variation of speed of synchronous motor due to frequency variation
variation of speed of synchronous motor due to frequency variation
(OP)
friends,
there r 2 big synchronopus motors of 16 MW and 12 MW in our plant which drives compressors.
The Grid here is not very stable thats why frequency variation is from 48 Hz to 51.5 Hz.Due to high frequency, speed of motor increases which causes it to get overloaded. To reduce the loading outlet damper requires throtelling which causes the production loss.
I want to know, is there any device available which can be used in this situation
there r 2 big synchronopus motors of 16 MW and 12 MW in our plant which drives compressors.
The Grid here is not very stable thats why frequency variation is from 48 Hz to 51.5 Hz.Due to high frequency, speed of motor increases which causes it to get overloaded. To reduce the loading outlet damper requires throtelling which causes the production loss.
I want to know, is there any device available which can be used in this situation





RE: variation of speed of synchronous motor due to frequency variation
RE: variation of speed of synchronous motor due to frequency variation
RE: variation of speed of synchronous motor due to frequency variation
This small a change in supply frequency should not affect motor operation. I would look for other causes.
Are your phase currents and voltages balanced?
Do you have high harmonics on the supply lines?
What else have you checked?
Lewis
RE: variation of speed of synchronous motor due to frequency variation
RE: variation of speed of synchronous motor due to frequency variation
50hz to 51.5 hz is a 3% increase in speed. Using centrifugal pump/fan laws, tthe power increases in proportion to speed^3
A 3% increase in speed corresponds to ~9% increase in steady state power and an approximate 18% increase in stator I^2*R heating temperature rise. On top of that, you may have to add the transient power required to accelerate the inertia from the lower speed (particularly if you swing rapidly from 48 Hz to 51.5 Hz).
RE: variation of speed of synchronous motor due to frequency variation
Mark Empson
http://www.lmphotonics.com
RE: variation of speed of synchronous motor due to frequency variation
RE: variation of speed of synchronous motor due to frequency variation
RE: variation of speed of synchronous motor due to frequency variation
RE: variation of speed of synchronous motor due to frequency variation
Buzz - Sorry for misinterpretting your meaning. I value your input on this board as I'm sure the others do. I only ask questions when I think I can learn something.
In the spirit of learning and exploring differences...
The range of frequency variation mentioned is very high in my experience. My indicated data from ERCOT grid in Texas, USA shows that frequency stays in a tight band of 60-60.1 [probably is 59.95-60.05] from December 2000 to present with only a few brief transients outside this range.
Are there any standards which govern acceptable ranges of frequency variation? (Don't tell me NEMA MG-1.... the allowance for frequency variations is not intended to imply motors should be allowed to operated above rated horsepower due to high frequency)
RE: variation of speed of synchronous motor due to frequency variation
Certainly from my experience, that is the limit normally applied for short term variations.
Mark Empson
http://www.lmphotonics.com
RE: variation of speed of synchronous motor due to frequency variation
I do not know where your plant is situated, but according to the large variation in frequency it seems to be situated in a small grid or in a developing country. Maybe you bought the equipment from an Eurpoean or American manufacturer who didn't know about the large variation of frequency at your site and designed to the standards applicable at his location.
The only solution which seems to be possible is to replace the rotor with three-phase wound rotor, install a small converter (3% of nominal power in your case) and operate it as double fed asynchronus machine.
RE: variation of speed of synchronous motor due to frequency variation
Lewis
RE: variation of speed of synchronous motor due to frequency variation
RE: variation of speed of synchronous motor due to frequency variation
Here was my reasoning:
- Current is approximately proportional to motor output horsepower (particularly near full load). => 9% increase in output horsepower equates to 9% increase in current.
- I^2*R heating is proportional to the square of current. => 9% increase in current causes 18% increase in I^2*R heating.
Temperature rise is proportional to losses (heating).
Approx 20% of full-load losses are constant (no-load) loss and the other 80% vary in proportion to I^2*R. As a rough approximation the losses (temperature rise) near full load can be approximated proportional to current-squared.
Watts is watts surely is true... but I^2R watts are not proportional to output watts.
RE: variation of speed of synchronous motor due to frequency variation
RE: variation of speed of synchronous motor due to frequency variation
Lewis
RE: variation of speed of synchronous motor due to frequency variation
http://lemewww.epfl.ch/ME/publications/1998/speeddrive_1998.pdf