Variable Frequency Drives:
VFD Wikipedia page: http://en.wikipedia.org/wiki/Variable-frequency_drive
PLC’s and PLC Programming:
Automation Primer: PLC programming topics.
PLC Academy : This site details some of the basics of PLC programming with a Ladder Diagram symbols overview and some programming examples. One unique post that would be highly recommended would be the Structure Text Tutorial. Great introduction to ST for anyone interested. The site owner Peter has spent time putting together great posts on those topics.
Contact and Coil: PLC programming topics from a practical perspective. My favorite being a topic I have tried to document for a long time- Patterns in PLC programming.
PLC Talk / PLCs.net: One of the more active forums on PLC’s online. At time of writing, the forum had more than 600,000 threads probably covering every PLC line. Also, a lot of great discussions. With 100+ users on there at any given time, you would probably get an answer (or at least a discussion going) on any industrial control related question.
General Industrial Automation
WISC-ONLINE Industrial Automation Tutorials: A series of videos ( more than 150 of them) on various topics related to industrial automation. Topics range from motor theory to PLC shift registers to servos..Maintained by the Wisconsin Technical College System.
TW Controls Articles Page: A good compilation of articles on industrial controls related topics. Specifically like the NPN-PNP description for sinking and sourcing input/outputs.
Industrial Automation Podcasts:
If you spend lots of time on the road, podcasts might be the way to get up to date on the field.
Automation World Podcasts: Covers various technical topics and automation industry trends..
InduSoft podcast: Interviews and trends in industrial automation.
Electronic Drives – AD/DC , Practical Applications, Troubleshooting and Repair – Robert S. Carrow