Main porpose of DOl starter and VFD on AHU

[anil130]

hi fresn..
what is then main purpose of using DOL Starter and VFD in a same AHU.? could any body make me clear about this system.

[r.bartlett]

I dolphink I know ...

[Grizzly]

Hi Anil130
I don't fully understand your question?
But basically some systems I work on have several scroll comps. where say there are 3, the first 2 are DOL.
With the 3rd one on VFD\ softstart.
Which when on full load the Voltage inrush of the 3rd compressor during start-up is softened. Which maintains system integrity and does not trip the incoming supply.
IE reduces the load on start -up and smooth's the current inrush!
Grizzly

[Glenn Moore]

Hi Anil
I assume this is driving the AHU fan motor. Often the DOL starter is placed in parallel with the VFD . If the VFD fails then the DOL starter will run the fan at full speed. If the DOL is in series with the VFD then its normally used for isolation purposes

[Magoo]

Hi Anil.
Soft starters quite often have a parallel contactor that is closed after motor is at full speed. VSD/VFD quite often have a contactor supplying the drive, as an isolation protection, depends on local electrical codes units are manufactured to.

[Rob White]

.

Could you mean 2 compressors? One on DOL and one on VFD?

If that is the case then it is to do with capacity control.
The Variable comp will start and run up to full load and then
the DOL will start. At this point the variable speed one will
back off or shut down until the load is high enough to call
for it and then it will run up to full load.

It is the nature of a VRF / VRV to "ramp" up and down the speed
of one or more compressors to cope with loads.

Or I'm wrong, that's not what you meant and discount all this rubbish :D

Rob

.

[anil130]

My Dear friends, many thanks for the sweet answer, i am just asking about why we used DOL starter if there is already VFD for the AHU.is it necessary to used DOL starter if there is already VFD.?if we used both then what is the reason behind this.i understand the function and purpose of VFD but i am not clear of DOL starter.

[Glenn Moore]

There is no real need for the DOL starter as the VFD has all the functions necessary to run the AHU fan motor safely as phase loss, overload etc is covered by the VFD . The DOL starter is simply a contactor to put a 3 phase feed to the VFD but allows the VFD to be isolated by a lock out button switch to prevent the motor being started accidentally.
The VFD has only electronic circuits to stop start and run the VFD but simply pressing the stop button on The VFD is not a safe way to work on the motor as external circuits could start the VFD so by having the contactor fitted allows the VFD to have the power removed from it to allow safe working on the motor. This may be local electrical regulations , but in theory the DOL is not really needed

[Rob White]

My Dear friends, many thanks for the sweet answer, i am just asking about why we used DOL starter if there is already VFD for the AHU.is it necessary to used DOL starter if there is already VFD.?if we used both then what is the reason behind this.i understand the function and purpose of VFD but i am not clear of DOL starter.

One compressor or two?

If one compressor you would not use both.
If two compressors you can have one on VFD and one on DOL.

See my post above.

Rob

.

[install monkey]

sometimes kit is installed as dol then the inverter is added to improve efficiency by running at a lower speed- say 45 htz and reduce electrical load on start up. as prev mentioned isolation purposes but you should have an isolator next to the fan/compressor that can be locked off so your not relying on the inverter for isolation.

[anil130]

many thanks to all of you for your valuable answer and i understand ..