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 Post subject: Motor Size
PostPosted: Wed Feb 18, 2009 6:49 am 

Joined: Fri Jan 30, 2009 9:17 am
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Does anyone have an idea what motor size needs to be included in the study? We have several large 480 MCC's that have a wide variety of motor sizes ranging from 3.5 HP up to several hundred. I have modeled the whole MCC in SKM, but which ones should I not include. Obviously maybe not the 3.5 hp motors, but what is a good stopping point. Plus, if we do have motor data and fuse/wire data for that MCC, why not put it in anyway? Thoughts on this?

Jack


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PostPosted: Wed Feb 18, 2009 6:56 am 
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Jnox wrote:
Does anyone have an idea what motor size needs to be included in the study? We have several large 480 MCC's that have a wide variety of motor sizes ranging from 3.5 HP up to several hundred. I have modeled the whole MCC in SKM, but which ones should I not include. Obviously maybe not the 3.5 hp motors, but what is a good stopping point. Plus, if we do have motor data and fuse/wire data for that MCC, why not put it in anyway? Thoughts on this?

Jack


Group them.


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PostPosted: Wed Feb 18, 2009 10:24 am 
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I would group the smaller motors <50 hp and leave separate the larger motors. With smaller motors there may be redunant motors so you can also put in a diversity factor and if they are not fully loaded, a load factor.

Some of your larger motors may have VFD's on them and if they are non-regen, then they don't contribute to the fault.

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PostPosted: Thu Feb 19, 2009 6:08 am 

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Grouping the motors sounds like a good idea. This may sound like a silly question, but when you guys say group the motors, are you talking about adding HP/Fuse sizes/wire lengths together?

Most of the MCC's contain motor overloads. Do we have to take that into account when performing arc flash study?


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PostPosted: Thu Feb 19, 2009 6:47 am 
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Jknox,

When grouping the motors, I add the hp, pick an appropriate wire/fuse size since the main purpose is to get the short circuit contribution from the individual motors. Does your software have an integrate feature for arc flash analysis?

Usually the overloads will not affect the arc flash as they are too slow to have any effect on the arc flash.

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PostPosted: Thu Feb 19, 2009 2:28 pm 

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I am using SKM to do DAPPER, CAPTOR AND arc flash.


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PostPosted: Thu Feb 19, 2009 7:08 pm 
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If you are talking of a MCC, and implementing the IEEE analysis method, then you rate the MCC section as one entity. It sounds as if you are rating the load side of the starter, downstream of the heaters. But when you open the bucket door, you also have exposure to the line side of the fuse disconnect or breaker. That line side has to be included. Since the line side is connected directly to the vertical bus without any other overcurrent device, the bus becomes the rating for all the buckets.

If I have a MCC that is MLO I rate the whole MCC lineup to the same rating. If the lineup had an incoming MCB, then I would label the MCB compartment to the line side rating, and the remainder of the MCC to the load side rating. But I would also implement special training so the techs no the difference between working inside a bucket versus taking off the back cover of a MCC (which is rarely if ever done).


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PostPosted: Thu Feb 19, 2009 7:34 pm 
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Motor Contribution

It was generaly accepted the A/C induction motors under 75 HP do not contribute enough to available currents to make a difference in the results. I would only include in the analysis motors of 75 Hp and greater and are likely to be running at a time of an Arc Flash. For instance a hydraulic system that has two motor/pumps one the main and the other the backup, only include one of them.
Synchronous motor have a greater effect on Arc-Flash the A/C induction motors.


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PostPosted: Fri Feb 20, 2009 7:53 am 

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RogO wrote:
It was generaly accepted the A/C induction motors under 75 HP do not contribute enough to available currents to make a difference in the results. I would only include in the analysis motors of 75 Hp and greater and are likely to be running at a time of an Arc Flash. For instance a hydraulic system that has two motor/pumps one the main and the other the backup, only include one of them.
Synchronous motor have a greater effect on Arc-Flash the A/C induction motors.


IEEE 1584:The study must take into account all sources, including utilities, standby and power generators, and large motors—those 37 kW and larger that contribute energy to short circuits.

37kW=50 HP.

I personnaly group <50 HP motors.


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PostPosted: Fri Jun 22, 2012 7:05 am 

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By grouping all motors less than 50hp together, wouldnt it result in an inaccurate interpretation of the situation seeing that each the curves for individual breakers would be replaced by a larger one where the kA and fault clearing time would vary?


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PostPosted: Sat Jun 23, 2012 3:33 am 
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Paper: what are you trying to analyse here, arc flash hazard at motor terminals? For upstream faults you don't need to model protective devices as they won't trip for arcing currents (or bolted fault currents for that matter).


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PostPosted: Sat Jun 23, 2012 4:29 am 
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Paper wrote:
By grouping all motors less than 50hp together, wouldnt it result in an inaccurate interpretation of the situation seeing that each the curves for individual breakers would be replaced by a larger one where the kA and fault clearing time would vary?


This has nothing to do with modeling them downstream of the MCC if that is your intent.

Generally they are grouped (add HP together) to estimate the contribution to arc flash due to the motor acting as a source of current (collapsing magnetic field/dumping energy stored in inertia) during an arcing fault. This only occurs for a few cycles. Larger motors have a larger effect individually so they are modeled individually.


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PostPosted: Tue Jun 26, 2012 6:28 am 

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Thanks for the replies.

After having performed a couple of test runs using SKM, the motor contribution to arc current for even a 50hp motor is far less than I had expected. Someone had mentioned that short circuit contribution from motors can be estimated at 6x the rated current. I imagine this is the reason why. I imagine this is the reason why unless I am totally off the ball.


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PostPosted: Tue Jun 26, 2012 6:03 pm 
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Paper wrote:
After having performed a couple of test runs using SKM, the motor contribution to arc current for even a 50hp motor is far less than I had expected. Someone had mentioned that short circuit contribution from motors can be estimated at 6x the rated current. I imagine this is the reason why. I imagine this is the reason why unless I am totally off the ball.


NEMA rated motors have a "Code letter" for LRA (locked rotor amps). This is the current that the motor pulls under locked rotor conditions after the coils have fluxed up. With small motors, the LRA can get pretty large but as you go up in horsepower (where it generally matters), it quickly comes down to around 6x FLA.

HOWEVER, watch out for IEC rated motors and energy efficient motors in general. They have no such limitations. In the quest to improve motor efficiency, the result is that the LRA gets very high. I've seen some Siemens motors get as high as 21x FLA and have struck them from the list of acceptable manufacturers for this reason...we can't even meet NEC Code for short circuit protection with those motors.


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PostPosted: Thu Jun 28, 2012 5:30 pm 
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There are two things to consider. The arc hazard at the motor (or local disconnect) and the motor contribution to the fault. You can lump the motors together to find the fault contribution, but you need to model the individual (or a typical) motor feeder fault current protection and cable impedance to find the arc hazard at the motor.


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PostPosted: Mon Jul 09, 2012 10:38 am 

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You can lump the smaller motors into a "lumped sum" calculation which typically uses 25% as the impedance for the lumped sum HP. This would be for the short circuit calculation portion. The 25% impedance (instead of the typical 17% for a single motor) included allowances for the lumped multiple wire and component impedances of the smaller motors.

The rest is my opinion and historical background:
I disagree with not modeling motors less than 50 HP. All motors should be included, either as individual models or as a lumped sum model. The practice of not including small motors was allowed in the ANSI standards developed for HV breaker short circuit calculations, when calculations were performed by hand. Today, using a computer program, only the lazy or ignorant would not include them in the model. In the real world, for example a textile mill, 1,000 - 1 HP motors installed on a bus would develop a fault current on a comparable basis to motors greater than 50 HP, when computed with the difference in impedances. The fault contribution from small motors can be significant when the quantity is sufficient.


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PostPosted: Mon Jul 09, 2012 3:44 pm 
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CSC wrote:
In the real world, for example a textile mill, 1,000 - 1 HP motors installed on a bus would develop a fault current on a comparable basis to motors greater than 50 HP, when computed with the difference in impedances. The fault contribution from small motors can be significant when the quantity is sufficient.


That's assuming ALL 1000 of the 1 hp motors are running at FLA at the same time on the same bus circuit which is highly unlikely.


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PostPosted: Mon Jul 09, 2012 4:23 pm 

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geh7752 wrote:
That's assuming ALL 1000 of the 1 hp motors are running at FLA at the same time on the same bus circuit which is highly unlikely.[/quote

Not true for a textile mill, which is the reference I cited. In fact, all motors running at one time, on a common switchgear bus, is the norm for a loom mill. Also, it does not matter if the motor is at full load or not, only if it is at full speed, in order to generate the full fault current. The predominance of fault current comes from the magnetizing energy stored in the motor windings.


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PostPosted: Tue Jul 10, 2012 6:53 am 
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A NEMA 1hp, 460V, 1750 RPM, 1.8 FLA motor produces 4A of fault current. Motor fault current decays to zero after the first 5 cycles.

Again assuming all 1000 motors fault at the same time and none of the starters have have PD devices (fuses, CB) and are across the line starters. No VFD's or line reactors.

So in a perfect world... not considering available system fault current, cable size, impedances losses, fuse or breaker clearing times... the worst case for 1000, 1 hp motors "lumped" is ~ 4kA Ibf peak for the 1st 1/2 cycle would be reflected back to the bus.

So... let's say all 1000 motors fault at the same time... that equals approx 2.6kA of Ia at a max of 5 cycles clearing time = 0.008 sec. or 0.062 cal/cm^@18" = 0 PPE.

This also assuming all 1000 of the motor t-leads are connected to a common point (or pretty close together) on the bus. Again... highly unlikely unless you have a real badly designed distribution system.

This is the trap of lumping motors <50 hp or lumping large quanities of low hp motors. It doesn't produce realistic results and doesn't change the incident energy enough to be significant.


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PostPosted: Tue Jul 10, 2012 10:06 am 

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To geh7752:

1. Not sure where you are getting "A NEMA 1hp, 460V, 1750 RPM, 1.8 FLA motor produces 4A of fault current", since both the NEC and NEMA Std. Pub. MG 1 give LRA as 15A, which must be multiplied by an asymmetry factor to get the fault current.

2. For the example I gave above, all motors are connected to the common switchgear bus for a bus fault, even though they may be downstream and fed from feeder breakers. They still contribute to the fault current. For example, one might have 4 MCCs connected to a switchgear bus. If the switchgear bus faults, the contribution from all the motors in those 4 MCCs add to the fault current at the switchgear bus.

3. The lumped motor fault current adds to and increases the incoming service fault current, so your calculation of 0 PPE makes no sense. Without the incoming power service and its fault current contribution, the motors could not have been running prior to the fault. For a supply source (through the transformer) contribution of 20 KA, the lumped motors could easily add another 5 KA, an increase of 25%. In industrial systems, it is normal for the running HP to be greater than the transformer KVA (they just aren't running fully loaded, but they still produce fault current).

4. There are cases where the lumped motor contribution is insignificant, when the total lumped sum of HP is very small. But then there are many cases where the lumped sum is significant. To give the blanket statement that the lumped motors are never needed, is erroneous and misleading, especially to those who may be new to fault current calculations. Not including the motors <50 HP would be choosing to ignore what actually exists in the power system in order give a lower fault current and potentially a lower PPE. IMO, that is not what an arc flash calculation should be doing.


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