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 ! I think/hope you will buy that thing and will not try to make it yourself ? Meanwhile they are cheap and reliable ! Greetings, Bertram -- Bertram Geiger <[EMAIL PROTECTED]> HTL-Bulme Graz, Austria -- Author: Bertram Geiger 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).
