Short Circuit MVA at Motor Specification sheet

Hv&Lv

Senior Member
I'm seeing "short circuit MVA at Motor bus" in Motor specification sheet? What is the significance of this?
What is Min Short circuit MVA and Max short circuit MVA
That’s you bolted fault current ratings
 

RD35

Member
As Hv&Lv stated, this refers to the fault current contribution the motor could make in the event of a short circuit happening on the line side of the motor feed. The motor will momentarily act like an A/C generator and contribute power to the short circuit making the event even worse. This usually only lasts for a cycle or two before the motors' magnetic fields collapse and die off. This is why fault current calculations include motor data.
 

MyCleveland

Senior Member
As Hv&Lv stated, this refers to the fault current contribution the motor could make in the event of a short circuit happening on the line side of the motor feed. The motor will momentarily act like an A/C generator and contribute power to the short circuit making the event even worse. This usually only lasts for a cycle or two before the motors' magnetic fields collapse and die off. This is why fault current calculations include motor data.
RD35
Are you sure this is the mtrs contribution or is it the min/max that the system can impose on the mtr itself without damage.

20-40MVA is a large range/value.
OP-what is the operating voltage and motor HP ?
 

RD35

Member
If I am missing something I apologize. I believe the system simply supplies voltage to the motor at a certain level and frequency. The motor resists that voltage via its inherent inductances/impedances thereby regulating amperage based on rotor speed. So if those MVA numbers are anything else, I don't know what they could be. I was thinking the range (20 vs 40) might be due to the motor being reconnectable for different voltages possibly. Could also be due to the need for both min and max number for different types of calculations. I hope an expert in this field will chime in and educate us all. I would also like to know more about this nameplate data range and what it is used for.
 

RD35

Member
Oh and I forgot to mention I was also assuming the "M" in MVA did not stand for "Mega". Figured it was Roman for (x1000).
 

Julius Right

Senior Member
The min.MVA is required in order to limit the voltage drop to 15%.
The max.MVA is required in order to limit the starting current [to maximum 92% for instance] with respect to infinite system power.
Let's say the rated voltage is 6 kV and the motor rated power is 450 kW,0.85 eff,085 p.f.[cosfi] then Sm=450/0.85/0.85≈623 kVA motor apparent power.
Let's say the start current is 5.5 times the rated. Then Smstart=5.5*623=3426.5 kVA=3.43 MVA.
The supply system impedance will be:
For min.MVA Zmax=6^2/20=1.8 ohm
For max.MVA Zmin=6^2/40=0.9 ohm
Motor impedance at start Zm=6^2/3.43=10.5 ohm
At start at Zmax Istart=6/sqrt(3)/(1.8+10.5)=0.282 kA
AT D.O.L. start the voltage drop [up to motor terminals] =sqrt(3)*1.8*0.282=0.8792kV[0.8792/6*100=14.65%.
For less than 20 MVA the voltage drop will be more than 15%
If we take the supply system power as infinite [Zsys=0] then start current will be [at D.O.L]=6/SQRT(3)/(0+10.5)= 0.33 kA.
If we limit the supply system power then start current will be [at D.O.L]=6/SQRT(3)/(0.9+10.5)=0.304 kA [0.304/0.33=92%]
 

MyCleveland

Senior Member
Charz
Please post additional detail on the MFG, serial number, voltage, HP.
I would like to contact factory for an explanation.
 

paulengr

Senior Member
That’s data from the customer, not motor manufacturer. Like telling you it is hazardous location, indoor installation, type of grounding, etc. you need the short circuit rating for the starter. AIC of the starter needs to exceed that. Has nothing to do with the motor except that it can affect starting and that it will affect the short circuit rating by contributing to it. That’s all starter data. During the first couple cycles of a fault the magnetic field in the motor core has to go somewhere...it causes a decaying transient on the motor bus until it drains away. The ratings are short circuit, not starting. Can be used for starting calculations though,

You typically get multiple ratings from scenarios. So if I have a generator, I get two scenarios. I might also want one with no motors running (recovery from power outage) and a “normal” load. Main-tie-mains add even more. So usually the power engineer just takes the worst and best cases.
 

paulengr

Senior Member
That’s data from the customer, not motor manufacturer. Like telling you it is hazardous location, indoor installation, type of grounding, etc. you need the short circuit rating for the starter. AIC of the starter needs to exceed that. Has nothing to do with the motor except that it can affect starting and that it will affect the short circuit rating by contributing to it. That’s all starter data. During the first couple cycles of a fault the magnetic field in the motor core has to go somewhere...it causes a decaying transient on the motor bus until it drains away. The ratings are short circuit, not starting. Can be used for starting calculations though,

You typically get multiple ratings from scenarios. So if I have a generator, I get two scenarios. I might also want one with no motors running (recovery from power outage) and a “normal” load. Main-tie-mains add even more. So usually the power engineer just takes the worst and best cases. Ccc
 

steve66

Senior Member
Wow. Answers are all over the place, and the all sound plausible.

Seems like a simple parameter - "Short Circuit MVA at motor bus", but there are different ways to interpret it.

Manufacturers aren't very good at making it clear exactly what they are talking about with any details that are even slightly non-standard.
 
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