Modulating Damper Circuit

[PHILSTO]

I have been given a small HVAC job to redesign and there is one circuit that the customer has used in the past and claims to have had no difficulty with. I am an old electrician with some HVAC experience in a new position with a consulting engineering firm so any advice would be appreciated. A modulating damper actuator (Belimo AF-24SR US) is being supplied 24vac from the grounded secondary of a class 2 transformer (X1 only ). X2 is grounded and not taken to the actuator. The analogue input to the actuator is supplied by an internally powered 15vdc 0-10vdc analogue output from a Distech ECP-510 Controller. The ECP-510 is supplied from a floating Class2 transformer as per manufacturer therefore the analogue output common has no ground reference. I am worried about floating voltages at the actuator. This is my first thread so be gentle with an old fella.

[ctrlguy]

I've had problems with grounded secondaries with some DDC controllers. I'd cut the ground, tie the commons together at the actuator and let them float.

[hanknerd]

Agree with ctrlguy about removing the grounded secondary (unless there's a code related issue with primary voltage or VA of transformer). Then wire the actuator and transformer per Belimo literature:

http://www.belimo.us/bellib/Damper_A...ring_Guide.pdf

You may have an issue with the 0-10V control from the DDc controller, the AF24-SR US is 2-10VDC control standard.

Other questions Belimo tech support is good and, believe it or not, answers the phone when called, (800) 543-9038.

[dapper]

If the primary voltage of this transformer is over 250V I believe the secondary must be grounded by code.

[orion242]

>removing the grounded secondary

Remove the ground and tie the commons of the 24v transformers together

[PHILSTO]

Thank you for your responses. I spoke to Belimo and I can ground the actuator motor feed as long as I respect the common from the controller and leave it floating. So the circuit I described apparently will work. I am forced by local code at this plant to ground any power circuit. Otherwise I would have kept the whole circuit floating as you suggested.

[nikko]

I also agree with ctrlguy. The info Belimo gave you will work for their product, but I believe that their signal ground and their X2 pole in the actuator is internally connected. If this is the case you've just pulled the -ve signal from the Distech controller to ground as well. This may or may nor be a problem.

Nikko

[dingman]

I agree with Nikko. I've used that actuator in the past and as a rule, I keep my signal (-) and 24vac common tied together and ungrounded to let them float together.

I haven't actually tested to see if those two leads are tied together internally, but I bet Nikko is right on that. I've seen it before on other actuators.

[lost0ne]

I have been given a small HVAC job to redesign and there is one circuit that the customer has used in the past and claims to have had no difficulty with. I am an old electrician with some HVAC experience in a new position with a consulting engineering firm so any advice would be appreciated. A modulating damper actuator (Belimo AF-24SR US) is being supplied 24vac from the grounded secondary of a class 2 transformer (X1 only ). X2 is grounded and not taken to the actuator. The analogue input to the actuator is supplied by an internally powered 15vdc 0-10vdc analogue output from a Distech ECP-510 Controller. The ECP-510 is supplied from a floating Class2 transformer as per manufacturer therefore the analogue output common has no ground reference. I am worried about floating voltages at the actuator. This is my first thread so be gentle with an old fella.

You will probably want to provide a signal common from the controller to the actuator. Usually twisted pair is used for the signal and it's return. Also,you must be concious of any other externally powered device that is connected to the same controller. If the secondaries on the transformers powering these devices are also grounded you should make sure that they are all in phase or else you run the risk of having ground currents flow through your controller.
You only want a single point of ground on your I/O subsystem. If you have multiple devices on the same controller you really want to float the external devices and ground the I/O section on your controller. Especially if the devices are located some distance from each other where they are possibly on a different leg of the AC mains.
You will have to check with the controller manufacturer, and follow heir recommendations.
It is common for the I/O common to be isolated from the power input of the controller.This allows the I/O subsystem to float relative to the controllers supply.