On 15.05.15 09:04, Kirk Wallace wrote: > I have a soft spot for these SCR speed controllers: > http://www.kbelectronics.com/manuals/kbic_manual.pdf > > They can run DC motors or AC universal motors, plus have a separate > field source if needed (see page 8). > > They use back EMF for speed feedback and regulate speed very well with > high torque. Isolated PWM can be used from LinuxCNC to control speed.
About a quarter of a century ago, before speed controls were built into electric drills, I built one with just an SCR and half a dozen components, for my 400w 240v drill. What gave it good torque at low speed was that the load was in the cathode circuit, not above the anode. That simple change allowed it to sense the motor back EMF, and crank up the herbs as the motor slowed under load. There was slight cogging down below 50 RPM, but the unit made it easy to push a 3/4 bit through steel without breaking my wrists. (Both gears were far too fast without electronic speed reduction, at that drill size.) Since the SCR only gave half-wave control, a switch and diode took it from half power to full in one step. I suppose I could have used a full-wave bridge to improve on that, but it wasn't needed. Apropos runaway in series dc motors: My recollection of the theory, from the Electrical Engineering uni course 43 years ago, is a bit foggy, but I have to concur that runaway of a series dc traction motor is dangerous. What is done to hobble a universal motor to avoid that, beyond running it on AC for lower torque, I don't know. Take a look at: http://en.wikipedia.org/wiki/DC_motor#Series_connection >>> A series motor should never be started at no load. With no mechanical load on the series motor, the current is low, the counter-EMF produced by the field winding is weak, and so the armature must turn faster to produce sufficient counter-EMF to balance the supply voltage. The motor can be damaged by overspeed. This is called a runaway condition. <<< That clearly doesn't apply to universal motors, since we do it all the time with power tools. (Though I'm not sure I'd like to run my power plane without the drive belt, to try it out.) Erik ------------------------------------------------------------------------------ One dashboard for servers and applications across Physical-Virtual-Cloud Widest out-of-the-box monitoring support with 50+ applications Performance metrics, stats and reports that give you Actionable Insights Deep dive visibility with transaction tracing using APM Insight. http://ad.doubleclick.net/ddm/clk/290420510;117567292;y _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
