What is RESET?

[chauhk]

Sorry, I am new in this area. But I am still don't understand some terminology. Can somebody explain what is reset, soft start ? Thank you.

[Roscoe]

Sorry, I am new in this area. But I am still don't understand some terminology. Can somebody explain what is reset, soft start ? Thank you.

Depends... could be a big explosion or large flash or more of what you already have ....uhhh nothing........ just jiving you man...... be a little more specific maybe.........;)

[refrtech]

A Reset is where the BAS system of a building overrides the Temp Setpoint of a unit .
So say you have a chiller that has a LWT setpoint of 42F . It can be set up so that the BAS can send it either a 0-10VDC or 4-20ma signal to raise the setpoint up 10F . It is used where your load drops off during a certain time and you may not need 42F water but can get away with making 52F water . It is used mainly to save energy when full load capacity is not needed .

Hope this helps .

[chauhk]

For example, If the VFD is used as a soft start, what does it mean?

And what is a discharge temperature reset? I am verifing a control program for a building. In the AHU control, It uses a linear reset schedule to calculate discharge temperature setpoint. The input is the outside air enthapy. However, the program is used for discharge temperature reset. What does that means?

[DPSwitch]

I post these things in the controls forum, and always get mucho help..... the same guys will probably find it here too though.

[Roscoe]

Dahhhhhhhhhh jesus sorry dude I eff'ed up ...again................ open mouth insert foot..........:o

carry on fellas..................:D

ah duh.............:o

[powerhead]

Discharge air reset is similar to chilled water reset.

If the space temperature coming back to the unit (via return air) starts to drop that means the load is dropping. The supply air can be raised a few degrees and stil cool the space. When the return air temperature starts to rise (space is getting warmer, more load) then the discharge air setpoint is lowered a few degrees.

I'm not too familiar with basing discharge air temp on outdoor air enthalpy, perhaps if the outdoor air has too much moisture you lose cooling erffect so the discharge air temp is lowered? I don't know, just a guess.

Soft start on a frequency drive starts a motor slowly and then brings it up to the speed required for the unit's parameters.

This saves the motor from getting a large spike or "inrush" of current when it tries to start full speed. Big inrushes of current cost money and also beat the crap out of a motor.

Imagine trying to push a car from dead stop to 60 mph in an instant verses gradually pushing it up to speed.

[refrtech]

A VFD is a Variable Frequency Drive . It will speed up or slow down a motor by using 0-60Hz ( 50Hz if in Europe ) . A Soft Starter does the same thing but limits the voltage allowed to the motor @ s/u by using SCR's . Think of applying line voltage to extremely high resistance resistors that heat up withen a few seconds then allows voltage to pass . This is usually done withen 3-7 seconds then a by pass contactor will pull in and the motor is given full voltage .

[GOVHVAC]

Dishcarge air reset for a air handler, as outside temp drops discharge air temp goes up.
Ex. outside temp = 80 degrees or higher discharge air temp=45 degrees
outside temp =55 degrees discharge air temp=55 degrees
outside temp =40 degrees or lower discharge air temp=65 degrees

[powerhead]

Dishcarge air reset for a air handler, as outside temp drops discharge air temp goes up.
Ex. outside temp = 80 degrees or higher discharge air temp=45 degrees
outside temp =55 degrees discharge air temp=55 degrees
outside temp =40 degrees or lower discharge air temp=65 degrees

Can I assume you are talking about a 100% outdoor unit? Possibly the enthalpy scheme mentioned above could be used here?

[propmanage]

It sounds like you are talking about the discharge set point resetting to a new set point due to the change in temperature. With little the information you provide this could also be anything from setting a new start up time schedule as the season changes or the supply air temperature for heating or cooling. Would need more info.

[DPSwitch]

Discharge air reset is a moving target to get a fixed result, the thing is the reset can be based on whatever the controls guy wanted.

[Randy S.]

A reset is an automated change to a set point in relation to a varying input or some other type of monitored signal.