I make the presumption that 240V single-phase is fed from a different transformer from the 208V 3-phase. These two different transformers may be connected to the single source supply.
2. It is only necessary to [estimate] the short-circuit at 240V single-phase bar and the 208V 3-phase bar (respectively); taking [motor contribution] into consideration. (Motor contribution) may be [disregarded on the source bar].
3. The 240V single-phase motors may be numerous in number but are of low wattage say <5kW each. These small motor are wired with small conductors and may be varies say 20-50m in length. In motor contribution [estimation] the conductor Z is usually ignored. In this case conductor Z plays a major part in damping the motor contribution.
4. Taking it as an [estimation] where all the motors are of low wattage and precise data are not available or too tedious to be factored in; a suggested factor of 4 is justifiable/acceptable, irrespective whether the motors are of high efficiency or standard.
Che Kuan Yau (Singapore)