We had a tricky technical question come through the AEG (Applications Engineering Group) last week. The customer (fairly adept with DCS800s) could not get past base speed on a tach feedback DCS800. The motor nameplate had a 1750 RPM base speed (armature voltage of 500VDC and a field of 3 amps) the drive would start to move the armature voltage past 500VDC to 540VDC and the field would stay stable at 3 amps and not decrease. The usual culprits are set-up like a fixed field while trying to do field weakening and etc. The drive was set-up correctly.

The issue was two-fold. The speed feedback assistant was not performed and the EMF speed feedback and Tach feedback were way off. These usually mean poor motor data or a rewound motor that no longer matches the nameplate (it happens…one-time we found extra brush holders in an attempt to change the commutation. . .)

The second issue was the Tach feedback signal was surging (like a sine wave) on the graph with the drive in EMF feedback at a stable base speed. The first course of action was to look at the tach. It turns out that the tach coupling was broken. After that the signal was stable.

The next step was to verify the field settings. Since this was a re-wound motor, the field stamped

nameplate data was a little off, so we adjusted the drive’s field down to 2.8 amps. We ran the speed feedback assistant again and calibrated the drive to the hand held tach reading per the ABB procedure and the tach and EMF signal started to overlap correctly.


The majority of DCS800 applications that B&D Technologies works on are retrofits. In the 300 we have worked on in the last few years, there were only four that were new applications. The problem with commissioning is usually the control wiring and lack of documentation.

DC drives are still a viable technology even in the elevated technological state of AC drives. Most AC drive companies either do not have a viable DC offering (namely the one that repackages the Italian DC drive with a few percent of global market share) or do not offer any at all.

Last Words

Do not think that AC is the only option. . . there are still reasons to keep DC.

C. Tolbert, DOE