Peter C. Wallace wrote: > >Dont your output filter inductors always have to carry full motor current, >regardless of PWM rate? > >(Well at a really low PWM rate you would have more motor current ripple so the >peak current would be higher) > > Exactly the problem. At some DC supply voltage around 120 V the enductor ramps up to above 20 A in one PWM cycle (20 us). If the current limit is set to 20 A, then the limit is reached as the inductor is already in saturation, and it is a tough condition for the power transistors to turn off at that stress.
If you slow down the PWM frequency, then the current ripple will exceed 20 A peak even when the average motor current is far below that - at least with low inductance motors. The only solution to this is to increase the inductor value. At 50% duty cycle into a dead short, the P-P current ripple is a couple of amps, if you cut the frequency almost by half, it will go up quite a bit. Jon ------------------------------------------------------------------------- Take Surveys. Earn Cash. Influence the Future of IT Join SourceForge.net's Techsay panel and you'll get the chance to share your opinions on IT & business topics through brief surveys - and earn cash http://www.techsay.com/default.php?page=join.php&p=sourceforge&CID=DEVDEV _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
