VSD 4-20ma

Read the operations manual for your VFD. There will be info on how to program its inputs and how to scale its response to those inputs. That is what the manual is for!

Because is not written in the manual that's y i'm here asking. anyway thanks for your reply.

It would have helped if you had of mentioned in your original post that there was no information in your manual that pointed you in the proper direction.

Action Instruments makes a line of great products. I have used many of them. Here is a link to their website.

https://www.eurotherm.com/

Thanks for the input. Much Appreciated.

A 'B-block' should invert the input, though checking with http://www.lee-dickens.biz/ is recommended before purchasing one (usual disclaimer), as there may be other solutions.

Excuse my ignorance all. VSD? Variable speed drive. But what are you all using those low power things for? Model race cars?

j.

The "4-20ma" referred to is the input "speed reference" to the variable speed drive from an upstream control device and not the output current from the drive.

What is All VSD? Is that a manufacturer? Assuming we are talking about Drives, most I have worked with have parameters that can be programmed for either the max. frequency or the max. percentage (referenced to base speed) at 20mA. (i.e. you put in 90Hz or 150%) Some of the simpler drives I have worked on have had a trim pot for this function so you could look for a trim pot that might be marked on the PCB.

Shawn

If your drive allows you to do -20ma to 20ma you could do the following:

1. Set the Drive for -20ma to 20ma

2. Swap you signal leads (this will give you -20 ma to -4 ma)

3. Set up the min speed at -20ma to be 10hz and then calculate an appropriate reference speed at max to give you your desired result at your -4 ma setting.

4. Then set a max cap for your speed (separate from the signal max setting) this will keep the drive from taking off if the signal goes to 0 amps.

line item 4 is critical. YOU COULD GET UNDESIERED RESULTS at 0ma say with a dropped signal line.

hope this helps.

Perhaps...

But, three years down the road when the drive fails and needs to be replaced, if the exact instructions on how the drive was coerced to work as needed is lost, is the tech who has to change it at 3:00 AM, going to be aware of the hoops needed to be jumped through to make the replacement work properly?

If it can't be done in software, then it needs to be done via permanent hardware such as a signal conditioner. Of course... this is IMHO.

This may not provide the exact information that you requested, however the Fluke recently developed process mA clamp meter. The Fluke 771 measures 4-20 mA without breaking the loop.

Sorry to disagree but all drives need to be programmed. It is the job of the integrator / plant engineer / maintenance mgr to keep up to data spec's on all drive configuration. I have put in more drives than I care to remember and all of them have needed to be configured. The -20ma to -4ma is not an unusual setup... neither is a -20 to 20ma. You are right about the conditioner... but you will also have to configure it as well. At 3am can a maintenance tech remember what jumpers to set on the conditioner without proper documentation. RE the drive, can same maintenance tech remember the drive spec's on a drive with 200 configuration parameters without documentation. I see the argument as mute.

Oh never worry about disagreeing, that's what makes this forum interesting. Many different opinions and viewpoints make a discussion much better.

While I agree with everything you said about who has responsibility for maintaining configurations and that every drive needs to be configured, I would still not use a drive in the way you mentioned.

In this day and age of serious fines for work place accidents, to use a drive that is connected so that it could possibly, given an error in programming and a failure in a 4-20ma control loop, run away and be out of control speed wise, is not something I bet to many corporate types would approve of. Though the nature of the driven system could make it irrelevant.

I have to concede that the method you propose is valid, and I have done similar in the past when had to, its the wisdom of it that I question given todays reality.

North of 60, Is your problem with the connection or with the use of 4ma as a max speed reference? No matter how you cut it, if Stanley uses a 4ma signal as the max signal and drops his line getting 0 ma he will have a problem unless he has the ability to clamped the out put of the drive. Some drives such as the Powerflex give the ability to swap min and max speed settings. With your logic you would still be in trouble if the drive lost its reference signal... unless the drives failure settings were not set to shut the drive down on 0 ma. This is a programmable setting so again we get ourselves into the issue of litigation because a tech forgot to program a drive properly. Similar issues happen when swapping direction control on servo amplifiers. you first have to swap the reference voltage and then swap "A" and "B" of your encoder. If the lack-less technician hooks a new amp up and swaps the control signal the drive will take off and wont stop until it either hits the wall or the stops. (that is unless the drive is programed to have a position error) oh wait we are again getting back to a programming error. Face it the job of a controls engineer is to attempt to eliminate the chance of "idiot" failure but there are times we just can't. So back out of my tirade, we should first ask is it mandatory to swap the control reference. If it is then how can we best resolve the issue. If you have a 4 -20ma inverter for Stanley that also has built in clamp to bring the inverting output signal to 4ma on the loss of signal from the input by all means he should use it. But with your own admonishment if there is a means to defeat this clamp via jumper settings it should not be used. Now re the Power Flex which has a means of internally swapping the reference signal (and possibly different drive manufacturers as well) he would have to guarantee that the speed clamp function was on in the drive. as well a a fault shutdown on 0 ma. As a side issue how would you guarantee that an electrician would not swap a -20 to 20 ma signal sending the machine backwards on startup on a bidirectional machine?

Now we have gotten way off of the initial question. Stanley, I did a little sleuthing for you, depending on your drive you may be able to do every thing internally in the drive to swap the signal, see the above statement re the PowerFlex drive. http://www.plctalk.net/qanda/showthread.php?t=24612

What drive do you have? Just make sure you have you drives set to fault at 0 ma. If you do not have a drive with this function look at converters with the ability to shut the signal down if it looses its input signal i.e. it goes to 0ma. I tried to find one of these for you but a cursory Google brought up no leads. Another solution would be to massage the signal in your PLC if you have analog inputs and outputs. Again make sure your code shuts the output down if the input signal goes to 0ma. As a side note (you have probably already done this) make sure your control of the drive is via more than the analog signal i.e. a stop start circuit and you have a full hard wired E-stop circuit built into the System that will defeat the drives operation.

Hi,

I startup and service VTAC 9 drives, during startup I ask the customer what their needs are what, type of signal etc. These particular drives provide adjustable parameters about reference input and resultant output frequency. It really is quite simple to do from the drives keypad. If you would supply the drive model someone may be able to further help you. The andwers are usually in the documentation.

As a side note that that indirectly relates back to the original issue, I have yet to find a drive I could not secure a manual for on the internet, as long as it was a digital drive, i.e. one manufactured within the last 10 years. Older than that, and it is probably not a good idea to start up a new application with it.

Stanley,

Post the make and model of your drive.