Need help with control panel

[PetrosA]

I have to design and price a control panel for a customer. It seems like a pretty simple one, but a little above my comfort level (which I need to get out of ). So far I've been dealing with Welder Dude (the customer) and Motor Sales/Repair Dude (his supplier) - not a lot of help. I'm hoping someone here can help me out.

The equipment is a manure chopper with two motors - one 5 HP with a VFD to power a chopper and a 2 HP with starter for a feed mechanism. They want to be able to adjust the speed of the 5 HP to work with the 2 HP. The unit is a portable one, designed to be hauled on a trailer hitch and powered by a 3? generator.

Our original talks included a start/stop that would control both motors together but after discussing it with Welder Dude, we decided that there should also be e-stops on either side of the machine (it's approx. 15' tall x 15' long and 8' wide, so running around or under it in an emergency isn't a good idea).

So here's where I'm at with my limited knowledge of controls:

Hooking up the E-stops:

Option 1

I could have a NC contactor to react to two NO e-stops so that pressing either would open the contacts. After that would be a NO contact for the start/stop controls that would feed the VFD and starter. What worries me with this plan is that e-stop is dependent on there being power - IOW, if one guy hits an e-stop, and another unplugs power from the generator, then they plug it back in, there could be power sent to the motors for an instant until the e-stop shuts the NC contactor off again.

Option 2

Run the start control through both e-stops NC so that pressing either would shut the NO contactor. This would only require one contactor for everything and would ensure that the motors are mechanically off until the e-stop is reset.


Coordinating the two motors:

Next question is about the VFD together with the starter controlled motor - Does the VFD need time to ramp up to whatever speed they preset? This would possibly affect whether I need a time delay on the starter controlled motor so it's not feeding into the chopper too soon.

What's painfully apparent to me is my lack of knowledge in this area, especially about what's available. I haven't found any supply houses around here with knowledgeable staff on the subject, I can't get a hold of any catalogs, and from what I've seen online, it looks like there may be better ways to make this work than what I've come up with. Relays, starters with integrated timers/contacts, VFD with contacts, etc. These are the things I don't know about and it's frustrating.

Any criticisms and better ideas you have are welcome.

 

[quogueelectric]

I strongly suggest and no offense intended is that you not design this control ckt as if you do and someone gets hurt from something unforseen you will assume responsibility for any problems. We are basically installers and unless you have training in design we are not the definition of a qualified person as per Osha regulations. Picture yourself in front of a judge and an attorney when he asks you what qualifications you posess for designing such a circuit and you get that sinking feeling in your stomach.

 

[ohmhead]

Well why do you want to use a contactor eliminate that contactor just use two simple emergency mushroom head pull on push in stop buttons put them in series wiring wise if you push in it stays off if you pull its on . Its down off your generator or no generator will never start it up again .


Heres what id do in addition to the series stop buttons which you could add more in series how about using them in addition with a contact in the VFDs there must be a aux contact in your VFD to shut it down sorry i cant see your system in person but think it out youll be fine . What are they model type ?

 

[Jraef]

NEVER use an E-Stop circuit that requires power to function. It needs to be "fail safe", so for an E-Stop, that means if a wire gets cut, the system becomes (relatively) safe by turning off. Think about your N/C contactor arrangement from that perspective. If the control wire were cut, the system would be running and you would not know the E-Stop would not work until you needed to use it!

The simplest circuit is to use N.C. contacts on the E-Stop buttons, wired in series, feeding the entire control circuit. The choice to use a main contactor is something that needs thought. VFDs don't like having their main line power cut while it is under load very often. In a true emergency once in a while that's OK, but remember that people are basically lazy and if you give them a big red button on both sides of the machine but ask them to go to the other side of the machine to use the normal Start/Stop buttons, they will eventually get tired of that and begin usng the E-Stop every time they finish a job. That will likely cause the VFD to fail prematurely.

Some newer VFDs nw have a built-in "Safe Stop" input (in quotes because every manufacturer calls t something different). These are recognized control input commands that cause the VFD to turn off rapidly if desired) and remain in a disabled state until a specific reset procedure is followed. This is often a better choice than a line contactor not only because of the VFD damage issue, but also because sometimes, a controlled decel with a VFD can actually stop the motor faster than if allowed to coast. So in an emergency, the VFD's ability to do braking may actually be better than just killing power.

As to the last issue, yes a VFD will amp to speed, although you can control the rate. But what you want to do s program an output of the VFD to change state on "Up to Speed", whatever the commanded speed may be. Then tie your Start circuit for the feeder motor starter to that output contact so that the starter control is only enabled once the VFD has reached the desired speed.

 

[ptonsparky]

Wondering why you want to control the speed of chopper. Something like that works best at full speed and then vary the amount of product fed to it.

 

[ohmhead]

Well as Jraef has mentioned the best method would be input signal the vfd as the dc buss will continue for a time during discharge so its still not a emergency stop . But in a real emergency id use a electronic brake thur vfd if it has the function to do it.

Meaning a constant dc stops motor now ! but the standard starter is another story which is more important the VFD burnt out or your life ?

When do you need a EM STOP! We did a orange fruit packing plant and each machine had two stops on each side and one main button in control room but this was controlled by the plant personal if you shut it down it better be important or you lost your job that day !

They didnt just use it for a smoke break or lunch its a stop for life safety only in that context by example .

I dont think this condition is but maybe sounds like it will be operated by anyone on the farm so whats safe ?

I know one thing iam not putting my hands in that manure or getting too close to machine during the process so i dont think its going to matter much in that sense.

 

[RETRAINDAILY]

Mcr

Mcr

We ALWAYS use a MCR (main control relay) a n.o contractor that all control circuits go through then out to start/stop.
so unless your emergency and safety latches are closed you will not have power at your buttons.

 

[ELA]

I second what quogueelectric has suggested.
Safety circuits can be as simple or as complicated as you decide to make them. How complicated they are made is usually determined by a risk assessment.

We always used "safety relays" that are specifically designed for safety circuits. Even though they add expense they can help make the design choices a little easier.

Have you searched any safety relay sites?
Here is one link to a manual download that might be helpful if you decide to go ahead with a design on your own.
http://select.phoenixcontact.com/phoenix/dwl/dwlfr1.jsp?lang=en
you can search for "safety relay application"

You can also try other safety relay sites such a Pilz.

Many people can design safety circuits but the question becomes are they really "safe" and to what level of safety?
You want to be sure they are safe to protect yourself as well as others.

 

[IMM_Doctor]

Risk Assesment

Risk Assesment

Google "Risk Assessment" / "Industrial Controls".
There are numerous factors to consider in a Risk Assessment Survey that you complete yourself.
How Severe would the injury be? Bump / Bruise / Laceration / Amputation / Death?
Frequency of Risk Exposure? Rare / Infrequent / Frequent / Constant?

There are other factors. Also consider a safety device, is a single point of failure going to lead to injury?
The ?old-school? single e-stop chain through N.C. contacts on e-stop buttons may fail to work, if the contact falls off the back of the button.
Consider a ?dual-channel? monitored circuit, and also have the safety relay monitor the ?force-guided? contacts on the power controlling device.
These devices are less than $200.00, and if you install them, you will demonstrate that ?Safety? was your key motive in the project.
My $0.02

 

[StephenSDH]

By modern standards would need redundant controls, if there is a risk injury more then a bruise(sounds like it). For SIL3, safety devices need 2 contacts, safety controller, and NC monitoring on relay/contactors, to prevent a reset when welded.

 

[PetrosA]

These are all excellent responses that I need some time to go through carefully. I finally found a supply house today that stocks the stuff I'd need to do the work and spoke with someone there who has experience in building controls. I think I have a good enough idea of what the costs will be to submit a heavily cushioned bid that will allow me to use the cushion for safety upgrades if need be.

That being said... My first impression of the customer's site is this - the safety controls they have on existing equipment are either non-existent or in horrible disrepair. This could mean a few things; either they have been using an electrician who hasn't shown them that it needs to be a priority, or the electrician doesn't have a clue and doesn't feel like doing his homework or they don't give a hoot about safety. My bid will either seem outrageous, in which case I leave the scene and dust my boots off at the door, or they'll realize they may have found a good business partner and I get down to serious planning and consulting to come up with the best option possible. Either way, I'm in it to learn something new and do the best I can.

Thanks for all your input!

 

[ty]

- the safety controls they have on existing equipment are either non-existent or in horrible disrepair. This could mean a few things; either they have been using an electrician who hasn't shown them that it needs to be a priority, or the electrician doesn't have a clue and doesn't feel like doing his homework or they don't give a hoot about safety.

It might just be, when they turn iton, it works.

I agree about the Start/Stop N.C.

I also would suggest an auxillary contact on the Chopper started to shut down the Feed starter. So in the event the Chopper stops running, the Feed stops running. That way the Chopper won't get filled up with too much excess.

 

[ty]

BTW, there are a few Rock Quarries near you.

You could go to one and possibly hire one of the Electricians there to help you design your control circuit.
A quick 'pop in' might save you some $$

 

[dmagyar]

Try automation direct for control devices

Try automation direct for control devices

I use automation direct, they have lots of stock and are located in Georgia, they ship promptly and are priced moderately.

I'm summarizing here so bear with me; With regards to controls I use a main disconnect to control power coming into the control panel ahead of everything. I include a control transformer, so that I'm using 120 volts for control. I use a main control holding relay to control the main contactor. I use an illuminated EMO mushroom head with a N/C contact (opens with button in) to control the control circuit, and a N/O (closes with button in) for the illumination of the button. This also indicates that you've got power to the unit. I like to use a holding relay in the control circuit to drop out if you have a power failure. It has a separate reset button with a momentary contact to restart the controls. That way when power is restored the equipment doesn't come back on until someone pushes the restart, no matter what situation the other controls are in "IE" HOA in the auto position. Oh yes, supplementary circuit breakers for all motors.

As far as advice, test your 120 volt control circuit with a drop cord, prior to installation and connection of your 3 phase power. If you have problems its better to be in a controlled environment to sort them out, than in front of your client.

 

[Jraef]

... My bid will either seem outrageous, in which case I leave the scene and dust my boots off at the door, or they'll realize they may have found a good business partner and I get down to serious planning and consulting to come up with the best option possible.

A good friend of mine did the exact same thing at a local re-bar fabricator. The previous electrician did everything as cheap as possible and "made it work" with no eye towards safety. My friend went in there on an emergency breakdown situation, got them running, but pointed out the safety problems they had. He has been steadily reworking each machine one at a time with proper safety controls for the past 2 years, and in this tough economy, that has been nice steady work he can rely on every month. So it never hurts to ask and taking the high road can sometimes lead to a nice relationship the benefits both parties.

 

[petersonra]

The real key to these kind of things is come up with some kind of risk assessment that will help focus you on what steps you need to take.

Often there are risks that are not obvious that should be taken into consideration and you need to account for them in the assessment.

Safety relays are a good idea not so much because they are always necessary, but because they are cheap enough that you can put one in and avoid spending the time and money on making the assessment of the reliability of the estop circuit.

One thing that is really important though is not to oversell what you are planning to do. You are NOT making the machine safe. You are improving the reliability of the estop circuit, and that is really all.

I am betting that the machine probably does not have guards, guard switches, cable stops, and all the other things that this kind of machine should have.

 

[x743]

Make sure you size the box to deal with the heat developed from the VFD. I had an oh by the way project that needed to be completed in 3 days. Got the box, installed the 20 horsepower drive, and had a box running a temp of 140F.

Needless to say that afternoon I was looking for a good box vent fan.

 

[Volta]

. . . One thing that is really important though is not to oversell what you are planning to do. You are NOT making the machine safe. You are improving the reliability of the estop circuit, and that is really all. . .

Good point. Pointing that out verbally and on your contract is a small thing to do, but goes a long way toward the customer/jury being reminded of the limited scope of your improvements.

Farms are dangerous.