3-ph Acim For Variable Frequency.
Posted 13 December 2007 - 04:21 PM
1. Use a current controlled DC source and that will protect our IGBTs in case, we make error in code.
2. Use any 3-phase AC motor available in market, that is of lower power (for cost reason).
The issue that we face is that our DC power supply can supply max upto 200V, while the final application needs 350V DC bus. hence for our testing, we thought of running the motor upto a particular frequency, so that we do not reach the "field weakening zone".
The motor that we got from market is rated for 230VRMS/50Hz (just a domestic motor and not a VFD type). And while we tested the motor, it does not generate enough torque. My assumption for the problem is: -
1. Motor wound for 230V/ 50Hz has thinner windings and more turns (high resistance and high impedence) and this makes it suitable for mains voltage and mains frequency.
2. If a motor is meant for variable speed, the rating of motor will be from 25V/18Hz to 350V/250Hz, and hence the winding should be of thicker winding and lesser turns.
3. And hence if we want to test our software by loading the motor, we should procure a motor which is suitable for variable frequency (upto our application frequency of 250Hz) and not just any domestic motor (rated for fixed 50Hz).
My query is -- Am I right in my assumptions above?
Posted 22 December 2007 - 09:21 PM
Welcome to the forum.
If the motor is not developing enough torque, it is due to insufficient flux in the gap, or the motor is too small.
The motor should develop rated torque at all speeds below rated speed provided that you have sufficient gap flux.
Either you are expecting a torque higher than the motor is capable or, or you algorithm is not delivering sufficient flux at a particular speed.
The sensorless vector algorithms should compensate for any resistances in the stator and rotor so they will not be an issue. That is the reason behind the use of the technology. If the resistances did not exist, we could use a standard V/Hz system and achieve high torque at low frequencies.
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