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 Post subject: Modeling Drives on Motors
PostPosted: Tue May 25, 2010 12:36 pm 
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Joined: Tue May 18, 2010 7:03 am
Posts: 53
I was reading a short circuit study by a large engineering firm. The author stated that only motors with drives that had bypass contactors were included in the study. Large motors on drives without bypass contactors were not considered because they did not contribute to fault current. Is this typical - wasn't covered in the class when defining starting devices.

If so, do you just open the disconnecting device in front of the drive before running the short circuity study? The model I am looking at has double ended 480V switchgear. Each 480V cubicle breaker feeds a starter panel (VFD) for a 200hp motor.

Thanks for any help.


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PostPosted: Wed May 26, 2010 4:37 pm 
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Location: Connecticut
I think you are confusing VFD's and softstart packages. Softstart type motor starting equipment have bypass contactors and are not true VFD's. Its ramps up (or down) a motor voltage at a given rate and at the end of the ramp rate the bypass contactor closes "bypassing" the softstart package and line voltage is applied to the motor. Hence the name softstart. The electronics in the softstart monitor the line voltage and amps.

In the event of a over current or fault condition the bypass contactor opens and the motor coasts to a stop. Any motor generated fault current is usually absorbed in the softstart braking resistors. One could argue that this fault current is not contributing to the line side fault current... so it becomes a moot point.

Therefore, in my experience with softstarts devices with bypass contactors I don't agree with the statement that drives with bypass contactor should be consider in the study. The only consideration is the AIC rating of the bypass contactor... that it meets or exceeds the available line side short circuit fault current.

VFD's continually modulate motor voltage and amps to vary the motor speed. Again, in the event of a motor fault, the motor generated current is absorbed by the VFD's dc bus and braking resistors and shouldn't make it back to the line side terminals.

In your case a 200 hp/1800 rpm motor generates about 1200 amps of peak fault current, that decreases to zero over about 5 cycles. As long as the VFD is sized for a 200 hp motor, properly fused, and meets the AIC line side available fault current, I wouldn't worry about bypass or no bypass for your sc study.


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PostPosted: Tue Jun 01, 2010 9:39 am 
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Thank you for the reply.


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PostPosted: Tue Jun 01, 2010 11:02 am 
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Joined: Wed Jan 28, 2009 2:19 pm
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Location: Georgia
Most AC inverter drives are non-regen types. This means that in a fault situation, current generated by the motor will not pass backward through the drive back to the supply grid. A large motor controlled by a drive will not affect fault current or arc flash beyond the drive because it will not contribute fault current back into the system.

If a drive has a bypass contactor, the fault current will pass backward from the motor through the contactor to the supply grid and will affect the arc flash energies.


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PostPosted: Tue Jun 15, 2010 7:11 am 
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Just an update. I spoke with tech support - When modeling the VFD, you can select the option to include motor contribution or not for the short circuit study. The software defaults to not include motor contribution.

Soft starts are modeled as standard starters unless you are doing a motor starting analysis. For short circuit soft starts are ignored.


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PostPosted: Sun Sep 19, 2010 12:35 pm 
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VFD's with bypass contactors and Soft Starts

Many years ago, when VFD's are not as reliable as they are now, it was commom practice to install a reversing starter (changing the output of the two contactors so they are the same phase rotation) to switch the output for a VFD onto the load (separate input power). When the drive failed, the load could still be run across-the-line at 60Hz by selecting the other contactor. I have not done that for probably 15 years except for recently with a critical load. The load is the forced draft fan for a boiler and it is a critical plant load. We are installing a 3 contactor (1 input/2 output) bypass panel that also has the VFD and output reactor in it. I will model this as an induction motor since the fan motor will contribute to the short circuit current when run in bypass mode with a damper, rather than through the VFD. I also model Soft Starts as induction motors in SKM since, after all of the staged contactors are through sequencing on start-up, you are then across-the-line and can contribute the typical ~5cycle duration short circuit current from the induction motor to the other short circuit components available at the faulted bus. SKM has a new VFD circuit element that allows for custom VFD electrical parameters (IEEE Red Book says VFD's typically don't allow motors to contribute to fault current, but they recommend checking with the manufacturer). I tried using the SKM VFD element a while back but discovered that several of the ASCII text report files (that I use!)disappear when you use it. Has any one else noticed that with SKM? I have not tried that again since the last couple of SKM update issues, though. They may have fixed that issue. I just got the SKM Hi-Wave module added to my SKM license to use for harmonics analysis and I suspect I will have to use the new VFD circuit element with it.


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