152 kw Motor HP

[A-1Sparky]

So, from what I've read about vfds and from what you guys are telling me, the electronics in the vfd are going to limit the fault current and prevent it from traveling upstream. Then where does the motor hp come into play as far as an arc flash study is concerned? I'm thinking this engineer is going to plug the motor's hp into a software program to determine available fault current. Doesn't he also need to input the specs of the vfd? Or at least determine if the vfd limits the fault current to 1.5X or 2X the load current?

 

[JoeStillman]

If he is using SKM, he can enter the motor data in HP, Amps, kW or kVA. You don't have to enter motor data the same way for all motors if you are using the latest version.

Be careful what you are calling a "drive". Most chillers in this size come with soft starters. There is a limited amount of speed control, but it is only on startup. Most of these use SCR's instead of a diode-bridge and IGBT's. The difference is the SCR drive can allow the motor to contribute to the SC current and the diode-bridge cannot.

I would give the engineer the nameplate data and let him figure out the HP if he wants to. He should know better than to expect a chiller to be rated in horsepower.

 

[kwired]

It is indeed controlled by a vfd, but the engineer wants HP spec regardless.

Why can't this engineer take the information on the plate and figure it out himself?

From what I was told, the motor was not inverter-rated, but had been in use with a vfd for about two years. When this unlucky guy went to start it up, the windings shorted and caused the arc flash. Sounded plausible to me, but I am by no means a vfd expert.

VFD's typically have pretty sophisticated protection that will open the circuit before the current even reaches levels that would be destructive in the drive or motor - especially during starting. A sudden fault after the motor is to speed could have more violent results, but if there is a fault at startup, often the motor may not even have enough power applied to turn the shaft before the protection sees the fault and shuts the drive down.

 

[kwired]

So, from what I've read about vfds and from what you guys are telling me, the electronics in the vfd are going to limit the fault current and prevent it from traveling upstream. Then where does the motor hp come into play as far as an arc flash study is concerned? I'm thinking this engineer is going to plug the motor's hp into a software program to determine available fault current. Doesn't he also need to input the specs of the vfd? Or at least determine if the vfd limits the fault current to 1.5X or 2X the load current?

The motor is not going to contribute to available fault current if it is supplied through a VFD. Power can not flow backward through such drive like it can for a conventional transformer. If it does, the rectifier at the front end is destroyed for it to even be possible.

 

[A-1Sparky]

Why can't this engineer take the information on the plate and figure it out himself?

You would think that he should be able to. I just decided to take it upon myself to figure it out.

VFD's typically have pretty sophisticated protection that will open the circuit before the current even reaches levels that would be destructive in the drive or motor - especially during starting. A sudden fault after the motor is to speed could have more violent results, but if there is a fault at startup, often the motor may not even have enough power applied to turn the shaft before the protection sees the fault and shuts the drive down.

That makes sense.

 

[Jraef]

So, from what I've read about vfds and from what you guys are telling me, the electronics in the vfd are going to limit the fault current and prevent it from traveling upstream. Then where does the motor hp come into play as far as an arc flash study is concerned? I'm thinking this engineer is going to plug the motor's hp into a software program to determine available fault current. Doesn't he also need to input the specs of the vfd? Or at least determine if the vfd limits the fault current to 1.5X or 2X the load current?

It's not really that the electronics limit the current to any specific value, it's that the very nature of what a VFD is precludes the concept of reverse power flow from the get go.

A VFD uses the line power as a resource to CONVERT the AC to DC, then uses the DC as a new resource to convert the DC back into AC for the motor. If the spinning motor becomes an induction generator because of the loss of power on the AC side, as in the case of a fault, then the regenerated AC might go from the motor back into the drive as DC on the DC bus. But a standard drive has no ability to then recreate AC back into the line source, it is a one way trip. So the VFD just trips off on over voltage and the potential energy in the motor stays there. So when doing an Arc Flash study, the motors supplied by a standard VFD are considered non-contributory to the available fault current. That means you ignore them.

There are, however, an increasing number of what are called "line regenerative" drives, also called Active Front End (AFE) drives. They have an inverter at each end of the drive, usually so that any regenerated enery IS pumped back into the line, typically for energy recovery during braking. They are also often used for what is called Active Harmonic Elimination (AHE) as an alternative to other harmonic mitigation strategies. Regardless of the purpose, AFE drives MUST be looked at as contributors to fault current in an Arc Flash study because the motor energy CAN feed back to the line source. It may be that unless someone proves to him that the VFD is not AFE, he is taking the tactic of assuming the worst.

 

[A-1Sparky]

It's not really that the electronics limit the current to any specific value, it's that the very nature of what a VFD is precludes the concept of reverse power flow from the get go.

A VFD uses the line power as a resource to CONVERT the AC to DC, then uses the DC as a new resource to convert the DC back into AC for the motor. If the spinning motor becomes an induction generator because of the loss of power on the AC side, as in the case of a fault, then the regenerated AC might go from the motor back into the drive as DC on the DC bus. But a standard drive has no ability to then recreate AC back into the line source, it is a one way trip. So the VFD just trips off on over voltage and the potential energy in the motor stays there. So when doing an Arc Flash study, the motors supplied by a standard VFD are considered non-contributory to the available fault current. That means you ignore them.

There are, however, an increasing number of what are called "line regenerative" drives, also called Active Front End (AFE) drives. They have an inverter at each end of the drive, usually so that any regenerated enery IS pumped back into the line, typically for energy recovery during braking. They are also often used for what is called Active Harmonic Elimination (AHE) as an alternative to other harmonic mitigation strategies. Regardless of the purpose, AFE drives MUST be looked at as contributors to fault current in an Arc Flash study because the motor energy CAN feed back to the line source. It may be that unless someone proves to him that the VFD is not AFE, he is taking the tactic of assuming the worst.

First, thank you for the very informative explanation. Second, let's say that a fault occurs at the motor and the regenerated AC does go back to the DC bus. Is this fault current negligible? Or does the motor size and its characteristics come into play? Third, are chiller motors of this size typically controlled by AFE drives or traditional drives and why (JoeStillman mentioned earlier that it is likely controlled by a soft-start rather than a drive)? Thanks in advance.

 

[rcwilson]

First, thank you for the very informative explanation. Second, let's say that a fault occurs at the motor and the regenerated AC does go back to the DC bus. Is this fault current negligible? Or does the motor size and its characteristics come into play? Third, are chiller motors of this size typically controlled by AFE drives or traditional drives and why (JoeStillman mentioned earlier that it is likely controlled by a soft-start rather than a drive)? Thanks in advance.

If the fault is anywhere from the VFD to the motor or in the motor, there is no energy fed from the motor back upstream. The motor will act as an induction generator and feed energy into the fault.

The motor contribution should be considered in analyzing the arc flash at the VFD enclosure. The motor contribution will be the Locked Rotor Amps (LRA), typically about 5 x FLA (maybe 3.5 maybe 8) reduced by the cable impedance. But it won't last long. Compare that current with the possible fault current from the switchgear or MCC.

If FLA = 225 & LRA = 850% as shown on that Siemens data sheet, fault current = 1912 amps neglecting cable impedance. Fault from the source would probably be 10kA +. The motor might increase the fault current at the VFD by 10% at most. But in an arc flash that 5% or 10% could be important if the arc flash level is near a category break point.

 

[A-1Sparky]

If the fault is anywhere from the VFD to the motor or in the motor, there is no energy fed from the motor back upstream. The motor will act as an induction generator and feed energy into the fault.

The motor contribution should be considered in analyzing the arc flash at the VFD enclosure. The motor contribution will be the Locked Rotor Amps (LRA), typically about 5 x FLA (maybe 3.5 maybe 8) reduced by the cable impedance. But it won't last long. Compare that current with the possible fault current from the switchgear or MCC.

If FLA = 225 & LRA = 850% as shown on that Siemens data sheet, fault current = 1912 amps neglecting cable impedance. Fault from the source would probably be 10kA +. The motor might increase the fault current at the VFD by 10% at most. But in an arc flash that 5% or 10% could be important if the arc flash level is near a category break point.

Thanks for the clarification...I appreciate all of the information.

 

[Jraef]

If the fault is anywhere from the VFD to the motor or in the motor, there is no energy fed from the motor back upstream. The motor will act as an induction generator and feed energy into the fault.

The motor contribution should be considered in analyzing the arc flash at the VFD enclosure. The motor contribution will be the Locked Rotor Amps (LRA), typically about 5 x FLA (maybe 3.5 maybe 8) reduced by the cable impedance. But it won't last long. Compare that current with the possible fault current from the switchgear or MCC.

If FLA = 225 & LRA = 850% as shown on that Siemens data sheet, fault current = 1912 amps neglecting cable impedance. Fault from the source would probably be 10kA +. The motor might increase the fault current at the VFD by 10% at most. But in an arc flash that 5% or 10% could be important if the arc flash level is near a category break point.

Bob,
Why would one consider the motor contribution to an upstream fault if there is a non-AFE drive? There is no way for the motor's energy to get back to the line to contribute.
I agree that in the absence of information as to whether or not the VFD is AFE or it has a bypass starter, one must assume there might be one or the other and consider it, but if you know there is no AFE or bypass, isn't it ignored as non contributory? You do this sort of work, I do not, that's why I'm asking. My information comes from what others have told me in project reviews and seminars on Arc Flash calculations.

 

[Jraef]

First, thank you for the very informative explanation. Second, let's say that a fault occurs at the motor and the regenerated AC does go back to the DC bus. Is this fault current negligible? Or does the motor size and its characteristics come into play? Third, are chiller motors of this size typically controlled by AFE drives or traditional drives and why (JoeStillman mentioned earlier that it is likely controlled by a soft-start rather than a drive)? Thanks in advance.

If the fault happens at the chiller motor side, the VFD will trip off on Short Circuit Current, they are required to provide that now by UL. So once the drive is off, there is no more regen from the motor after the existing magnetic field collapses, which happens in a fraction of a second after power is removed by the drive.

Chiller motors USED to not have drives because applying a VFD to a chiller is a tricky business due to the physics involved in chillers. But the chiller mfrs have that all figured out now, so a lot of them ARE now coming with VFDs because the energy savings potential for them is huge. So if the mfr says it has a VFD, you have to take them at their word. There is no need to ever brake a chiller motor, so the only reason they might use an AFE drive is to mitigate harmonics. But an AFE drive is essentially 2 VFDs back-to-back, and the cost reflects this. The HVAC industry is well known for being as cheap as possible on everything, so if they are going to offer a VFD at all, it is likely the cheapest version they can get away with and let the end user deal with the harmonics issues after the fact. If you know the make and model number I can tell you immediately.

 

[jim dungar]

Bob,
Why would one consider the motor contribution to an upstream fault if there is a non-AFE drive? There is no way for the motor's energy to get back to the line to contribute.
I agree that in the absence of information as to whether or not the VFD is AFE or it has a bypass starter, one must assume there might be one or the other and consider it, but if you know there is no AFE or bypass, isn't it ignored as non contributory? You do this sort of work, I do not, that's why I'm asking. My information comes from what others have told me in project reviews and seminars on Arc Flash calculations.

As I read Bob's comment: The motor is a source of current for a fault internal to the VFD cabinet.

When doing fault calulations you need to consider (BTW this also applies to any electronic invertor such as a UPS):
1) a fault upstream from the VFD (no motor contibution through VFD)
2) a fault internal to the VFD (contribution from both source and motor)
3) a fault downstream from the VFD (negligible contribution from source, usually less than 1.3x rated output)

 

[rcwilson]

Jraef - I was responding to A1sparky's comment that implied the motor would backfeed power upstream for a fault in the motor. Of course, that is not possible. Energy from the spinning motor could only go into the fault at the motor, not upstream.

In doing Arc Flash studies, I do ignore contribution from motors on VFD's unless it is a regenerative drive, same as when calculating short circuit contributions (reference IEEE 551, paragraph 8.4).

When a VFD enclosure is included in the arc flash study, I determine the short circuit level at the input terminals not including the motor contribution. I check the arc flash level and if it is close to a PPE Level boundary, I check the arc flash including the motor.

 

[Jraef]

OK, I got it. Thanks.