VSD Question 5

[JRLAKE]

I recently supplied a 250 HP pump to a quarry and included the pump manufacturer's 250HP VSD drive to control it.

At startup I learned that the electrical supply (lines, transformer, 0.5 s time delay fuses) were rated for 200 HP. I can control the max amp draw using the PLC on the VSD to prevent an overload. However, I am concerned that if a condition occurs suddenly in the pump to cause it to lock up, a sudden rush of current could cause problems. Basically, the motor is capable of drawing more amps than the transformer can handle and all that we have preventing this from happening is a setting on a PLC. I don't want the transformer to blow up, if this is possible.

I am a mechanical engineer and I am weak in the electrical area. The quarry's electrician says that my concerns are not possible because of the PLC setting that limits amp draw. However, this is the same guy that wired for 200 HP when 250 HP was stamped on the pump motor and VSD.

Do I have a legitimate concern?

 

[dV8r]

Fuses need to to be rated for Amps not HP, how many Amps does your motor draw at max load?

 

[JRLAKE]

This is part of my concern, that he sized everything assuming a "Typical Amp Draw" for a 200 HP motor which is 230 Amps.

The actual FLA for this 250 HP motor is 292 Amps. Everything in this system is rated for 250 Amps.

My concern is that the PLC is not fast enough to catch a sudden upset pump condition.

 

[davidbeach]

PLC isn't fast enough, but if the VFD has an amp limit function that may well be fast enough. Still though, it would be much better for the infrastructure to be upgraded for the actual motor supplied.

 

[CJCPE]

Any time a motor that is connected to a VFD suddenly has the load lock up, the motor is capable of drawing enough current to blow up the VFD. For that reason, it is a primary design requirement that a VFD must have a very fast capability to limit the output current and protect itself. If the VFD adjustment is changed to allow the motor to run at full load, the branch circuit protection should protect the under-sized transformer etc.

If you check the overload rating of the VFD, you will find that it can supply perhaps 150% of the motor FLA for 1 minute, perhaps less than that. If it sees more than about 200% of motor FLA for even a fraction of a second, it will shut down. If it can’t do that successfully, it will blow up.

As long as the branch circuit protection is sized to protect the transformer, conductors etc. feeding the VFD it doesn’t matter that it is under sized except that you can’t get rated power out of the motor.

 

[JRLAKE]

Thanks CJCPE. I appreciate the help.

 

[Ctrlfreak]

First off, it's obvious that this is not being installed to code. A 250Hp motor under normal conditions is rated at 302amps. I'm assuming 480Vac@60Hz. Sizing for protection is not done by nameplate values. You say the circuit feeding this drive is rated for 200HP(240Amp,at least 300kcmil Cu wire,or parallel runs)? Let's look at an example: Toshiba G7 running 300amps recommended parallel 2/0 input wires,and parallel 4/0 on the output. Pumps are very demanding of VFDs. If you program the VFD to limit current, what's going to happen is a lot of nuisance tripping. At the minimum you are going to need protect the VFD by semiconductor fusing. Basically it sounds like your electrician wants to use the VFD as a protection device. Looks like trouble ahead. Weight the cost as to which should be replaced.

 

[instrument1717]

The Primary Level of protection must be within The drive .
(Protections with External /other units (like PLC )are not an alternative for this .)
Yet ,One can have addtional secondary protections in external systems ( Like PLC ).
In the event of primary detection failure the secondary will act.But proper High speed Data logging is required for Trouble shooting (To Know about ... Who, when ,why .. made the trip )

The PLC can Perform Very fast Depending Up on the way it is programmed .

For Imediate performance ,Standrad PLc's provide Interupt Program routines with direct acces of periferal I/O's ( So Very Fast Performance is possible .)
If programmed for Cyclic scan ,Then the scan time Typicaly lies between 100ms to 500ms.

 

[davidbeach]

The PLC can Perform Very fast Depending Up on the way it is programmed .

However fast you can get the PLC to run, it won't function at protection speeds - 4ms scan rates - so anything that is depending on a PLC for electrical protection will become toast.

 

[JRLAKE]

How long does it take to toast a transformer?

 

[ozmosis]

JRLAKE
If you have a VFD between your transformer and motor then I really do not think you need to be worried about toasting your trx. As mentioned already, all drives have overload protection. The power transistors used in the VFD will have inherent protection and if all else fails,any number of devices in the VFD will fail before the transformer fails.
If there is a problem, your VFD will not wait for the PLC to tell it there's a problem, it will trip itself. I guess also you will have fuses/CB's between trx and VFD.

 

[JRLAKE]

The VSD does have built in protection devices, but it protects for anything above 300 Amps. I am told the transformer can only handle 250 Amps. Therefore the VFD won't "think" there is a problem between 250 Amps and 300 Amps unless the PLC tells it there is a problem.

I have 250 amp time delay fuses. However I have seen many VFD's toast with these fuses in place. But during these instances the transformers have always been correctly sized. Therefore my question is, if the same thing happens what goes first? The VFD (not a big deal) or the transformer (big deal)?

I am not worried as much about protecting the drive as I am protecting a person that may be standing by the transformer.

 

[instrument1717]

davidbeach,

I can agree with you only if the protection programms are put in cyclic scan ...In that case Typical scan time lies between 100ms to 500 ms.

But
It can perform very fast protection functions (while online) with an inturrupt programm which temporarily hault cyclic program scan ( Never to use as primary protection , but as secondary protection if needed. Primary protection should be self contained by the unit and should not be depended to functions of external devices.)

For an interrupt Driven progrm routine The performance time is too low , is equivalent or even better ( in case of speed )to a microprocessor based protection device.

The major functions with interrupts are as follows.

1. Hault current cyclic scan / sub system comunications.
2. Load accumulators stack.
3. Run Interrupt Routine to give the desired logic out put.

There after it comes back for continuation of its cyclic scan , whereit was halted .PLC -manuals have details about Timings for each logic operations .

So the PLC, if programmed specicifcally for protections , It can Perform . conventional microprocessor based protection devices are specific to aplication ,with less no's of I/O's and Built in softwares, So They have less functions to perform hence seems fast.


The PLC is not a design meant for unique functinal protection in power systems even though it can ,instead for low speed mutiple logical applicaions. Implementation of secondary protection (for units like drives) is an option for End user .

 

[CJCPE]

The transformer must be protected by fuses or circuit breakers in the primary and secondary. Whether it is overloaded a little or a lot by the VFD or a shovel shorts the wires, that protection should be expected to prevent toasting the transformer.

Fuses ahead of a VFD, even semiconductor fuses, are primarily intended to get the VFD disconnected from power without a fire if the electronic VFD protection fails. Semiconductor fuses may or may not prevent internal destruction of a VFD, but they should prevent the effects of VFD destruction from extending outside the VFD enclosure. If a VFD manufacturer can demonstrate to UL that ordinary fuses or a circuit breaker will provide the proper limitation of VFD destruction, they will often not recommend semiconductor fuses.

The original posting seems to imply that the VFD doesn’t need to run at rated output to produce the required maximum pump head and flow. If there is an adjustment that will prevent the pump from operating at a point that will overload the transformer, that should prevent nuisance fuse blowing. The only difficulty with that is that someone may someday decide that they want more flow and adjust the drive to get it. When fuses start blowing repeatedly, they may put in bigger ones without checking the sizing of the transformer, conductors etc. feeding the drive. That will ultimately cause the transformer to be toasted.

Electrical code probably does not permit under-sizing the branch circuit for a dive like this, but it might be allowed if the drive and motor were to be re-nameplated for the intended operating load.

 

[CJCPE]

JRLAKE:

When you say "I can control the max amp draw using the PLC on the VSD to prevent an overload," do you mean that there is literally a PLC that has overall control of the drive or are you referring to the drive's internal operational adjustments that are set using a keypad and display on the front of the drive? Even if a PLC has overall control, it is probably the drive that controls the speed of reaction to deviations from setpoints transmitted from the PLC.

 

[JRLAKE]

We have both actually. However I am referring to the drive's internal operational adjustments that are set using a keypad and display on the front of the drive (ABB ACS800).

 

[Drivesrock]

One useful fact is that the VFD input current should be less than the motor current due to the drive input being near unity power factor and certainly at motor speeds below 60Hz where the motor volts will be lower (proportionally if in straight forward V/f mode) even if the motor is drawing FLC. You may well find that the input current is much closer to 250Amps than you would have expected.

Drivesrock

 

[JRLAKE]

Does this hold true for direct torque control rather than scalar speed control?

 

[jraef]

VFDs on motor circuits pose a somewhat unique set of circumstances when it comes to circuit capacity. Because of that, it is necessary to separate current flow into two categories; Operating Current and Fault Current.

I seriously doubt that there is any programming in the PLC that is intended to provide circuit fault protection via the VFD. At best, you can do serial communications with the PLC to the VFD so that you can read and set limit values. That however would only be a replacement of commands, i.e. instead of doing it through the keypad, not a functional interactive protection scheme at the level of circuit protection per the NEC.

On the other had, the VFD can be programmed to limit the operating current beneath whatever value you need to avoid. For all intents and purposes, you can turn that 250HP motor into a 125 HP motor if you so desire, so making it perform like a 200HP is only a matter of knowing what you are doing with programming and what you are willing to live with in order to maintain that current (i.e. some drives do so by reducing speed). But this is a FUNCTIONAL issue, not a protection issue. You still cannot rely upon the VFD to be your Short Circuit Protection Device (SCPD) as per the NEC.

Now if you have provided the proper SCPD based upon your transformer capacity, it will be LOWER than what the VFD is capable of, which really isn't a problem in terms of protection. The VFD cannot "force" the circuit to deliver more current than what that SCPD will allow. So in your case, if you have put 250A fuses in front of the VFD, you are adequately protecting the circuit (I didn't check that with the NEC rules by the way), but you may end up with the fuses blowing on you under relatively normal operating conditions for a 250HP motor.

THAT IS something that you can control with the VFD! If you program the VFD to never allow it to deliver more than 250A, it will do so. The consequence may of course be that your motor is not going to be able to run your pump at full capacity, but YOU ALREADY HAVE THAT PROBLEM by having too small of a transformer. If there is a fault in the circuit downstream from the VFD which attempts to pull more current faster than the VFD can react, the 250A fuses may blow somewhat prematurely, but what harm is there in that other than the nuisance and replacement fuse cost?

JRaef.com
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[Bestboy]

Lateral thinking: - My concern about adapting the protection of 250Hp motor to that of 200HP is that years to come new electricians and engineers will replace the current crew. For some reason the load requirements might increase and a need for motor upgrade will surface. They guys will look at the motor and notice that it is underrated. They will possibly increase the overload settings to allow the motor to handle the additional load. And possibly they will forget to check the ratting of the transformer. I have seen similar happenings so many times. So please solve the current problem and think about the future as well.