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 Post subject: VFD's on TCC's... Do you model the motor inrush?
PostPosted: Fri Nov 09, 2018 7:53 am 

Joined: Thu Nov 08, 2018 7:07 am
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I have searched pretty extensively on some literature regarding the TCC of a motor and it's in-rush current when being controlled by a VFD. Now, I understand that the VFD's essentially eliminates the motor inrush, but is there a current accepted standard regarding VFD's on TCC's? I have some motor circuit protectors feeding to cable, then to a VFD, then to a motor. On the TCC, I have all of these shown except for the VFD. As (was) typical, I have the motor inrush curve shown (I use ETAP 12.5/14/16). I know that the actual motor inrush with a VFD doesn't quite match the shown in rush (at 8x), so what do you guys do to account for the VFD?


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 Post subject: Re: VFD's on TCC's... Do you model the motor inrush?
PostPosted: Fri Nov 09, 2018 5:15 pm 

Joined: Tue Mar 20, 2018 11:55 am
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I'm not sure for ETAP, but SKM includes functionality to perform a transient motor starting study utilizing a motor controller of your choice. The study would generate a specific motor damage curve based on the method of control. In short, you would select a VFD motor controller and input your particular drive settings. The study would generate a motor damage curve, and you would include this curve on your TCC drawing in lieu of a standard motor damage curve.


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 Post subject: Re: VFD's on TCC's... Do you model the motor inrush?
PostPosted: Sat Nov 10, 2018 10:36 am 
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With a VFD the TCC is much more flexible because you can basically dial in the VFD to limit current. For example with a large multimeter common bus system I was involved with recently (14 roughly 2000 HP motors organized into 3 axes plus 2 more on a fourth switched axis) to put it bluntly it was trivially easy to exceed pretty much any realistic feed to the system. So we dialed it in including some fiddling around with cable impedance to give a reasonably robust system under most conditions. But that's not a typical system.

The first thing to recognize with VFD's is that the fusing and/or circuit breaker protecting the front needs to be very fast in the event of a short circuit fault. If you do not do this, it will quite literally launch pieces of components out of the drive and if the cabinetry isn't sufficient (good luck determining this), it WILL penetrate metal covers and launch pieces out of the drive. This is no joke and no place to play around. The VFD manufacturer will have very specific requirements here.

Second consideration is whether or not there is a bypass contactor present with a starter. In this case you can just model the motor as a motor and forget about worrying about the PLC. This is really important for arc flash and less so for short circuit considerations. This is a common practice in chemical plants and especially water/waste water plants but the value of the extra contactors is questionable in new applications...not arguing reliability concerns here just how to model it.

Third consideration is the overload/overcurrent side of things and this gets into TCC's and coordination. If it's a bypass system as mentioned above then just model it as a conventional starter and we're done. But if it's just a VFD then the VFD can be tuned as mentioned in the first paragraph to almost an arbitrary size. It's here that we have to consider the application. For instance conveyors pull about 140% of their CEMA rated horsepower for about 30-40 seconds on longer conveyors until they get up to speed while fans and pumps for instance pull current proportional to roughly RPM(RPM so they don't hit a peak until maximum output...assuming that the user doesn't want to sort of "overexcite" and drive it at maximum torque to get it up to speed as fast as possible...this is important in high inertia large fan wheels which take a while to get there. Either way, consider the VFD application first to determine requirements and TCC shape.

Finally under fault conditions in a VFD-only application the VFD can pull about 150-500% of name plate for a period of time (it's in the documentation) and this is primarily a controls issue...a gate chip fries and stays locked in so the VFD shorts but in silicon, so the semiconductor (it is after all not really a conductor) limits current. This might be something to avoid tripping on (or not).

Generally if you just size it as a motor circuit as far as TCC's go you won't get into trouble. But if you try to aggressively limit current sooner or later you will start nuisance tripping. This might be a benefit though in say generator powered applications where the whole reason for soft starting or VFD operation is to current limit to avoid buying a huge generator. But as far as modelling goes....it's easy to just pick what you want it to do and then set your TCC for the application rather than being limited to the downstream equipment's characteristics.


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