Please Help me Understand Motor Contribution in SCCR Calcs......

dionysius

Senior Member
Location
WA
Motors are NOT generators of electricity. They consume electricity. So what sense do these statements make which confuse me to no end as I read about Short Circuit Calculations?????? Can any of you help me to understand this.

PHP:
"Motor short-circuit contribution, if significant, should be added at all fault locations throughout the system. A practical estimate of motor short-circuit contribution is to multiply the total motor full-load current in amperes by 4. Values of 4 to 6 are commonly accepted"
PHP:
"Motor Contribution Limitation: The first critical requirement limits the application of a series combination rating where motors are connected between the line-side (protecting) device and the load-side (protected) circuit breaker.  NEC® 240.86(B) requires that series ratings shall not be used where the sum of motor full load currents exceeds 1% of the interrupting rating of the load-side (protected) circuit breaker."
PHP:
" Motor Contribution If motors are connected between the series rated devices, is the combined full load current from these motors less than 1% of the downstream circuit breakers’ interrupting rating (individual or stand alone interrupting rating) per 240.86(B)? "
 

GoldDigger

Moderator
Staff member
Motors are NOT generators of electricity. They consume electricity. So what sense do these statements make which confuse me to no end as I read about Short Circuit Calculations?????? Can any of you help me to understand this.

PHP:
"Motor short-circuit contribution, if significant, should be added at all fault locations throughout the system. A practical estimate of motor short-circuit contribution is to multiply the total motor full-load current in amperes by 4. Values of 4 to 6 are commonly accepted"
PHP:
"Motor Contribution Limitation: The first critical requirement limits the application of a series combination rating where motors are connected between the line-side (protecting) device and the load-side (protected) circuit breaker.  NEC® 240.86(B) requires that series ratings shall not be used where the sum of motor full load currents exceeds 1% of the interrupting rating of the load-side (protected) circuit breaker."
PHP:
" Motor Contribution If motors are connected between the series rated devices, is the combined full load current from these motors less than 1% of the downstream circuit breakers’ interrupting rating (individual or stand alone interrupting rating) per 240.86(B)? "
When you put a short circuit on a spinning motor it certainly does become a generator. For a short time.
Just as generator which is driven by an external power source will try to act like a motor and try drive the prime mover (engine) to over speed. No mass of iron and wire is *just* a motor or "just* a generator.
 

dionysius

Senior Member
Location
WA
When you put a short circuit on a spinning motor it certainly does become a generator. For a short time.
Just as generator which is driven by an external power source will try to act like a motor and try drive the prime mover (engine) to over speed. No mass of iron and wire is *just* a motor or "just* a generator.
Is this to do with the fact that a motor is acting as a coil or inductor???? So when the current changes very fast like the points and ignition on automobile a large emf or voltage is generated??? What is the short circuit you are "putting on the motor"......is it not the coil in the motor is shorting to case??? The short is not external???

Sorry for the dumb questions but this is not making sense and it is bothering me.
 

GoldDigger

Moderator
Staff member
The current contribution from a motor is in response to a fault somewhere else in the wiring system, not a fault in the motor itself.
If you have a ground fault of a single hot conductor to ground just after the service disconnect and no loads are turned on in the system the entire fault current comes from the POCO service transformer.
If the same fault happens while large motors are running, the same current goes into the fault from the service, but additional current comes from the motors.
The source of that current is that you have some rotating magnetism in the rotor cutting through the field coils of the motor. This current will continue until the rotor magnetism decays or the shaft stops rotating, whichever comes first.
 

iwire

Moderator
Staff member
Location
Massachusetts
Is this to do with the fact that a motor is acting as a coil or inductor???? So when the current changes very fast like the points and ignition on automobile a large emf or voltage is generated???
No.

For a short amount of time the motor acts like a generator.
 

topgone

Senior Member
Is this to do with the fact that a motor is acting as a coil or inductor???? So when the current changes very fast like the points and ignition on automobile a large emf or voltage is generated??? What is the short circuit you are "putting on the motor"......is it not the coil in the motor is shorting to case??? The short is not external???

Sorry for the dumb questions but this is not making sense and it is bothering me.
The energy still present in the rotating motor (stator winding still cutting the flux provided by the rotor) will flow into the faulted section until the flux decays and cannot export current. We're talking about fractions of second here!

The amount of fault current a motor gives were estimated to be 25% of its FLA (you divide the motor FLA by 0.25, and you get the motor contribution; or another way of saying is you multiply the motor FLA by 4; 1/0.2 = 4).
 

dionysius

Senior Member
Location
WA
The current contribution from a motor is in response to a fault somewhere else in the wiring system, not a fault in the motor itself.
If you have a ground fault of a single hot conductor to ground just after the service disconnect and no loads are turned on in the system the entire fault current comes from the POCO service transformer.
If the same fault happens while large motors are running, the same current goes into the fault from the service, but additional current comes from the motors.
The source of that current is that you have some rotating magnetism in the rotor cutting through the field coils of the motor. This current will continue until the rotor magnetism decays or the shaft stops rotating, whichever comes first.
Thank you for your input.

There are a couple of problems here as I see it. First the action time for the short circuit is just a cycle or two. So there is only a 1/30th of a second window for the crazy energy to flow before the show is all over. Then as a 2nd observation the energy from the POCO outstrips anything the motor rotation could supply during that short window. The motor contribution is the tail and the POCO is the dog. The tail does not wag the dog.

Thus I conclude that the spinning motor load acting as a generator is not worth considering in practice since it is relatively so minuscule.

You are trying to recognize stored energy in the system at a location which is downstream from the point of damage (the short cct). In my opinion that downstream player will never make it to the site of the damage with enough ammo since the site is already devastated by the big tsunami. The protection device upstream from the site has failed to do its job by first detecting and then blocking the tsunami fed from the POCO.

To make any sense here the motors would have to be so great as to be able to compete with the POCO generators. This is just never close to the case.
 

iwire

Moderator
Staff member
Location
Massachusetts
There are a couple of problems here as I see it. First the action time for the short circuit is just a cycle or two. So there is only a 1/30th of a second window for the crazy energy to flow before the show is all over. Then as a 2nd observation the energy from the POCO outstrips anything the motor rotation could supply during that short window. The motor contribution is the tail and the POCO is the dog. The tail does not wag the dog.
You are right, in most if not all cases the utility contribution is much greater but that is not important. You are trying to find the max current.

Let's say you have a bridge and it can support 10 tons. A utility truck weighing 9.5 tons is parked on it. Now a truck from a motor shop weighing 1 ton tries to cross it.

Does it matter that the utility contribution is much more than the motor shop?

Thus I conclude that the spinning motor load acting as a generator is not worth considering in practice since it is relatively so minuscule.
With respect your conclusion is wrong.
 

dionysius

Senior Member
Location
WA
You are right, in most if not all cases the utility contribution is much greater but that is not important. You are trying to find the max current.

Let's say you have a bridge and it can support 10 tons. A utility truck weighing 9.5 tons is parked on it. Now a truck from a motor shop weighing 1 ton tries to cross it.

Does it matter that the utility contribution is much more than the motor shop?



With respect your conclusion is wrong.
You paint a good picture here. It gives me an idea. Why not place the equivalent of a diode backflow preventer on the input side of the motor to block any backflow from that motor ????? In fact do that for all loads that store energy.
 

iwire

Moderator
Staff member
Location
Massachusetts
Why not place the equivalent of a diode backflow preventer on the input side of the motor to block any backflow from that motor ????? In fact do that for all loads that store energy.
Well AC current has to flow both directions so any simple method is out. Perhaps a device could be made to do this but it would literally have to work instantly for it to have an effect.

But why add complexity and parts to fail to a system when it can be handled with calculations?
 

dionysius

Senior Member
Location
WA
Perhaps a device could be made to do this but it would literally have to work instantly for it to have an effect.
The diode I describe is always acting to permit energy flow in the forward direction only. It takes ZERO time to act. It is like going through a turnstile with very fine granularity. The problem with the devices we have now is that they do not always detect the damage fast enough and they allow energy through before they shut the gate.

I must hit the sack now but thank y'all for the input so far.
 

Sahib

Senior Member
Location
India
The diode I describe is always acting to permit energy flow in the forward direction only. It takes ZERO time to act. It is like going through a turnstile with very fine granularity. The problem with the devices we have now is that they do not always detect the damage fast enough and they allow energy through before they shut the gate.
In that case you may consider providing current limiting fuse at any electrical faulted equipment to prevent further damage.
 

kwired

Electron manager
Location
NE Nebraska
The diode I describe is always acting to permit energy flow in the forward direction only. It takes ZERO time to act. It is like going through a turnstile with very fine granularity. The problem with the devices we have now is that they do not always detect the damage fast enough and they allow energy through before they shut the gate.

I must hit the sack now but thank y'all for the input so far.
I'm sure that can be done, but at a higher expense then to beef up the withstand rating to handle motor contribution in the gear in question.
 

Tony S

Senior Member
Is this to do with the fact that a motor is acting as a coil or inductor???? So when the current changes very fast like the points and ignition on automobile a large emf or voltage is generated??? What is the short circuit you are "putting on the motor"......is it not the coil in the motor is shorting to case??? The short is not external???

Sorry for the dumb questions but this is not making sense and it is bothering me.
You have to consider the residual magnetism in the rotor, it doesn’t just go away when the supply is cut. A short circuit on the supply lines will indeed open the OCPD but the motor still has kinetic energy to dissipate somewhere. Your figure of 4 to 6 times FLC ties in with the starting current of 4 to 6 times FLC.

Where it gets interesting is when you have a group of motors, think about it.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Engineer
The motor contribution is the tail and the POCO is the dog. The tail does not wag the dog.

Often motor fault current is 'injected' at multiple points in the system where the utility fault current is expected to be falling off due to impedance or current limiting devices.
 

ron

Senior Member
The diode I describe is always acting to permit energy flow in the forward direction only. It takes ZERO time to act. It is like going through a turnstile with very fine granularity. The problem with the devices we have now is that they do not always detect the damage fast enough and they allow energy through before they shut the gate.

I must hit the sack now but thank y'all for the input so far.
A VFD (that does not have a bypass) will act as such a diode.
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
A VFD (that does not have a bypass) will act as such a diode.
This has been the subject of debate, but yes, that is the general accepted concept at the moment, ASSuming the drive is not line regenerative. Technically, because there is a generic statement in IEEE standard 141-1993 (Redbook), section 4.2.5 that says "adjustable speed drives can contribute current from the motors to a short circuit...", most calculation software packages such as SKM Power Tools and ETAP will assume the worst (that he drive MIGHT be regenerative) and go ahead and leave the motor contribution in. But in reality if it is a simple VSI drive with a diode front-end rectifier, it cannot contribute fault current backward from the motor unless it completely melts down and becomes it's own bolted fault, which at that point becomes its own new problem.

As to the earlier concept of "just put a diode to block it" (sic), remember it's AC, so you would need TWO diodes to block anything, and then if it is blocked in both directions, nothing works! The VFD does because the diodes are configured as a RECTIFIER to DC, effectively blocking anything from going the other way. So technically it COULD be done if VERY motor were on a VFD (non-regen of course), something we VFD manufacturers would highly promote...:thumbsup::thumbsup::thumbsup::thumbsup:
 
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