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# Minimum and Maximum Reference VLT HVAC Basic Drive FC 101 (Danfoss)

02/19/2017 4:32 AM

Hi members ,

I am currently auditing several commercial buildings and as a part of this project , the cooling towers in such buildings use a Danfoss VLT HVAC Basic Drive FC 101 drive.

Its a 1.1kW 3phase motor , 1440 rpm IM .

When i click on the drive to understand different setting , i came across the following as displayed on the screen.

3-02 Minimum Reference 27.0000

3-03 Minimum Reference 50.0000

No units being displayed with the reference values , which complicates the thing.The minimum freq. is set to 15Hz and maximum to 50Hz. The analog signal is in form of voltage (0-10V) and current (4mA-20mA) from a RTD situated inside the cooling tower.

Could someone help me with how one can design the minimum and maximum reference ?

Please feel free if you need additional information or clarification from my end.\

Thanks

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Location: Richland, WA, USA
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#1

### Re: Minimum and Maximum reference_VLT HVAC Basic Drive FC 101 (Danfoss)

02/19/2017 4:42 AM

It's probably Hertz. The fourth-place precision is weird, though.

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#2

### Re: Minimum and Maximum Reference VLT HVAC Basic Drive FC 101 (Danfoss)

02/19/2017 12:49 PM
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#5

### Re: Minimum and Maximum Reference VLT HVAC Basic Drive FC 101 (Danfoss)

02/19/2017 10:25 PM

The manual is not clear and doesn't explain properly how to choose/design the minimum or maximum reference.

Power-User

Join Date: Jul 2016
Posts: 217
#7

### Re: Minimum and Maximum Reference VLT HVAC Basic Drive FC 101 (Danfoss)

02/20/2017 8:26 AM

The minimum reference is set by the user, typically. It can be set for a fan or a centrifugal pump at the lowest speed when the machine actually does useful work.

For a centrifugal pump, it is often over 20% speed before liquid moves from the discharge, for a typically installed and sized pump. Perhaps someone figured out the fan was not creating flow in the cooling tower until 27 hertz.

Setting the minimum speed also allows a PI controller to eliminate a long response when recovering from 0% output, winding up until the machine actually does something measurable.

Power-User

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#9

### Re: Minimum and Maximum Reference VLT HVAC Basic Drive FC 101 (Danfoss)

02/20/2017 11:50 AM

The manual is determined not to reveal the units, or the basis to set the units in either of those two clamp parameters. The units may be implied by your choice of reference, which is (in the PWSlack refrain) not visible from here :)

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#3

### Re: Minimum and Maximum Reference VLT HVAC Basic Drive FC 101 (Danfoss)

02/19/2017 2:27 PM

Have a look at the manual, it is very likely Hz which represents the drives output voltage (motor RPM).

You will need to perform the basic conversion from Hz to your voltage and current inputs to figure out what the reference Hz values should be for what motor speed you want (but that is not hard, and the manual generally has detailed information to help).

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#4

### Re: Minimum and Maximum Reference VLT HVAC Basic Drive FC 101 (Danfoss)

02/19/2017 4:28 PM

Why is there an RTD in the tower? An evaporative cooling tower will only ever achieve local wet-bulb air temperature, unless, of course, it is overloaded.

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#6

### Re: Minimum and Maximum Reference VLT HVAC Basic Drive FC 101 (Danfoss)

02/20/2017 5:33 AM

The RTD is in the leaving water outlet of the cooling tower. See paragraph 2.6.16 on page 20 of the Manual which describes the control.

He tells us the minimum motor frequency is set for 15 Hz. However, the Reference Minimum of 27 suggests that one or more other parameters are restraining the parameter "4-12 Motor Speed Low Limit" from going below this higher value.

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#8

### Re: Minimum and Maximum Reference VLT HVAC Basic Drive FC 101 (Danfoss)

02/20/2017 10:57 AM

Jraef is good at this.

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#10

### Re: Minimum and Maximum Reference VLT HVAC Basic Drive FC 101 (Danfoss)

02/20/2017 1:58 PM

Wow, that is about as unclear and confusing as they could possibly make it...

The values in Min and Max Ref are HEXIDECIMAL values, where 4000 = 100% commanded speed in Forward and -4000 = 100% commanded speed in reverse. Why they would use a hex value is beyond me... The decimal equivalents to those would be 16384, so maybe they thought that would be even more confusing? Why the heck that needs to be so high is a puzzlement, but I THINK it is because the drive is CAPABLE of a maximum of 400Hz, which if based upon a 50Hz design as they are, is 8x the 50Hz frequency. So if you WERE using the drive at 400Hz, this scales the input speed command from the analog input reference to make the range wider. 50Hex is 80 decimal, so that implies that the maximum speed command from the analog input reference will be 80% of the base output frequency.

Convoluted though if you ask me...

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#11

### Re: Minimum and Maximum Reference VLT HVAC Basic Drive FC 101 (Danfoss)

03/08/2017 4:43 AM

You can Check the manual at here: http://dieuhoapanasonic.vn/

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