"At speed" vs "motor running" input from a VFD

My opinion is this.

"Aux input" should mean that the contactor is pulled in. If you want to add another input to the PLC to say "at speed" then add it as an extra input, not as a replacement to the aux input.

Now, the question:

How do I support my opinion?
Is my opinion valid?

Many customers don't realize the implications of changing the parameter on the Variable Frequency drive from "running" to "running at speed" after the program has been written. They think that just increasing a timer in the "Motor Failed to Run" section of my plc program is a cure all. Some times this is not the case.

While trying to follow the "Customer is always right" philsophy, should I write my programs with the intention that this contact could perform either way?

It was a bigger deal when I was working in the packaging industry. Many conveyors that were moving parts into position would never get up to speed. The plc need to know that the conveyor was moving. I was able to support my argument very well then, and could pretty much demand that the parameter be changed back to drive running.

Now, I am in the gas and oil industry. This is a new industry for me, and I may be making a big deal out of nothing. The thing that bothers me the most, though, is when I write the program thinking "drive running", only to find out later that the contact is really "running at speed".

So, I need to know if I should write my programs for either scenario. (is this even possible?).Or, should I argue my case and demand one or the other. Am I making too big a deal out of nothing? Has anyone else ran into the same problems?

One example:
I can think of one example from an earlier post where Allen Case mentioned to place the N.C. state of the aux contact from the FWD starter in series with the Motor Run Coil of the REV starter and vice versa, instead of just using N.C. of the opposing PLC Run Coils. I think that this is a good idea. I think that this would prevent the PLC from trying to energize the REV coil if the FWD coil was stuck in, or if someone was holding it closed with their fuse pullers. Even though I believe that these reversing contactors are usually mechanically interlocked. - Now, how would this logic work if the "at speed contact were being used".

Sounds to me like you have TWO (2) different needs..1 to know motor is running...2 to know motor is running at optimal/required speed. Depending on the application these two may or may not equal the same thing.

If its an "absolute" necessity to know the precise speed of a device than you should use an encoder or analog tach, dont rely on a digital input of "RUNNING". Use the digital AUX input to show its ON/Running and the encoder/tach to enable a bit to show its at speed. I wouldnt endeavor to explain how to use either to most of you because I think you probably understand that better than me.

To me what you state...your opinion...makes perfect sense.

I think Ron Doran has said it exactly right. The two bits are similar, but not the same.

If I had a timer on the "Running" contact time out, I would know that the drive failed. If so, I might generate a Controlled Shutdown of the rest of the system, since without that drive, I won't be able to continue.

If the timer on the "At Speeed" timed out, but the "Running" didn't, I might generate a Hold for the rest of the system instead, since the drive is doing something, just not what I want.

But that's one application. In another, a drive that's not running at speed is just as no good as a drive that's not running at all.

But only you can make the call. The answer won't be the same in all cases, so you won't be able to have a cover-all rationalization ready. When you need it, tell the customer. If you don't need it, live with it.

It's an Engineering decision. Be an Engineer.

To me, the answer to your question should always be determined at the initiation of the project during the stage where you are assessing the customer’s needs. When the characteristics of the machine(s) are identified, the use of “up to speed” or “running” should be discussed and agreed upon.

So…when to use which.

I have found that the “up to speed” input is usually <u>not</b> what customers want. Normally they only want the machine to identify the condition when the motor has stopped. These types of applications generally see short ramp up/down and do not involve close synchronization between motors. In these cases, the customer is usually looking to know only that the motor is not running.

Applications where “up to speed” is desired tend to have the VFD programmed with long ramp times and/or require some sort of synchronization between motions so that the next step of the sequence can safely start.

As with so many, I have not found an easy overall answer to this question.

Like Alan said, it always seems to me to be an application-by-application question and one that you, as the programmer, will probably need to suggest the answer to each and every time.

That’s my 2 cents.

Steve

Sorry to all about the underline...html booboo.

Steve

As a long time variable speed application engineer with precious little PLC experience except for what it sends me for outputs and requires back as inputs, I find this question a bit baffling. Run and Up-to-Speed are two completely different signals, as mentioned.

Usually, a Run signal from the drive back to the PLC is used as a confirmation that the Run signal from the PLC to the Drive was acted upon. If the drive were faulted, run inhibited, or if there were safety or interlock contacts in the Run signal wires going from the PLC to the drive that were open, the result would be no Run signal back when there was a PLC output instructing the drive to run.

Up to Speed is used far less often, in my experience, and usually to indicate that the motor is running at a speed sufficient to perform some process function successfully. An example would be a stamping press drive where, if the flywheel is turning too slow, the top die would stick down in the bottom die--not a good thing, for sure! Another example would be a process where material is sorted by centrifugal force. This requires that the element that spins the product be up to adequate speed to do the sorting or else cross contamination or poor sorting would result.

Up to Speed does not necessarily have to mean above a set speed. It can mean below a set speed just as well. An example here would be a process that requires the application of a mechanical brake to stop and hold-in-place a rotating member. It is common to inhibit brake application until the drive (using regen or snubber braking) pulls the load down below a set speed. When below this speed the brake is enabled and a stop occurs.

Another use for Up to Speed is in applications where the control system (PLC usually) sends a preset fixed speed to the drive and the drive has a separate input programmed to respond by forcing the motor to a preselected speed. Up to Speed can be used for confirmation that preset speed has been reached, however, you must set the Up to Speed program parameter just under the preset speed or you can get hunting, especially on drives that are not tightly speed regulated.

My apologies on letting this get a bit long but my purpose is to be helpful, just as you PLC guys have been more that helpful to me.

Just today I was working on a program where I have to synchronize two motors; only one of which is on a freq-drive.

Neither motor is "aware" of the other. The motor without the drive is running at its speed producing one cycle-per-second. The motor on the drive starts at 0-rpm. The motors need to be in synchronization within the first 1/2-cycle after the drive starts.

The RUN SPEED (Actually, "CONSTANT SPEED-1 = FAST SPEED")of the Drive motor is set to equal the running speed of the non-drive motor.

The device driven by the drive is physically positioned 1/4-cycle ahead of the normal sync-point.

When the device driven by the non-drive motor is at Point-X, the drive starts the motor - "RUNNING".

The acceleration during START is based on 0-3600 rpm in 4.5-seconds. This is fairly fast. The target speed is 1600-rpm.

So, at Point-X, the drive starts the motor and the motor comes upto speed in approximately 1/4-second.

Having reached the target speed ("AT SPEED"), the drive motor then goes into REGULATE_MODE where the accel/decel is based on 0-3600 rpm in 600 seconds. After a successful start, REGULATE_MODE applies either REG_UP or REG_DOWN corrections at that rate.

There are sensors that track the current position within the cycle.

Now, here's where the "AT SPEED" signal shows its value...

When the drive gives the "AT SPEED" signal, the program checks the current position status.

If the drive-device is in position when the "AT SPEED" signal comes on, then I know that the drive is producing the desired effect.

If, however, I see that the drive-device is EARLY, I know I need to reduce my RUN SPEED ACCELERATION Rate.

If, on the other hand, I see that the drive-device is LATE, I know I need to increase my RUN SPEED ACCELERATION Rate.

So, for this particular application, the "AT SPEED" signal is a diagnostic tool as well as a signal to go to REGULATE_MODE.

The "RUNNING" Signal indicates an attempt being made while the "AT SPEED" Signal is used to determine the success or failure of the start-up.

Wait a minute...lets not get all ENGINEER here.

UP TO SPEED is a simple term...lets leave it there.
There are 2 (maybe more than I have thought of here/now) ways to KNOW if UP TO SPEED. I will acquiesce ( I like that word too Rick) that UP TO SPEED can have different meanings depending on application...BUT as an example lets say UP TO SPEED is 1750 RPM for a motor. You could use a timer to say the motor has been ON/RUNNING X amount of time so MUST be UP TO SPEED. How reliable is that though?

Some applications can use that info. Some need more info...ie the actual speed.

I am just a TECH but lets look at what the term "UP TO SPEED" implies. To me it means it has obtain a certain RPM ..FPM etc. If this was used as a comment for a plc that is how I would take it.

This is a case of semantics and application/needs combined. Apply the terms to the applications...try to match the term to the application.

BTW acquiesce is not YES...its more of a "I give" or " I dont object to what you say/do"
1. To rest satisfied, or apparently satisfied, or to rest without opposition and discontent (usually implying previous opposition or discontent); to accept or consent by silence or by omitting to object; -- usually followed by in, sometimes by to.
2. To concur upon conviction; to assent to; usually, to concur, not heartily but so far as to forbear opposition.

What ever happened to gearing, clutching and super-imposed moves?

to Terry Woods/RS Doran

That's another very interesting use of Up to Speed, and definitely as seen from a PLC'er's perspective. One question tho: If the drive is programmed for 0-3600 in 4.5 seconds, then usually, 0-1600 would be

4.5 X 1600/3600 = 2 seconds

I don't know how you are getting faster accel time unless the drive behaves differently than those I'm familiar with.

As far as semantics go regarding Up to Speed, it seems to me that semantics are the key to everything. Maybe the best example I can give is to read an unfamiliar instruction manual (PLC, drive, or whatever). Until you discover how key words are used, the manual is generally not very understandable.

Back in a former life as Apparatus/Equipment Engineer for Bell Telephone Labs, I remember an almost laughable lengthy argument over whether a Stop button on a control panel should be labeled: STOP, HALT, INHIBIT, DISABLE, etc, etc, and memory fails me how many more!!

We settled on STOP, sure enough!!

So, in light of all that, I guess Up to Speed can mean "when you get there, do something" or, "when you get there, stop doing something"

Mercy! How do we put THAT in the instruction manual!!

Lots of drives allolw the user to configure two separate contacts. Most of the time one of them is fault though (but doesn't necessarily have to be). I personally like the running above a certain frequency contact. I use it on fans when the minimum speed is, say 25 Hz. Then set the above freq value to 24. Then u know when the thing has ramped up. I use the first contact for "running". If you tell it to run, and you don't see the "running" contact, it's faulted.

1) Most of the time you can't just ask a customer what they want - you also have to find out why they want it and what they REALLY want and what the basic objective is.

2) A guy taught me a long time ago that you control as directly as possible the paratmeter that you are interested in. If you want to verify that the VFD is started, monitor a "Run" contact, which essentially functions the same as an auxiliary contact on a starter. If you want to know if the drive is running at the set speed, use an analog signal from the VFD back to the PLC. That will not only let you pick up delays for accel/decel, it will also let you know if the VFD actual frequency doesn't match the PLC speed command. This happens because the motor is at the amp limit, or the speed command is in "skip" range, or becuse the VFD is set up with max speed below the command speed, or ......

3) Remember that the "Run" contact on many VFDs will maintain its closed status on normal VFD shutdown until the motor has been decelerated to "zero" speed on a controlled stop. This is often useful information, because it can keep you from reversing at the wrong time, or re-starting the motor on the fly (although a lot of VFDs can catch a spinning motor).

I normally use the Run contact and not At Speed contacts.

DickDV,

What the hell can I say... Out to sea without a compass?

I don't know why I wrote 4.5 sec. for this particular exercise. I am using the 4.5 Sec value in another routine.

Anyway... screw-up aside... you got my point.