hi chaps. i want to ask you about the role of the air-gap flux in the motors with regard to v/f control, everywhere it is written that v/f is kept constant to keep the magnetizing current constant, but why??? what is the relation between flux and torque? I also wonder the air-gap flux and torque relation , how can we show this by formulas..thank you in advance...
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about v/f control
- Thread starter turkoman
- Start date
Skogsgurra
Electrical
The flux is kept constant because there is no economy in trying to go beyond sturation flux in the iron. It is usually the flux in the back iron of the stator that gets saturated first.
The simple math is that flux in Vs/m2 is proportional to voltage times time (volts times seconds, hence Vs). Since the area of half a period of a sine wave is proportional to period length, it follows that you have to reduce V when period length is increased (which is identical to decreasing the frequency.
It is possible to elaborate that simple relatioship ad nauseam and many text-books do.
Gunnar Englund
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
The simple math is that flux in Vs/m2 is proportional to voltage times time (volts times seconds, hence Vs). Since the area of half a period of a sine wave is proportional to period length, it follows that you have to reduce V when period length is increased (which is identical to decreasing the frequency.
It is possible to elaborate that simple relatioship ad nauseam and many text-books do.
Gunnar Englund
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
- Thread starter
- #3
excuse me but i ask you what is the relation between the fact that we keep v/f and hence flux constant and we do a speed control , what if we dont keep it constant? cant we control the speed ? also pls tell me what about the relation between torque and the airgap flux? it is so difficult to solve this dilemma
Skogsgurra
Electrical
Speed control means that we change the frequency of the drive. Speed is proportional to frequency. A motor running at 1800 RPM at 60 Hz will run at 900 RPM at 25 Hz and so on. The speeds mentioned are at no load. When loaded, the speed decreases one or two percent, but speed is still essentially proportional to frequency.
Since we need to change the frequency to change the speed, we also need to keep the V/Hz ratio constant so that the motor operates at optimum flux at all frequencies.
There are many basic texts covering this. Just google on and be educated.
Gunnar Englund
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
Since we need to change the frequency to change the speed, we also need to keep the V/Hz ratio constant so that the motor operates at optimum flux at all frequencies.
There are many basic texts covering this. Just google on and be educated.
Gunnar Englund
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
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