VFD Deceleration for Centrifugal Blower

nhee2

Senior Member
Location
NH
I don't work a lot with VFDs - have an application where I observed some existing 75 HP centrifugal blowers (VFD driven) are configured with 40 second deceleration time period in the drive. Since on shutdown the process valves/equipment shuts down immediately I am wondering what the benefit of having a deceleration ramp is in a blower application vs. just shutting off and coasting down. The blowers end up slowly ramping down in a dead-head condition as currently programmed.

Is there a benefit to ramp down in this application?
 

nhee2

Senior Member
Location
NH
I let mine coast to stop, but that means little. Why use any power if it’s not needed?
Ramp down can cause. an overvoltage issue if it’s too short of time.
thanks- this is what I was intending to suggest, but wanted to make sure I was not missing something. In that case, is anyone familiar with the Danfoss VLT drive and where I turn the deceleration ramp off? I am not seeing it in the manual, unless you just set the time to zero.....
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
thanks- this is what I was intending to suggest, but wanted to make sure I was not missing something. In that case, is anyone familiar with the Danfoss VLT drive and where I turn the deceleration ramp off? I am not seeing it in the manual, unless you just set the time to zero.....
Usually there is a parameter that controls how the drives comes to a stop when the enable is turned off. It will be something like zero equal coast one equal ramp.
 

SSDriver

Senior Member
Location
California
Occupation
Electrician
thanks- this is what I was intending to suggest, but wanted to make sure I was not missing something. In that case, is anyone familiar with the Danfoss VLT drive and where I turn the deceleration ramp off? I am not seeing it in the manual, unless you just set the time to zero.....
Its a little bit weird to do that on a lot of the Danfoss drives. Danfoss calls that function COAST. There is not a setting to tell it to choose from COAST or DECEL. Parameter numbers might be slightly different depending on the model of drive you use. I usually use an FC202.

Option 1: If you want to start/stop the drive from the buttons on the drive you are very very limited. The only thing you can do is change parameter 1-81(Min Speed for Function at stop[RPM]) or 1-82 (Min Speed for Function at stop[HZ] depending if the drive is controlled by RPM or HZ) and increase that number up as high as it will let you. Sometimes that number is restricted to a maximum of maybe 20 HZ. The drive always ramps down and then when it hits a target number (either 1-81 or 1-82) it will then coast. I think the factory default is usually 1 hz. So basically it will ramp down to 1 hz and then coast. If you increase that number up to the max at 20Hz( I think) it will ramp down to 20hz and then coast. I absolutely hate that I can't just program the drive to coast to stop and have it function from the stop(off) button the the LCP screen.

Option 2: Program one of the unused digital inputs to "[2] Coast Inverse" (Terminal 27 usually, parameter 5-12). This will be a true Coast to stop function. This works inverse of a normal digital input. Once the input signal is removed it will coast to a stop. An easy way to achieve this is to jumper your digital input for the start command (usually terminal 18) and apply that same signal to your programmed Coast Inverse (usually terminal 27). This way when you apply the start command it tells the drive to start and NOT coast. Once that start command is removed it also removes the input for Coast inverse and initiates a Coast to stop.

EDIT: I should add do NOT just turn your decel time down to zero or ONE (not sure you can actually set it to zero). It will try to stop the motor in 1 second and then coast. The will cause overvoltage issues with the drive.
 
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nhee2

Senior Member
Location
NH
Its a little bit weird to do that on a lot of the Danfoss drives. Danfoss calls that function COAST. There is not a setting to tell it to choose from COAST or DECEL. Parameter numbers might be slightly different depending on the model of drive you use. I usually use an FC202.

Option 1: If you want to start/stop the drive from the buttons on the drive you are very very limited. The only thing you can do is change parameter 1-81(Min Speed for Function at stop[RPM]) or 1-82 (Min Speed for Function at stop[HZ] depending if the drive is controlled by RPM or HZ) and increase that number up as high as it will let you. Sometimes that number is restricted to a maximum of maybe 20 HZ. The drive always ramps down and then when it hits a target number (either 1-81 or 1-82) it will then coast. I think the factory default is usually 1 hz. So basically it will ramp down to 1 hz and then coast. If you increase that number up to the max at 20Hz( I think) it will ramp down to 20hz and then coast. I absolutely hate that I can't just program the drive to coast to stop and have it function from the stop(off) button the the LCP screen.

Option 2: Program one of the unused digital inputs to "[2] Coast Inverse" (Terminal 27 usually, parameter 5-12). This will be a true Coast to stop function. This works inverse of a normal digital input. Once the input signal is removed it will coast to a stop. An easy way to achieve this is to jumper your digital input for the start command (usually terminal 18) and apply that same signal to your programmed Coast Inverse (usually terminal 27). This way when you apply the start command it tells the drive to start and NOT coast. Once that start command is removed it also removes the input for Coast inverse and initiates a Coast to stop.

EDIT: I should add do NOT just turn your decel time down to zero or ONE (not sure you can actually set it to zero). It will try to stop the motor in 1 second and then coast. The will cause overvoltage issues with the drive.
Thanks, was looking through the manual this afternoon and agree it did not look as simple as just a single configuration setting.

I had thought 1-80 might work if set to '0'. but you are suggesting that even if set to '0' (coast), the drive will not coast until ramp down speed is less than the value in 1-81?. This was not the way I was initially reading it, but now that I've read your explanation, that seems to make sense.

Based on your experience - does the Option 1 solution also work when using the remote start command to terminal 18? While Option 2 will work, ability to get in there and make wiring changes myself or to instruct others to make changes is limited right now. So a configuration setting change is preferable.
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
It should be noted that DECEL has two main functions: how the drive reacts to a STOP command (the obvious one), but also how it responds to a CHANGE in the speed command. So let's say you are running at full speed and something changes to command it to run at 30% speed, the DECEL rate determines how quickly (or slowly) the drive takes the motor DOWN to that lower speed. So turning the DECEL off or setting it too short can have implications on the PROCESS that the drive is controlling. THAT might be the reason why someone set it to 40 seconds, knowing that when shutting a centrifugal load OFF, that 40 seconds is irrelevant once the valves close.
 

SSDriver

Senior Member
Location
California
Occupation
Electrician
Thanks, was looking through the manual this afternoon and agree it did not look as simple as just a single configuration setting.

I had thought 1-80 might work if set to '0'. but you are suggesting that even if set to '0' (coast), the drive will not coast until ramp down speed is less than the value in 1-81?. This was not the way I was initially reading it, but now that I've read your explanation, that seems to make sense.

Based on your experience - does the Option 1 solution also work when using the remote start command to terminal 18? While Option 2 will work, ability to get in there and make wiring changes myself or to instruct others to make changes is limited right now. So a configuration setting change is preferable.
Option 1 is only a half option. It doesn't really function as a true coast to stop. It will decel down to what ever you set 1-80 or 1-81 and then coast the rest from that point on. Note: it will only look at one of those parameters. If you adjust the speed of the motor on the LCP and you're changing the setpoint in HZ it will follow 1-82. If you are changing the setpoint in RPM it will follow 1-81. And yes I believe it will follow this from the keypad or by removing the digital input for the run/start command. I usually don't use this option and control almost everything from inputs even if that is just a simple switch in a box mounted off to the side. I don't like operators touching the VFD at all if they don't have to. This usually saves a lot of call backs and random issues. Literally all you need is a switch from terminal 12 or 13(both are 24+ DC out) going to terminal 18 and a jumper to terminal 27(or what ever digital input is not used). Just confirm that new digital input you use is programmed for Coast Inverse.

And what Jraef mentioned is true. There are multiple reasons to not turn down DECEL to almost nothing
 
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