Harmonics and FVD's

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djtazjr

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Now here is something that I really would like to have a answer too.

A FVD will cause harmonic distortions while operating to control any giving motor at any giving amp rating. What should an electrician do to accommodate for the harmonics, especially if you have a line of FVD's?

If the source voltage is 120/208v and the drive is for 10 HP motors, what calculations should be made for the conductors with harmonics in mind?

This is just a question not a real situation mind. I was reading the lasting issue of EC&M and this came to mind. If anyone would like to add to this situation or would like to carry this a little I would really enjoy the challenge (if it is indeed a challenge to all the experts in here) :D
 
roger

roger

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Re: Harmonics and FVD's

Basically an electrician wouldn't worry about it.

Not that we as electricians shouldn't be aware and knowledgeable of the problem.

This would be a design issue and could be accommodated by a K rated or an isolated (no neutral) transformer to serve just these loads.

I am assuming these motors at this size are strictly poly phase.

Roger

[ November 04, 2003, 08:04 PM: Message edited by: roger ]
 
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djtazjr

Member
Re: Harmonics and FVD's

Maybe an electrician shouldn't worry about this Roger, but what if there can be design flaw as we know there always is. :)

The motors will be poly phase by the way.I have been working with the schools down here and they use FVD's with there air handlers off of
chillers. Now I know they use K-rated Transformers but thats with there computers and electronic devices.
The system the Schools use is 120/208v Panelboard and there is mixed branch circuits with the FVD's; VAV's, damper controllers, EMC cabinets, 20 amp branch circuits suppling a variety of other things.

The point being, there is a large mix with the FVD's.

Keep in mind please that my question is just a scenario. This is something that I have been thinking about for a few.

Thanks for the reply Roger! More is appreciated.
 
roger

roger

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Re: Harmonics and FVD's

DJ,
but what if there can be design flaw as we know there always is.
Click to expand...
you are a brave soul. :D

Roger
 
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djtazjr

Member
Re: Harmonics and FVD's

Maybe I shouldn't have said that about designs. My honesty has gotten me into trouble, an so therefore I apologize now for offending anyone reading this Forum. :(

I appreciate your Post Ron and looking forward to it.

Anything further?
 
dereckbc

dereckbc

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Re: Harmonics and FVD's

If it is a poly phase system; what is the problem? Last time I checked a "K" rated transformer has a grounded circuit conductor (neutral) on the output.

I agree with Roger in part, it is a design issue. There are several ways to deal with it, all expensive, and requires a train driver. :D
 
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djtazjr

Member
Re: Harmonics and FVD's

Dereck,

The FVD's are not on a K-rated transformer.

Also, I am just curious trying to get some kind of satisfaction.

Isn't that how you better yourself? ;)

All the same I really do appreciate your info on this.
 
dereckbc

dereckbc

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Re: Harmonics and FVD's

The point I was trying to make is if the motors are poly-phase a K rated transformer would be of no use on the VFD system other than trap triplen harmonics, but you could get the same effect with a delta/delta isolation transformer. What could be done at the service is to divide and conquer. Make one feeder to the VFD, and another feeder to a K rated transformer to supply single phase loads, thereby isolating the VFD system and single phase loads. There are additional steps, but again expensive, such as filters or ferro transformers. Sorry for the confusion.

Dereck

[ November 04, 2003, 09:54 PM: Message edited by: dereckbc ]
 
dereckbc

dereckbc

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Re: Harmonics and FVD's

The ultimate solution if expense is of no concern is an M-G. Guaranteed to eliminate any harmonic problems generated by the VFD.
 
roger

roger

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Re: Harmonics and FVD's

Just make sure it's not an antique M-G with no schematics.

I did get it working though.

Roger
 
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tony_psuee

Senior Member
Location
PA/MD
Re: Harmonics and FVD's

Throwing in my opinion on dj's original question of what can an electrician do. I would say make sure you follow the VFD manufacturer's recommended instructions for grounding of the drive. In the brief few years I have been working with drives a large majority of the problems I have seen have been caused by improper grounding. I was recently at a job site with a cluster ground and when we separated it and followed the instructions in the drive manual the harmonic distortion was reduced by almost 50%. :eek:

It is an interesting topic I would like to be a sponge about it and absorb as much as possible.

Tony
 
charlie b

charlie b

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Re: Harmonics and FVD's

When my company issues a design package that includes motors with Variable Frequency Drives (VFDs), we throw the burden of harmonics mitigation on the VFD vendor. Our specs require that the vendor provide a method of preventing the VFD from inflicting (i.e., on the power distribution system) harmonic voltages or currents above specified limits. How the vendors choose to accomplish that task is their business. In other words, the need for harmonics mitigation is a design constraint, but the method of accomplishing mitigation is not a design issue.

There are several methods available. Just selecting a transformer with a K-rating is not one of them. K-rated transformers are over-built, so that harmonic currents would not damage them. That makes them heavier and requires more room and more ventilation, but does not make harmonics go away. I?ll not endorse any product line nor any technology, but I?m sure an Internet search could located several alternatives to K-rated transformers, alternatives for blocking, and not just tolerating, harmonic currents.
 
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j_braddock

Member
Re: Harmonics and FVD's

if i remember my vfd basics, all drives create harmonics. most suppliers have a do not exceede distance before requiring line reactors. when you go past this distance, the reflected waveform in most cases will damage/destroy the vfd unit itself. that is the reason for the reactor. if you implement a policy of all vfd require line reactors, your harmonic problem will be greatly reduced.
i know of one company that requires all motors 2 hp or larger have reactors.
hope this helps
 
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greg1

Member
Re: Harmonics and FVD's

Speaking of reactors I had to put external fans on them to cool them down. They are taking beating, especialy when undersized. Any preventive maintenance should include temp measurment.
Greg
 
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pwhite

Senior Member
Re: Harmonics and FVD's

if they are heating up, then they are probably rated standard duty. they should have been rated as heavy duty usage. this means that they are sized up one or two sizes for heavier on/off applications.
 
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mclain

Member
Re: Harmonics and FVD's

At our location we run 6, 1000 HP DC Torque motors off a SCR drive system. The electronics of the control system is designed to handle the harmonics and noise produced by the system. The transformers are oversized to compensate for harmonic current as well. The most noticeable problem is that a UPS installed on a desktop computer will only last about 6 month. One solution we are looking at is a rotating UPS. Which uses a electric motor to drive a generator to provide a separately derived source for computers and instrumentation. I would note that we generate our power from three 1050 KW generators.
 
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