HVAC Control Panel and Power Panel

[nevinjohn]

Hi guys!

I've never worked with controls and this is the first time I am experiencing it and I am confused.

1. HVAC Control Panel.
I see most of the actuators and minor sensors are connnected to it. How about the power for these actuators? Although these would be in the range of 20-30Watts, would it be powered from the Control panel itself? If not, is there a requirement of additional power panels for these devices?

2. For Air Handling Units /
How are these interfaced with the HVAC Control Panel? If the unit has to be shutdown, it should be powered from the HVAC Contril panel. Am I correct, or is there something else?

3. How much would be the approximate power consumtion for a HVAC Control Panel?

Thanks
Nevin

[Brian_UK]

Most devices are powered/controlled from the control panel. It is just a means of bringing all items into a central point.

As to the power consumption, well it all depends on how big the plant and equipment is.

[nevinjohn]

In the scheme its clear that power to actuators are fed from the control panel as it requires just 24 - 48 V. How about the AHU fan motor? This requires a 3 phase connection and so connected to the local distribution board (DB). So, how can this control panel take control of the unit?

Lets say I need to shut down the unit. How is this heavy duty motor interlinked with the HVAC Control panel?

[Balance Piston]

The simple version.

Some control panels will use 24v as their control voltage to control signals such as digital outputs, digital inputs and analog outputs / inputs. Digital outputs and inputs are considered on off devices (1 or 0 in binary) such as switches, relays, and solenoids. Analog devices are transmitters, transducers, positioners, etc. They can be 0-5 vdc, 1-5 vdc, 0-20ma, 4-20ma, etc etc. Some analog devices need an independent power supply and others are considered loop powered. All transmitters / transducers (pressure, temperature, etc.) have a range that corresponds to their output and some are adjustable using a communicator such as a Hart.

When the controller calls for a compressor to run it sends a 24v digital output signal to a relay which energizes a coil and closes the contacts. There are two different voltages at the relay, (typically) the contacts will have the higher voltage and the coil will be considered the control voltage, lower voltage. The voltages on either side can vary, they will not always use 24v for the coil, or 120/240v for the contacts. Large horsepower compressors can use 24v for the coil and 16kv for the contacts, just depends on the design.

Suggest reading the below link if you are unsure how a relay works.

http://en.wikipedia.org/wiki/Relay

[moideen]

The power connection of 3 phase motor will not connect to direct DB. It is connected from control panel through 3 pole contactor. Then the contactor will be controlled / looped with the ON/OFF switch and safety devices such as motor overload.

[Balance Piston]

The power connection of 3 phase motor will not connect to direct DB. It is connected from control panel through 3 pole contactor. Then the contactor will be controlled / looped with the ON/OFF switch and safety devices such as motor overload.

That is somewhat a generic statement.

Modern control panels do not have safeties in series with start signals. The safeties are either a digital input or analog input to the control panel. The program will determine whether or not to remove the start signal to the motor starter. The only signal that will remove power, or for lack of a better word, de-energize all outputs directly, without going through the program is the ESD (emergency shut down) button and this is only valid if it's wire it to be fail safe per most standards.

[Balance Piston]

That is somewhat a generic statement.

Modern control panels do not have safeties in series with start signals. The safeties are either a digital input or analog input to the control panel. The program will determine whether or not to remove the start signal to the motor starter. The only signal that will remove power, or for lack of a better word, de-energize all outputs directly, without going through the program is the ESD (emergency shut down) button and this is only valid if it's wired to be fail safe per most standards.

Let me rephrase what I typed because most on here might be more familiar with typical AC systems which due use safeties in series with the start signals. Residential is really not my background so my response is based on my experiences. My explanation is basically to broaden the discussion beyond residential AC systems as some people may read that and assume all control panels are wired that way. As the field of automation progresses and begins to move into residential I would not be surprised to see them follow the standards in other industries. If your controls are modern enough then you want as much information as possible, if safeties are wired in series, the controller is unable to know what caused the shutdown or more specifically, which safety caused the shutdown.

[Balance Piston]

Grammar.......due = do......

They should add an edit button!!

[nevinjohn]

moideen, could you post a schematic diagram?

[moideen]

That is somewhat a generic statement.

Modern control panels do not have safeties in series with start signals. The safeties are either a digital input or analog input to the control panel. The program will determine whether or not to remove the start signal to the motor starter. The only signal that will remove power, or for lack of a better word, de-energize all outputs directly, without going through the program is the ESD (emergency shut down) button and this is only valid if it's wire it to be fail safe per most standards.

Hi,balance,
Yes, you are absolutely correct. But mr.nevin john asked the basics of electrical control system. So I explained briefly.

[moideen]

moideen, could you post a schematic diagram?

Of course, not now, next morning, I would draw and post a simple schematic drawing with control and power wiring typically using for hvac system..(Only relay logic, as balance said, the analogue digital is the next step after understanding the simple relay logic)