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 Post subject: Modeling for aggregate motors in SKM
PostPosted: Tue Dec 19, 2017 5:06 pm 
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Let's say I've got 15 motors all rated under 20 HP on a single bus. I have lumped them into one combined motor totaling 150 HP. How do I model the protective device PD since this load is theoretical? There is no specific PD manufacturer and model, and there are no specific relay settings. It doesn't seem correct to exclude a protective device. So what can I do? Thanks for the discussion! -Josiah-


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 Post subject: Re: Modeling for aggregate motors in SKM
PostPosted: Wed Dec 20, 2017 8:26 am 
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There is absolutely no need to include a protective device. It serves no purpose.


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 Post subject: Re: Modeling for aggregate motors in SKM
PostPosted: Wed Dec 20, 2017 8:39 am 
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A protective device interrupts current flowing from the motor to the fault. Without it, we get an unrealistic length of time for fault current contribution. Could you please expound on your comment, JBD?


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 Post subject: Re: Modeling for aggregate motors in SKM
PostPosted: Thu Dec 21, 2017 10:15 am 
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Josiah Halverson wrote:
A protective device interrupts current flowing from the motor to the fault. Without it, we get an unrealistic length of time for fault current contribution. Could you please expound on your comment, JBD?


You do not have a single motor. The fault currents from the group of motors will not be flowing through a single device.

Motor contribution, at 480V, is all but negligible until you get to single motors >100HP. Small motors contribute fault current for only a few cycles.

Have you tried running any simulations?


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 Post subject: Re: Modeling for aggregate motors in SKM
PostPosted: Thu Dec 21, 2017 11:00 am 
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Case1: Lumped motor, no relay, Bus energy is 6.2 Cal/cm^2
Case2: Lumped motor with relay, Bus energy is 7.47 Cal/cm^2
I think we can call this difference negligible. I'm going to take that puppy out and not worry about it. Thanks for the discussion, JBD.


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 Post subject: Re: Modeling for aggregate motors in SKM
PostPosted: Thu Dec 21, 2017 12:54 pm 
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Are you modeling this as 1 large motor or multiple motors of the same size? For example (1) 100HP or (10) 10HP.

Make sure you understand, and accept, the affect on the short circuit contribution, before you try interpreting AF results.

I always run AF cases with and without the motors. Many people are surprised to find their AF incident energy can be significantly higher when all the motors are 'off-line, like during maintenance.


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 Post subject: Re: Modeling for aggregate motors in SKM
PostPosted: Fri Dec 22, 2017 12:51 pm 
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ANSI and other standards for calculating short circuit current recommend modelling motors over a certain size (generally 100 HP or larger) individually and small (generally 25-99 HP) motors by summing their HP together and modelling as a single lumped element. This takes into account the assymetrical fault current so your approach is correct so far. This models the fault contribution of the motor inductances but it's not how you model the overcurernt protective device at all.

I'm not really sure what you are trying to do here. In an industrial control panel or an MCC the incident energy concern is at the incoming busbars/cables, NOT between the motor breaker/fuses and the starter or outgoing cables. So you model that and the motor loads are simply there to represent a load. The upstream over current protective device will be modeled using the actual device characteristics. It is possible to have say ten 50 HP loads (500 HP) but a breaker set for only say "200 HP" in the event that diversity applies (not all motors run simultaneously) but again the same concept applies so it doesn't affect the outcome of the study.

If you to model say a starter with a secondary disconnect connected to a small motor (20 HP) and want to model incident energy at the secondary disconnect then the "load" is a single 20 HP motor. Otherwise you're lumping different busses together.


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 Post subject: Re: Modeling for aggregate motors in SKM
PostPosted: Thu Dec 28, 2017 1:41 pm 
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Motor contribution to a fault diminishes over time as the motor spins down. SKM can be set to take this into account. Because of this any motor OCPD will (almost?) never operate based on the energy provided into a fault on the other side of the OCPD due to the motor energy as it generates into the fault.

For us, we model 50HP and larger motors individually and lump all smaller motors. We use the OCPD for the 50HP+ motors to look at motor protection and coordination, but we don't concern ourselves with their effect on incident energy.


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