Bacnet and Digital Inputs for VFDs

8-01 parameter.jpg

A customer called today and was asking about a VFD that would only start AFTER a power cycle, run for a while and then would stop.

They were starting the drive with a contact closure on digital input 18, but also connected over bacnet.

In these cases where both comm and hardwired points are connected, it is possible to get multiple start or stop commands conflicting that result in unreliable start/stop behavior. This happens when the bacnet BV start point is placed on a graphic or in the schedule, while the BO is also scheduled or commanded.

Here is how you can fix this:

If you are ONLY going to start the VFD using the hardwired point:

Set parameter 8-01 “control site” to "[1] Digital Only”. Now bacnet commands to start or stop the drive will have no effect, only the hardwire digital point will work.

Danfoss VFD "Common" Start/Stop

Hi-  Recently a colleague asked about "common" start/stop for VFDs that have the integral bypass--  Basically, the drives come wired with the ability for a single start/stop signal to start the motor when the SELECTOR SWITCH IS IN EITHER DRIVE MODE OR BYPASS!

The reason that you would want common s/s--> It allows the operator to select bypass and the motor will NOT take off at full speed, without dampers/valves etc being positioned in the correct place.  Motors running at full speed when the system isn't ready have the bad effect of blowing out ductwork and dampers.   When selector switch is in bypass you MUST also have a s/s from the EMCS as a backup to make sure the system is ready to start.

The reason that you DON'T want common s/s-->  You want operators to be able to select bypass in an emergency, and want the motor to run immediately without any other control signals required.

Below is a picture showing the examples, as well as a section from the manual on how to change it-- WARNING- the manual excerpt says that you should cap off the "red" wire after jumping X55 3/4, that Danfoss wiring is probably blue from the factory.  See the final picture for how it should look in the panel.

Common START/STOB ENABLED

Common START/STOB ENABLED

COMMON START/STOP DISABLED

COMMON START/STOP DISABLED

manual common ss.jpg
BLUE DANFOSS WIRE FORMERLY CONNECTED TO TERMINAL 18, WIRE NUTTED OFF WITH CONTROLS SS NOW ON 12/18.

BLUE DANFOSS WIRE FORMERLY CONNECTED TO TERMINAL 18, WIRE NUTTED OFF WITH CONTROLS SS NOW ON 12/18.