Hello, I had a question pop up today in an interview about the low volt side of a VFD. Now, forgive me if my verbiage is not clear here as I am trying to remember the question as it was given as I was very nervous. First off, I have installed about a dozen VFD's in my career but limited to install and connecting power. Nothing to do with the control side. The question asked was something to the effect of, "what of the following three control methods is the prefferred method to control a VFD and why?" The only 1 of the 3 methods I remember is 0-10 volts. Can someone please help me here? This was an interview for a electrical maintenence position and being a construction electrician, I am very limited on troubleshooting and control methods such things as VFD's. Thank you very much.
VFD control side interview question
- Thread starter Sherwood
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- Status
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
Hard to respond without knowing the entire question and context, but I'll try.
Assuming they were referring to how to control the SPEED signal to a VFD, the 3 most widely used methods would be 0-10VDC, 4-20mA or a remote potentiometer. For all intents, a remote pot is essentially the same as the 0-10VDC, so it's really about those other two choices. As a gross generalization, 4-20mA is a preferable choice in a speed command signal compared to 0-10VDC because of two things; it is inherently more immune to noise from outside influences and voltage drop over a distance is less of a problem. The noise immunity is based on the fact that most electrical "noise" results in changes in the voltage of a signal. If what you are READING is the voltage, the noise can result in changes to that signal Those changes can be filtered out, but in doing so, you lose some attenuation of that signal (signal loss). If the signal is current based, the current is unaffected (less affected) by any noise that it picks up, so you need less filtering and less signal loss.
If you have a DC voltage signal, the longer the distance is for the circuit, the more voltage drop you end up with. If the maximum voltage drops to, let's say 9VDC over the distance, that's not a bid deal because you can still adjust the offset and gain of the signal a little. But if the distance is REALLY long and the voltage drop is 5V, then you are only left with 0-5V as the span of your signal and when you adjust your offset and gain, an small change in the circuit, like from temperature of the wire, can show up in the response of the device (in this case the VFD speed). If you use a current based signal like 4-20mA, per Kirchhoff's law, the current is the same anywhere in the circuit. So no matter how long the run its (within reason), the current, and therefore the signal, remains relatively constant.
Assuming they were referring to how to control the SPEED signal to a VFD, the 3 most widely used methods would be 0-10VDC, 4-20mA or a remote potentiometer. For all intents, a remote pot is essentially the same as the 0-10VDC, so it's really about those other two choices. As a gross generalization, 4-20mA is a preferable choice in a speed command signal compared to 0-10VDC because of two things; it is inherently more immune to noise from outside influences and voltage drop over a distance is less of a problem. The noise immunity is based on the fact that most electrical "noise" results in changes in the voltage of a signal. If what you are READING is the voltage, the noise can result in changes to that signal Those changes can be filtered out, but in doing so, you lose some attenuation of that signal (signal loss). If the signal is current based, the current is unaffected (less affected) by any noise that it picks up, so you need less filtering and less signal loss.
If you have a DC voltage signal, the longer the distance is for the circuit, the more voltage drop you end up with. If the maximum voltage drops to, let's say 9VDC over the distance, that's not a bid deal because you can still adjust the offset and gain of the signal a little. But if the distance is REALLY long and the voltage drop is 5V, then you are only left with 0-5V as the span of your signal and when you adjust your offset and gain, an small change in the circuit, like from temperature of the wire, can show up in the response of the device (in this case the VFD speed). If you use a current based signal like 4-20mA, per Kirchhoff's law, the current is the same anywhere in the circuit. So no matter how long the run its (within reason), the current, and therefore the signal, remains relatively constant.
Thank you!! That is exactly what I was referring too. 4-20mA and potentiometer now sound very familiar now that you mention it and I'm no longer got a million things going through my head from my Zoom interview a couple hours ago. I searched high and low online but could not find what you just described. Hence the reason why I reached out here. You explained it perfectly and now understand the preferred method. Thank you very much. I really appreciate it.
petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- engineer
The best way is via Ethernet. Pots and other analog control is old school.
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
True, assuming there is a PLC or other controller involved, and now, there usually is.
But I still see a LOT of places that are resisting the ways of the world and wanting to still "kick it old school". I just had a meeting last week with an engineer for an oil company because they are going to shut down their local refinery and switch it to making "bio fuels". They will gut the 60+ year old plant and start fresh with all new electrical gear. We discussed using "smart" MCCs and Ethernet control of the VFDs. He was all for it, but it came back that the on-site crews is insisting on sticking with hard-wired controls.
"You can lead a horse to water..."
petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- engineer
Well I don't have a lot of experience with Smart MCC's but what I have seen is unimpressive. The device net ones are painful at best to work with. The ethernet ones are better but the price is insane for what you get.
I'm kind of a fan of blended MCCs with VFDs that are ethernet and hardwired contactors but with the IO local to the MCC so it can be wired in a shop and not out in the field. It's the most cost effective thing and it tends to work the easiest and set up pretty quick.
I'm kind of a fan of blended MCCs with VFDs that are ethernet and hardwired contactors but with the IO local to the MCC so it can be wired in a shop and not out in the field. It's the most cost effective thing and it tends to work the easiest and set up pretty quick.
GeorgeB
ElectroHydraulics engineer (retired)
- Location
- Greenville SC
- Occupation
- Retired
First, I 100% agree with your statement giving the advantages of 4-20mA over 0-10V. Relating the "lead a horse", I do not like 4-20mA where bidirectional control is desired. I find, in the field, that the horse doesn't understand 12mA for standstill where with +/- 10V they understand 0V for standstill. Yes, I've gone beyond your 0-10V to -10V to +10V. When technology has every common "Fluke meter" with a mA sensor, my feeling will change, but not likely before.
RumRunner
Senior Member
- Location
- SCV Ca, USA
- Occupation
- Retired EE
0-10 volts signal still is a mainstream input signal preferred by designers. As stated, 0-10 volts is not going away anytime soon.
Each format does have its advantage.
The old school (0-10 volts) that's claimed-- is invaluable for machines that require close human intervention during operation like production machines for food processing for example.
The ethernet type has its edge over 0-10 volts because it can be mounted from a greater distance.
The binary signal that it uses for control-- can be located to a max distance of 4000 ft and immune to noise using only two twisted pair of wires.
From a safety standpoint--achieving safe operation and monitoring a machine from a distance of almost a mile away is unheared of.
This may not be even endorsed by OSHA.
Yes--a horse cannot read 4-20 milliamp signal but it knows it can raise a 550 lbs load to a distance of one foot in one second.
Perfect for a workhorse to keep busy.
Each format does have its advantage.
The old school (0-10 volts) that's claimed-- is invaluable for machines that require close human intervention during operation like production machines for food processing for example.
The ethernet type has its edge over 0-10 volts because it can be mounted from a greater distance.
The binary signal that it uses for control-- can be located to a max distance of 4000 ft and immune to noise using only two twisted pair of wires.
From a safety standpoint--achieving safe operation and monitoring a machine from a distance of almost a mile away is unheared of.
This may not be even endorsed by OSHA.
Yes--a horse cannot read 4-20 milliamp signal but it knows it can raise a 550 lbs load to a distance of one foot in one second.
Perfect for a workhorse to keep busy.
petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- engineer
0 to 10 volts is convenient because it's just a voltage divider using a pot. It's been done millions of times and works really well particularly if the pot is close to the VFD and the VFD is supplying the 10-volt signal the pot is being used to make a voltage divider of.
At the analog signal has to travel some distance a 4 to 20 milliamp signal makes more sense because it is far more noise resistant.
I just don't use either much anymore. If it's something integrated into a system it's going to be ethernet. If it's a stand alone drive it's going to have a him module mounted on the doors so that the drive can be controlled from the him module and no pot or start stop buttons are going to be needed because they are built into the him module.
At the analog signal has to travel some distance a 4 to 20 milliamp signal makes more sense because it is far more noise resistant.
I just don't use either much anymore. If it's something integrated into a system it's going to be ethernet. If it's a stand alone drive it's going to have a him module mounted on the doors so that the drive can be controlled from the him module and no pot or start stop buttons are going to be needed because they are built into the him module.
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