Induction AC motors are an interesting sort of beast. They rely on a "slip frequency" between the rotating field in the stator and the position of the rotor to produce the current that magnetizes the rotor. The slip rate varies with the torque, so even with the same frequency applied to the motor (power is fed to the stator coils) the output RPM will vary with the load (torque). At zero torque the slip approaches zero hertz and the rotor runs at nearly synchronous speed. As load (torque) increases the rotor slows down and the increasing slip frequency couples more power into the rotor. Note the rotor speed dropped while the applied power stayed at the same frequency, but the current drawn increased. Slip frequencies are typically such that rotor RPM trails synchronous speed by 2 to 5 percent, thus a typical motor would be rated at 1750 RPM when the ideal 60 Hz synchronous speed would be 1800 RPM. The 1750 is not a guaranteed speed, but rather the expected speed at the rated torque or horsepower. Actual speed would vary over a probable range of 1700 RPM to almost 1800 based on actual load.
If the 5% or so variation in speed of an induction motor is acceptable, then perhaps a closed loop would not be needed. If tighter speed control is needed, then I would recommend using a "Vector" style VFD speed controller. These monitor the actual motor speed and make multiple adjustments to the applied power to achieve the desired results. Some require an encoder, but there are modern versions that get the required feedback from the back EMF of the motor coils. These are call "sensorless vector" drives. Trying to close a speed control loop around a non-vector type VFD might work, or it might result in a control loop that constantly hunts. You takes your chances...... Regards, Steve Stallings > -----Original Message----- > From: [EMAIL PROTECTED] > [mailto:[EMAIL PROTECTED] Behalf Of Anders > Wallin > Sent: Monday, December 11, 2006 3:41 PM > To: Enhanced Machine Controller (EMC) > Subject: Re: [Emc-users] DIY closed-loop VFD ? > > > > hmm, answering my own post here... > > > A DIY way of closing the loop would be the following: > > -Control the VFD using its 0-10V analog input (use the 4th DAC on the > > m5i20 card) + direction input. > > -Fit an encoder to either the motor shaft or the spindle > > -close loop in EMC using PID > > there might be a flaw in this logic since the analog VFD input controls > the output freqeuency, not the voltage/current. > > When the motor is loaded I would assume that we need more > voltage/current through it, not just an increased frequency. On the > other hand the V/f drive will increase voltage along with frequency... > > still not sure if this will work or not ?? > > AW > > ------------------------------------------------------------------------- > 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 > ------------------------------------------------------------------------- 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
