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Freuquency variations

Freuquency variations

Freuquency variations

(OP)
Hi there,

I case of critical low frequency of a generator, say less than 47 Hz - would this impose a risk and why for destroying electrical equipment/components ?

What is the critical time duration for drop in frequency related to f.ex. motors ?
 
regards, hhhansen

RE: Freuquency variations

I am assuming that you are concerned with smaller islanded sets.
Motors, transformers and other inductive loads using iron cores don't like low frequencies. Inductive reaction is proportional to frequency. As the frequency drops so does the inductive reactance and current rises. Iron cores saturate and the current then rises abruptly.
But, the Automatic Voltage Regulator on many islanded sets has an Under Frequency Roll Off feature. UFRO typically kicks in at 3 Hz below rated frequency or 47 Hz for a 50 Hz set. Below 47 Hz the AVR's UFRO feature will lower the voltage in proportion to the frequency and maintain an acceptable Volts/Hertz ratio. This protects equipment by avoiding magnetic saturation of iron cores and lwers the load on the gen-set to allow the prime mover to more easily recover to the proper frequency.
Many AVRs have a jumper selection to set the UFRO feature for either 50 Hz or 60 Hz operation.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Freuquency variations

(OP)
Thanks a lot waross.
I can follow your point, but does your conclusions also apply to short duration of under frequency, say 3-5 sec. ?.

B.R. hhhansen
 

RE: Freuquency variations

Yes. Temporary overloads due to large motors starting is a common event on many residential standby sets in hot climates. When the last of several air conditioners kicks in, it is common for the set to pull down and drop speed and frequency until the compressor is up to speed. These events are typically within the 3-5 second range that you mention.
This may be one of the most frequent UFRO events. In an area where power outages occur several times a week, most owners are happy to accept a dependable standby set and accept the power dips as their air conditioners star back up. Given that a good diesel set may have cost $15,000 plus installation and a larger set that would still have smaller voltage dips when the A/C kicks in may cost an extra $3000 to $5000, many customers chose to live with the A/C starting voltage dips.
The UFRO got a workout almost each time an A/C started and worked well.  

Bill
--------------------
"Why not the best?"
Jimmy Carter

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