AC Frequency Drive

First things first. Please don't use all caps (capital letters) in your post. In blogs, forums and emails, they mean that the person is shouting. On the practical side, it's more difficult to read all caps.

Now to your question.

You need to clarify a few things.

• What do you mean by "variation in temp"? If I assume that you mean temperature, whose temperature is being varied? the PLC? the drive? the motor?
• What is the relationship of the variation? Does it increase or decrease with speed?
• Is the variation happening only when the PLC is controlling the speed? If you vary the speed directly from the AC drive, is the temperature still changing.
• By "AC drive" do you mean a "variable-frequency-drive" or a "soft-starter"?

If you have a critical question to ask, it is important to include the details so that we can make a good analysis and come up with good answers.

SIR,

WE ARE USING UNITRONICS PLC AND THAT ONE IS 570 MODEL THERE WE ARE CONTROLLING HEATING AND COOLING WITH SAME PLC WE ARE CONTROLLING SPEED OF AC FREQUENCY DRIVE 0 TO 10 VOLT FROM PLC WE ARE TAKING AND WHEN WE ARE STARTING DRIVE AT THAT TIME IN PLC IF TEMPRATURE IS 50 THEN IT WILL DECREES UP TO 10 DEGREES EVEN WE ARE ALL READY PROVIDE SHIELD CABLE BUT WE NOT UNDERSTAND,PLEASE SUGGEST ME THANKS

Let me rephrase my questions:

• What are you heating and cooling?
• What is your drive controlling? If it's a pump, what is the pump's purpose?
• When you start the drive, the temperature goes down before going up? The temperature of what? your drive? your motor? your process?

You're being too short with your post. I suggest you give more detail about your system so that we can understand what is wrong and what needs to be done.

I'm being very patient but if someone else comes here and reads your post, you'll get the same comments that I gave and probably some more that you won't like.

Then, once again, please do not use all capital letters.

sir,

i all ready explain before that we are using plc 570 model for controlling heating cooling and we are taking out put 0 to 10 volt output for ac frequency drive and when we are starting drive same time even its connecting or not temp decrees in the plc even we are not giving reference to the drive,

pls find out the solution

Just check your PID in PLC logic it might have defirent addresses

Sorry, but your statements are not easy to understand, please make short sentences for each point, maybe we will understand you better then.....sorry.

It is very hard to understand exactly what you are asking. Even though you already explained it before.

Most people can read between the lines, but in this case, it's just one huge line.

One suggestion would be to add correct punctuation where it is needed. This will help everyone know where one statement ends and where the next begins.

These two aren't so bad:

"i all ready explain before that we are using plc 570 model for controlling heating cooling"

"and we are taking out put 0 to 10 volt output for ac frequency drive"

But the next is open to interpretation:

"and when we are starting drive same time even its connecting or not temp decrees in the plc even we are not giving reference to the drive"

Is this supposed to say:

That you are starting multiple drives at the same time?

Even if it's connecting or not the temperature decreases in the PLC?

Even if we are not giving reference to the drive?

There are a few ways to interpret your statements, and nearly impossible for someone to give you adequate advice without know exactly what you mean.

Like the other post says, please break your staments into shorter ones to help us understand what you're writing.

very simple sir in one line i will explain we are using variable frequency drive,and when we are starting the drive means supply going and drive start to work plc reading is changing,means some cases its going dawn in some cases up if drive in stop condition then no ptoblem all is ok no variation in temp,we are using simple pid control from unitronics plc. pls suggest me now

I and others, have suggested that you only write short statements, obviously you feel that this advice is not needed for you to be understood fully.

WRONG!!!!

Your English is mixed up (I am sure my Punjabi would be too!! which is why I only write in English) and not easily and clearly understood by CR4 members....

PLEASE PLEASE write short simple statements ONLY.....

Then we might be able to understand and help.......

Hi guys

Is the plc running off an external power supply?

Is the (frequency drive ) not actually a analog phase controler? possibly controling a heater bank as opposed to a motor?

Thanks

BCD.

actually you do not have mind because 24 volt plc output cannot control heater ok our problem is all ready solve

thanks

No, it is not clear.

If I interpret your question as you say it, it seems that your PLC temperature is decreasing. From where I come from, that is not a bad thing.

Now, if you say that it's the temperature value in the PLC that's decreasing, that would make more sense. However, I'm not sure if that's what you mean.

Please read what I interpret your statements to be.

1. You have a PLC that is reading the temperature of some process.
2. Your PLC sends a 0 to 10 volt signal to your AC drive to control its speed.
3. I don't know if your AC drive controls the temperature of your process. You do not mention this.
4. When you start the drive, the temperature of the PLC decreases? the temperature of the drive decreases? the temperature of your process decreases? which one is it?
5. If I assume that your drive controls the temperature of your process, is the temperature decrease real (it's really decreasing) or is the temperature indication decreasing? This is important.
6. Now you say that if the drive is started, even when it is not connected to the PLC, the temperature in the PLC decreases. This is how I understand your post.

I don't care what PLC your using. It's not terribly important at this time. I do need to know how your system is set up. If you cannot, or will not, provide this information then I cannot help you.

sir,

very simple sir we are using variable frequency drive and if speed is controlling by plc or direct same affect even there is no link between the plc and drive then also starting time plc reading of m.a. is changing( milli ampere reading is changing) even we are using shielded wire also.

Hi Joginder,

I just want to say first of all that when I set up my forum name I didn't think too clearly about what I was doing. Now it is too late to change it, but I just want everyone to know I'M NOT SHOUTING.

If I understand clearly, I think that when your PLC calls the motor drive to start the motor, the PLC temperature drops by some degrees. I can't say I've ever noticed anything like this myself in such a system, but I probably wouldn't be monitoring for it. I'm not familiar with your PLC, does it have a cooling fan? If it does maybe the action of driving the 'run' signal, or the analogue speed signal, to the motor drive prompts the fan to start. Are there any other problems with the set-up?

no sir, there is no cooling fan,its very simple if you under stand my problem that if even we are not controlling the drive by the plc then also if we are independtly controlling the speed by potntiometer then also our m.amps signal is dropping its surprise i understand little we not used isolate supply by the same way we are trying to solve the problem if you have any solution pls suggest me thanks.

It sounds like this user needs to use shielded cable for his connection from the drive to the motor and from his temperature sending device to the PLC. It sounds like whenever he runs the motor, it is inducing an error into his temperature feedback. Maybe just separating, or running in separate conduits, the wiring to the two devices could help reduce the induced error.

It sounds like this user needs to use shielded cable for his connection from the drive to the motor and from his temperature sending device to the PLC. It sounds like whenever he runs the motor, it is inducing an error into his temperature feedback. Maybe just separating, or running in separate conduits, the wiring to the two devices could help reduce the induced error.

This is basically the same thing I understood.

Questions ... how are you sensing temp back to your PLC? Thermocouple .. 4-20ma .. millivolt...?

As said above... twisted shielded wiring ...grounded at only one end.

How far from drive to motor. If above 240v... the further they are apart the greater the reflected is.

And of course..you didn't run the sensing leads in with your drive wiring...

I think that our enquirer is saying that the process temperature variable registered as an input to his PLC drops when the motor starts.

This could be caused by the temperature measurement system being sensitive to the voltage drop in the main power supply,because the voltage drops below the regulation capability of the sensing system,or because of a fault in the sensing system.

Perhaps it would be discriminatory to suggest that in his country the power supply might be a little fragile.

To prove or disprove this theory,run the measurement system from a UPS.

joginder,

i think your problem is due to interfeariance of motor.

as you say your temperature indication changes up to to deg, when motor start, check your probe is isolated from your actual macine it may shorting to body on your machine and thats why it shows errer, this is because of improper grounding, what you are trying to say is not clear. simply ans follwsjust yes no type.

you are using plc for temp indication controlling === y / n

same plc is controlling your ac drive ============

your ac drive input signal is0 to 10 v dc ==========

what is your range of operating frquency =========

what is your minimum speed of conveyor==========

for what you are using this ====================

doyou set your temp controlling parameter properly===

does your out put is isoleted====================

any other load connected with the same incomingline suppply ====

due to that load your temperature changes============

this problem is linkedwith ground loops==============

from which contry state you are ===========( india/ punjab)

may this help to solve your problem.

thank you very much to all for the same collaboration i solve the same problem actually all supply was not islolated from same supply i was using control and plc also due to variable frequency drive all milli ampere signal was disturbing beacause all temp signal in m. a. thanks

His "mish mash" of English was such that I could not understand the problem 100% and even the fix he wrote was a half mystery.

Thank God (all Gods!) that its fixed.......!!!

thank you very much sir,

this is the first time i am askiing any question in my life actually i work in gulf for 10 years i also visit south korea and hope sir by the grace of god we will solve all problem

Sir,

I appreciate the struggle you are having in expressing your problem in English. I believe a very likely cause for your temperature signal's error can be from electronic noise being imposed on the DC signal from the fairly high-frequency of the VFD's carrier wave. Three suggestions for you:

1. Make sure that the motor cable's earthing (ground) conductor is connected only to the earth terminal at the motor and the earth terminal on the VFD's terminal strips. If the VFD has one for line and a separate one for load, make sure that you are using the load earth terminal. It must not connect to any other earth termina(s). If you twist (spiral) the power and earth conductors where they are within the panel, this would also help.

2. If you are still seeing too much noise, then within the PLC's program you can write some logic to average the signal over a short period of time (perhaps a half second or so). Depending on how you do this, it can be a moving average, which will cause the least lag in response. You may find the PID parameters to be so hard to set that a more simple proportional-only control could be more stable.

3. One time, when I was routing a 0-10VDC signal over a twisted pair on about a mile of commercial telephone cable, the noise from the voice signals was about twice the amplitude of the DC signal I was monitoring. I did the moving average mentioned above and also connected one winding of a control transformer as a series reactor to give a partial low-pass filter.

Others' posts have mentioned the need for a shielded cable for your DC signal (but ensure that the shield is earthed at the PLC end only). Also, the routing of the DC cable needs to be isolated from that of the VFD power cable to the motor, by at least 15cm wherever possible within the enclosure and in a separate conduit outside (but it can cross the path of the power cable at right angles). You can obtain shielded power cables; their use may be necessary if the above steps do not solve your problem.

--John Mueller

ok sir in one place you write ensure you connect shield wire at plc side why sir not other side pls explain

Sir,

Two reasons why: 1) Connect the shield (or "screen") at only one point. If the shield is grounded (or "earthed") at more than one point it forms what is often called a "ground loop". Almost always, the ground potential will vary from location to location, depending on many local conditions such as soil types, nearby electrical equipment, earthing locations for power or lightning protection, building materials, and others. The purpose of the shield around the signal conductors is to intercept any electromagnetic radiation from other sources such as power conductors, switching transients, radio-frequency transmissions, and so on. It will then drain them safely to the ground. If the shield is grounded at more than one point, it becomes a conductor in the ground circuit, which will itself be radiating electromagnetic waves to the signal conductors inside the shield. This is just the opposite of its intended function.

2) The single location you choose is often a matter of preference or policy. A control system with a PLC often has more than one shielded cable. Process equipment may be moved; a reliable bonding ground path to this equipment may be interrupted at times. Sensors may be changed. The PLC is less subject to these types of interruptions and probably has a reliable grounding path. Even if the grounding path to the PLC is poor, it is handling both the input signals and outputs related to them. It is probable that the people servicing the various pieces of equipment are less-skilled or have less knowledge than the person who works on the PLC and its enclosure. For these reasons I prefer a policy of grounding all shields at the most central location which is usually the PLC.

I will also usually heat-shrink a small piece of shrink-fit tubing around the stripped end of the shielded cable where it is not going to be grounded. This will make the installation neater and decrease the likelihood of accidental or unintentional grounding of the shield at these locations. In addition, if the shielded cable is on a multi-drop run, at each intermediate point I may bring out the drain wire (or twist a braided shield together into a wire) and insulate these wires. I will connect these to a terminal which is clearly designated for this purpose and is not bonded to ground.

I hope this helps. -JMM

I've misunderstood the question AND the answer,can someone please translate into English/Pidgin.OK just the answer.

Be kind, as I've just escaped with a severe belt caused by someone failing to earth a metal clad isolator.

Maybe I have missed something also here? I thought the original problem was that the PLC temperature (itself) was falling, when the drive was started, not the process it was measuring? If this is the case, then the PLC is probably mounted in the same cubicle as the drive. When the drive is off, the anti condensation heaters are on, and the cubicle temperature rises. When the drive is started, the anti condensation heaters are switched off automatically, and the drive cooling fan also starts automatically, drawing cooler air into the cubicle from the outside. I too am having difficulty following this thread?