×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

over size generator alternator for VFD load???

over size generator alternator for VFD load???

over size generator alternator for VFD load???

(OP)
guys, when using generator as back up power. is it critical to oversize the alternator in the generator to run VFDs??? if so why??? I spoke to an VFD tech, and he mention about over size the generator and its got something to do with the high power source impedance of the generator being 6 to 8 times higher than the Utility ... does anyone knows what he mean??  

RE: over size generator alternator for VFD load???

Rectifiers like to draw current in short bursts near the peak of the voltage waveform. Generators do not like this. Marginally sized generators especially don't like this. The usual effect is severe flattening of the voltage peak, which is a result of the relatively high internal impedance of the generator source. A good AVR will go some way toward combatting the problem but if you need a clean sinusoid then a bigger machine is generally an option, as is a 12-pulse (or higher) rectifier, or a harmonic trap, or an active filter.
  

----------------------------------
  
If we learn from our mistakes I'm getting a great education!
 

RE: over size generator alternator for VFD load???

Yes - if you look at the current waveform there is about 60 degrees of dead time when the current is zero, then about 120 degrees where current flows. I think this drive likely has a DC link reactor which is causing the current to be drawn over a longer period than I expected, but that is possibly also an effect of the highly distorted voltage waveform. The voltage waveform is unusually bad - if you superimpose a sinewave of the appropriate scale onto the trace you will see how much the voltage crest has been flattened by the high current drawn near the voltage crest.
  

----------------------------------
  
If we learn from our mistakes I'm getting a great education!
 

RE: over size generator alternator for VFD load???

As the generators get bigger this issue diminishes because the regulators are more capable. What size VFD or motor are we talking about?

Keith Cress
kcress - http://www.flaminsystems.com

RE: over size generator alternator for VFD load???

(OP)
drives are 20Hp and the motors are 10hp

RE: over size generator alternator for VFD load???

So you are in the 20~40kW generator realm.  You would want to go larger.

Keith Cress
kcress - http://www.flaminsystems.com

RE: over size generator alternator for VFD load???

(OP)
the drive manufacture said that the generator impedance was too large relatively to the drives, thus limiting generators voltage devlier capablity, and since the drive has the voltage supervison settin, therefore the drive try to maintain the voltage, as tradeoff the current went down which explains the frequency drop.

how do I varify what he said is true ??? anyone got an idea???  

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources