Trickier than that. Depending on the corner point it may be able to develop the same hp at half speed w/ twice the torque. One would think that an induction motor that needed variable speed would be designed to top out somewhere on the const hp part of the curve than right on the corner point. Induction motors can naturally field weaken so that can have a 2+:1 const hp regions.
FWIW, permanent magnet machines can do the same but there are two 'gotchas' to be careful with. 1) Too much field weakening can demagnetize the magnets limiting the useful range and 2) if there is a fault and you have to turn the drive off while in this mode, the field weakening will also disappear and the back EMF will jump suddenly, rectify back through the bridge, and make the bus voltage jump (possibly by a factor of 2+!). You may need a way to mechanically disconnect the motor from the bridge in a fault to protect the vfd from overvoltage. Been there, done that, have the t-shirt! Another thing to note. Space Vector Modulation (SVM) is a bridge modulation scheme - a type of PWM. Can be used in V/Hz drives or flux vector drives which are both control methods. They are not directly related. You can have a V/Hz control using SVM or a flux vector control using something like hysteresis modulation. SVM is typically the superior modulation (most efficient). (neglecting state space control) SMD On Tue, Nov 3, 2015 at 2:12 PM, Przemek Klosowski < przemek.klosow...@gmail.com> wrote: > On Tue, Nov 3, 2015 at 10:02 AM, John Kasunich <jmkasun...@fastmail.fm> > wrote: > > > But of course the speed will be lower. For example, if it is designed > for > > 380V, 240Hz, 14,400RPM, > > the volts-per-hertz ratio is 380/244 = 1.5833. If the VFD can only > > deliver 208V, then you will be > > limited to 208/1.5833 = 131Hz, and the top speed will be 7860 RPM. It > > will still deliver rated torque > > at rated current, but since the speed is lower the kW will be lower - > only > > about 3.8kW. > > > > It's not my specialty so I am trying to go over the numbers > methodically---please check if I am getting it right. > At nominal 380V the thing runs at 6kW6kW * 208/380 and 60*240 Hz, i.e. > 14400 rpm. At 208V it should do 208/380*14400, or 7880 rpm, close enough. > But for power, P=omega*T, 6kW * 208/380, or 3.28 kW. amirite? > > ------------------------------------------------------------------------------ > _______________________________________________ > Emc-users mailing list > Emc-users@lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/emc-users > ------------------------------------------------------------------------------ _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
