On Monday 03 February 2003 12:29, somebody wrote > Declan Moriarty schrieb: > > On Sunday 02 February 2003 13:28, somebody wrote > > > > > > The Dc motors had running current of ~80 - 100A with stall curent > > limits at ~400A and the A.C ones are slightly more powerful, but I > > have no specs at all. I would like to know enough to specify an > > A.C. motor controller. Specifically, the max current per phase and > > current limit to equate with the figures above. That's what I need > > to know. > > > > Here's my analysis: > > > > The Ac motors will have a higher applied voltage, because > > 1. Driving with a squarewave instead of a sinewave raises the rms > > value of the waveform. > > 2. The fact of 3 phases instead of 1 also adds to the power > > applied, reducing current. > > 3. A further increase in that many of the DC motors were series > > wound (ie field in series with armeture). I believe the > > > > Speed is not an issue: The DC motors were geared to suit the > > dynamics of the motor. It appears the A.C. motors were downgeared > > by comparison, as this allows higher frequency lower current > > operation. I can soon confirm that. > > mech. ize of motors is similar on the bigger ones. Except small DC > servodrives (high power for short times) are much smaller the AC3~ > > What is approx. motor power ? > > > I would proceed as follows: > > 1. determine the power of the former DC motor, might be its labeled > on the device. Or just Pel = U * I as you have DC. Keep in mind that > the labeled power is mechanical power on the shaft so you have to > correct for efficiency (0,6...0,8 ?) > 2. Select a AC 3Phase motor with about the same nominal speed (3000, > 1500, 1000, 750,...) with the same or slightly higher Power. > 3. Select a 3~ motorcontrol corresponding to that motor size > > There are 3~ Motorcontrols for 3~ line (380V) and 1~ line (220V) > input, all with 3~ output. Compared to DC motors there is not such a > high starting torque available with ac 3~ motors. > I think you have not to care so much about the current, as > controllers are specified for the motor (Power) you intend to use. > But if you have selected a motor, you find the current in the specs > or labeled on the case. or: Pel = U * I * cos(phi) * sqrt(3) > > If the nominal speed of the ac motor is higher then you want/need, > then you can reduce speed with the motorcontroller. But doing that > also reduces the nominal power too (P ~ M * n). So it may be > necessary to select the next higher motor size to compensate for > that. So it is convenient, when the nominal speeds do not differ to > much. > Or the other way round: if you have 380V line connected to a 220V > motor you can gain 70% power by running at 87 Hz with same motor size > !
Thanks Bertram. That answers most of what I needed to know. > I think/hope you will buy that thing and will not try to make it > yourself ? Meanwhile they are cheap and reliable ! Buying the right thing is always easier - unless you are being totally ripped off. I am tied by confidentiality from saying much more. -- Regards, Declan Moriarty. -- Author: Declan Moriarty INET: [EMAIL PROTECTED] Fat City Network Services -- 858-538-5051 http://www.fatcity.com San Diego, California -- Mailing list and web hosting services --------------------------------------------------------------------- To REMOVE yourself from this mailing list, send an E-Mail message to: [EMAIL PROTECTED] (note EXACT spelling of 'ListGuru') and in the message BODY, include a line containing: UNSUB CHIPDIR-L (or the name of mailing list you want to be removed from). You may also send the HELP command for other information (like subscribing).
