Motor drive experts......

Thread Starter


Joined Nov 12, 2008
Hi all. I am a noob to this forum, but not new to forums. I am a technician and have worked at various factories as well as the military. Now that i've introduced myself I have a question that might be answered here.

I have always troubleshot machinery with with bad drives down to the drive, but now i'm supposed to start fixing the drives themselves, all of various manufactures.

It pertains to drives, old and new:

1. Is there a general, or common theory of operation amongst all drives in ac circuits? Motor drives to be specific.

2. What are the internal devices in drives used to control amperage or frequency outputing the drive to the motor? SCR's?

3. On DC drives, is current outputing the drive linear of is it pulsing?

Basicly, I need to know theory of operation of AC and DC drives and major devices used in them. I've searched a little in the forum, but didn't find what I am looking for.


Joined Jul 5, 2006
Depends upon vintage and sophistication of the drive. AC or DC motor drives can use SCRs, bipolar transistors, MOSFETS, or IGBTs, depending upon how old they are, the power level, the required control range, etc.

If you are being assigned to repair electronic motor drives, it sounds from your questions like you might be well advised to ask your employer to send you to training for the task or, if they don't want to, take a course or two on your own dime.



Joined Oct 29, 2008
Another idea would be to contact the mfgr. or visit their website to see if they have any documentation available.

BE CAREFUL! If these are VFDs they potentially have enormous capacitor banks inside and take time to "bleed off".