4-20mA or 0-10v for VFD speed control ?

[milemaker13@att.]

I want to be able to dial up/down my speed while running the mill. I can program the VFD for either 4-20ma or 0-10v.
How does either one work, and what device is used?

My guess is a pot for 0-10 or a signal generator for the 4-20...

 

[sfriedberg]

Both of these are pretty standard control "protocols". Crudely generalizing, the tradeoff is that 4-20mA can consume more power (strictly control circuit power) than 0-10V, but in return has greater immunity to electrical noise and a definite indication of broken control circuits. 4-20mA current control is the gold standard in a lot of industry segments.

The VFD is going to sense current when configured for 4-20mA, and sense voltage when configured for 0-10V. To have a knob provide either type of control signal, you will need both a DC power source (a 9V battery can work) and a potentiometer, and probably another resistor to match your power source voltage to the control signal limits.

To choose a potentiometer and limit resistor, you need to know Ohm's law relating voltage, current and resistance. Let's say you pick a 12V power source. For 4mA current, you put 3K ohms in series with the power source and the VFD control terminals. For 20mA current, you put 600 ohms in series. So, you could pick a 600 ohm limit resistor and a 0-2400 ohm variable resistor, putting both in series. (I am not even going to attempt to draw a schematic with ASCII art.) This scheme will draw up to 240mW for a 20mA signal, 48mW for a 4mA signal.

For 0-10V, let's pick a 5000 ohm potentiometer. You'd need to put it in series with a 1000 ohm limit resistor, put both across the 12V power source, and take the output voltage from the sliding contact on the potentiometer. This scheme will draw 24mW, more or less continuously.

Obviously, there are more complex ways to skin this cat. But all you need is a knob, not a signal generator.

 

[CalG]

The various VFD's that I have all contain an output suited for device control.

0-10V is very easy with only a 5-10k pot needed for speed.

Simple is nice.

 

[milemaker13@att.]

Thats what I thought, using a simple 5k pot to control 0-10v. I suppose that depends on the drive supplying the 10v, right?

 

[Scottl]

4-20 mA has the advantage that if a wire breaks or the signal is otherwise interrupted it triggers an error. That is why it was developed a long time ago for industrial processes.

However, for a VFD a simple potentiometer used as a voltage divider is simpler and probably all you need.

If the VFD manual does not have control circuit examples there may be application data on the manufacturer's website.

 

[Tony Quiring]

If controlling ONE vfd then use the pot.

Most units should have connections available for same meaning just connect.

Control source, control ground and control, translate "control" to whatever term your device uses.

If controlling more than one, then the other is good as many have an OUTPUT of this signal that can be connected to the SLAVE units to control them.

Our AB13XX unit has this output that can be configured to output based on frequency or load, we set ours to speed and added a simple meter movement to indicate speed.

Just a simple indicator, center scale is roughly 60 hz.

Sent from my SM-G781V using Tapatalk

 

[gustafson]

IIRC isn't 4-20ma good for distance related devices? Like if in the pre digital age, you wanted to control a conveyor belt system that had 3 motors several hundred feet apart, 0-10 volts would not be useful.
Most VFD installs we are speaking of have a pot within 3 feet of the box, so noise and resistance losses are minimal.

 

[johansen]

the issue i have with 0-10v is what happens when you have some leakage current into the wire, and your pot goes open circuit.

a 4-20ma system is looking specifically for a 4 to 20ma current source, which is harder to fake.


a 0-10v system can be made more robust by loading the wiper at the vfd with say, a 50K resistor, then using a 1K pot. if the pot goes open circuit and you have some leakage, the 50k resistor will pull the volts back down to ground and your system won't run away at full speed.

I fixed a labratory rotary evaporator last week. open circuit on the pot was full speed.. flinging hot oil at the operator.. dumb design. full speed should have been a completely shorted pot, not open.

 

[JST]

Some VFDs (at least) offer the option of direct or inverted control, so 0V can be full speed, or zero speed.

 

[Phil in Montana]

There is a 0/10 volt supply in the vfd, if 4/20 ma is wanted just takes a pot and a limit resistor, no big deal...Phil

 

[rons]

When I worked at a instrument company the current loop was the way it is done in the food processing industry.

The Science of 4 to 20 mA Current Loops - Application Note - BAPI

For personal use, what is best is what is simple.

Mikhail Kalashnikov said that he read this somewhere: "All that is complex is not useful. And all that is useful is simple". That was his whole life model.

My model is try to use what I already have in stock, as long as the goal of 1st class is used. For temporary stuff it doesn't matter.
I don't need to say I'm compatible with the food processing industry. I use pots, 2-wire, 2-wire control.

There is too much concern over what should not be a concern. That is speed control and getting to that exact rpm. Metal cutting speeds don't need readings like 59.750. What difference is that from 60. Not 60.000, but 60. But I do like the science aspect of the idea.