VFD general operation question

Status
Not open for further replies.
I

Isaiah

Senior Member
Location
Baton Rouge
Occupation
Electrical Inspector
I have a general question regarding VFDs. Does the Drive frequency vary according to a linear 4-20mA feedback from field mounted transmitters such as temp,pressure,level etc??


Sent from my iPhone using Tapatalk
 
It can be a manual input, voltage, mA, ethernet..,my applications it was ethernet from a PLC, analog inputs to the PLC
 
Isaiah said:
I have a general question regarding VFDs. Does the Drive frequency vary according to a linear 4-20mA feedback from field mounted transmitters such as temp,pressure,level etc??


Sent from my iPhone using Tapatalk
Click to expand...
It depends entirely on what you setup the system to do.
 
tom baker said:
It can be a manual input, voltage, mA, ethernet..,my applications it was ethernet from a PLC, analog inputs to the PLC
Click to expand...

So inputs to PLC from the field instruments then outputs from the PLC to the Drive to vary the speed/frequency right?


Sent from my iPhone using Tapatalk
 
Is this a general question or do you have a particular model of VFD you're working with? If the latter, what is it and we can provide more detailed feedback.

There probably aren't too many applications where feeding a transmitter signal directly to the analog input of a VFD would work well. A PID loop running on a PLC would provide better feedback control.
 
EC Dan said:
Is this a general question or do you have a particular model of VFD you're working with? If the latter, what is it and we can provide more detailed feedback.

There probably aren't too many applications where feeding a transmitter signal directly to the analog input of a VFD would work well. A PID loop running on a PLC would provide better feedback control.
Click to expand...
Depends. We have a lot of irrigation systems that only use direct connection to the VFD.
 
Is possible you set parameters for 4 mA input to run at a certain speed, and at 20 mA you run at another speed, everything in between is likely at linear ratio on the speed. The 20 mA reference could be either the high or low reference as well - all in how you set the parameters.

Then for some applications you may use PID functions to lessen surges in output - the PID function predicts what will be needed next and increases or decreases output in anticipation of what will be expected to happen next. (it will start backing off of a high setpoint before it is reached to prevent overshooting or same for low setpoint. Get it tuned right and you can hold something at a steady output rate, flow, pressure, etc. even though other aspects are changing that will effect whichever item you are trying to keep stable. In our discussion flow may change but you are trying to maintain a constant pressure even though flow changes.
 
Isaiah said:
I have a general question regarding VFDs. Does the Drive frequency vary according to a linear 4-20mA feedback from field mounted transmitters such as temp,pressure,level etc??


Sent from my iPhone using Tapatalk
Click to expand...

You are asking a GENERAL QUESTION which “generally” pertains to how VFDs are controlled.
There are two ways in accomplishing this task.
Explaining this in its simplest terms make the effort easy to understand--than going into tangents--and what is experienced in the field.
Telling OP that his unit being maintained is directly fed to PLC or how it’s done -- depends on how it was setup--doesn’t give a hint how it is accomplished-- let alone how it works.
Controlling VFD is done either by reference voltage of 0 to 10 volts or as you mentioned 4 to 20mA analog input.
It is a “gold standard” for electronics engineers to use 4 to 20 mA feedback signal.

The advantage of using 4 to 20 mA scheme is (among other things) you can use only two wires to the sensor and can be immune to noise.
You can run feedback signal with long runs without worrying much for exposure to electrical noise.
On the other hand, 0 to 10 volts is ideal for short runs like process control, monitoring scales like load cells for monitoring weight and identifying different composition of liquids etc.

Problems arise and confusion due to NOT understanding analog references and ground loops.
Basic knowledge is essential in understanding and sorting out cacophony of complex topics.

Responses are often coming from people lacking formal training that often result in unreliable source that learner should be aware of.
They are often conflicting and incomplete.
This is the downside of grabbing bits and pieces gleaned from the internet.

These people think that the web is a substitute for formal education.

Fish
 
Fishbrain said:
You are asking a GENERAL QUESTION which “generally” pertains to how VFDs are controlled.
There are two ways in accomplishing this task.
Explaining this in its simplest terms make the effort easy to understand--than going into tangents--and what is experienced in the field.
Telling OP that his unit being maintained is directly fed to PLC or how it’s done -- depends on how it was setup--doesn’t give a hint how it is accomplished-- let alone how it works.
Controlling VFD is done either by reference voltage of 0 to 10 volts or as you mentioned 4 to 20mA analog input.
It is a “gold standard” for electronics engineers to use 4 to 20 mA feedback signal.

The advantage of using 4 to 20 mA scheme is (among other things) you can use only two wires to the sensor and can be immune to noise.
You can run feedback signal with long runs without worrying much for exposure to electrical noise.
On the other hand, 0 to 10 volts is ideal for short runs like process control, monitoring scales like load cells for monitoring weight and identifying different composition of liquids etc.

Problems arise and confusion due to NOT understanding analog references and ground loops.
Basic knowledge is essential in understanding and sorting out cacophony of complex topics.

Responses are often coming from people lacking formal training that often result in unreliable source that learner should be aware of.
They are often conflicting and incomplete.
This is the downside of grabbing bits and pieces gleaned from the internet.

These people think that the web is a substitute for formal education.

Fish
Click to expand...
The other thing though is for variable speed drive and say controlling pressure created by a pump, 4 mA may commonly be the low speed reference and 20 mA the high speed reference, but the drive only sees it as percentage of output and not actual pressure, so if your pressure sensing device is putting out 4-20 mA it could be that that high (or low) output signal is at zero PSI, 20 PSI, 100PSI, 1000PSI....

My point is it isn't exactly plug and play unless you are using some sort of OEM system that was already configured by manufacturer for the application. Generic or all purpose drives and sensors you will need to set up parameters to suit your application and can have several possible ways to do a particular installation depending on what accessories you might have at your disposal.
 
kwired said:
The other thing though is for variable speed drive and say controlling pressure created by a pump, 4 mA may commonly be the low speed reference and 20 mA the high speed reference, but the drive only sees it as percentage of output and not actual pressure, so if your pressure sensing device is putting out 4-20 mA it could be that that high (or low) output signal is at zero PSI, 20 PSI, 100PSI, 1000PSI....

My point is it isn't exactly plug and play unless you are using some sort of OEM system that was already configured by manufacturer for the application. Generic or all purpose drives and sensors you will need to set up parameters to suit your application and can have several possible ways to do a particular installation depending on what accessories you might have at your disposal.
Click to expand...
OP’s question is too broad a question to answer.
Yes, VFDs require varying signal to be able to control according to customers’ need or process requirement.
To fully grasp its dynamic nature, one has to have a comprehensive
understanding how VFDs are controlled.

Unlike the old scheme motor control, DC motors were used to enable this preferred way of doing things.
Speed can be controlled either electrically or mechanically.
In controlling AC motors during the early days of VFDs, (also known as PWM) pulsed width modulation were used.
Tweaking a trim pot manually to adjust speed were used in those days.
The primary components like SCR (now done with IGBT) the DC bus are the given part. . . meaning they are the essential components.
To make an easily understood way on how this is accomplished, I will explain the basic control using SCR.

The SCRs are turned on at a certain angle in the SINE WAVE.

Hold it for a minute. . . I will make it so we can get away with many jargons and equations.
I will try to avoid invoking the vagary of engineering test books, graphs and its accompanying complexities.
After all, people are asking questions to clarify what is not understood.
Using jargons and math equations only make things more convoluted.

As I I said in another post--generally, VFD control uses 4-20 mA signal. . . . the most preferred way.
You may find that this signal is derived from a source of varied feedback signals-- ie pressure, temperature, level etc.
These signals are then converted to 4 – 20 mA signal that the VFD can understand.

So, to answer OP’s question:
“Does the Drive Frequency, voltage etc vary according to the linear 4-20 mA?”
Yes, 4 to 20 mA and its signal variation is used for controlling VFD’s. entire functionality since current and voltage work together.
Depending on the nature of the load ( capacitive or inductive) voltage and current could lead or lag.
That’s the phenomenon that engineers are harnessing to our own advantage.

Fish
 
Fishbrain said:
So, to answer OP’s question:
“Does the Drive Frequency, voltage etc vary according to the linear 4-20 mA?”
Yes, 4 to 20 mA and its signal variation is used for controlling VFD’s. entire functionality since current and voltage work together.
Depending on the nature of the load ( capacitive or inductive) voltage and current could lead or lag.
That’s the phenomenon that engineers are harnessing to our own advantage.

Fish
Click to expand...
Answer is yes, along with- but there is more to defining what that signal represents and how the drive it to respond to it.

You can have a pressure transmitter that sends 4-20 mA that represents 1-100 inches of water column or you could have one that represents 0-100 PSI, 0-500 PSI, etc. Now say you are going with the 0-100 PSI transmitter but want the drive to run at full speed at say 75 PSI, you probably will need to tweak some parameters so that 16 MA or greater is what input would be when you are to output full speed. Or if the transmitter is programmable tweak it's output to match the needs of the application.
 
Status
Not open for further replies.
Top