On Tue, 2010-11-09 at 12:25 -0500, Brian wrote: > If you use PWM, doesn't that make it an AC motor? I suppose unless > you are just modulating half of the polarity, then you could still > consider it DC. I am not immediately familiar with the bldc > components, so if this is just my ignorance speaking, then ignore it. > > Brian
PWM is just used to produce controlled DC or AC voltages. My understanding of the difference between BLDC (Brushless DC ) and PMAC (Permanent Magnet AC) motors is the way the rotor magnets and the stator coils are configured. The terminology is influenced by motor designs of the past. Brushed DC motors are run on DC and have their coils energized by commutator pads, which are basically switches. So each rotor coil is turned fully on or off with a fixed DC voltage. The design of the motor is configured to try to get a constant torque from this arrangement. The better motors have a large number of coils so that the effect of one coil leaving or entering an energized group will have less impact. Also the rotor core can be stacked in a slight helix relative to the stator coils to soften the magnetic "edges". The BLDC is configured to have the stator coils run on mostly DC and have the torque be constant as the rotor poles pass the stator coils. Usually the coils are energized with a linear ramp from zero to full, but regulated, DC voltage, dwell at full DC, then ramp down. High frequency PWM is used to give the effect of a controllable DC voltage. If one could efficiently produce a true variable DC voltage with the required current and voltage, then one would not need PWM. The PMAC motor is very similar to the BLDC motor, except the stator coils are configured to use a PWM generated sine wave, or AC. Since a sine wave constantly varies in a precise way, the motor driver needs to know where the rotor is to a high degree, so these motors tend to have high resolution absolute encoders. Since the BLDC motor coils are either turning on, staying on, then turning of, three simple digital Hall sensors are used. So basically, BLDC = DC on coils, Hall sensors PMAC = AC on coils, Absolute sensors PWM used to produce and control DC or AC Corrections welcomed. -- Kirk Wallace http://www.wallacecompany.com/machine_shop/ http://www.wallacecompany.com/E45/index.html California, USA ------------------------------------------------------------------------------ The Next 800 Companies to Lead America's Growth: New Video Whitepaper David G. Thomson, author of the best-selling book "Blueprint to a Billion" shares his insights and actions to help propel your business during the next growth cycle. Listen Now! http://p.sf.net/sfu/SAP-dev2dev _______________________________________________ Emc-developers mailing list Emc-developers@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-developers
