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Old October 22nd, 2007, 07:41 AM   #16
brucechase
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Quote:
Originally Posted by PeterW
The only contactor required for an inverter, would be the power on contactor, which could be the safety contactor, which would be BEFORE the inverter.

Doug is quite correct, you should not put a contactor between the inverter and the motor. You use the control signals to RUN/STOP the motor, not the contactor.

Overloads also go before the inverter as you are protecting the inverter, the inverter protects the motor.

Where the inverter supplies multiple motors, each motor should have its own protection after the inverter (in addition to the inverter protection), but if one trips the inverter should be stopped.

We had a HMI selection where we selected if a drive was enabled or not, therefore if one had a fault and tripped the feedback would be different to selection and the inverter commanded to stop. If the motor had a fault, then the HMI would be used to de-select that drive to enable restart without it. The breaker would be locked out. If someone turned on a breaker that was de-selected then again the inverter stopped.
Completely disagree on this!

Several drive manufacturers have instructions on putting contactors AFTER the inverter and how to control them. I do this when safety application demand it. In some situations, I've installed 2 contactors after an inverter. I have NEVER had an inverter fail because of this.

All overloads go between the inverter and the motor - not before the inverter. (I'm not sure why you would use one if you are only controlling 1 motor since all drives I've seen have an overload function built in.) As a matter of fact, most overloads won't work properly before an inverter since it has a dc bus that can charge and discharge. If there are multiple motors, each motor get an overload and that is between the drive and motor.

A circuit breaker or fuse goes before the inverter to protect the inverter in case there is a SHORT CIRCUIT. The circuit breaker protects the wiring and components in the drive and motor.


There are many other things I disagree with when it comes to the OP. Switching the negative in NOT illegal, especially if the DC common is not grounded (as I have in many cases). How would this apply in sinking/sources applications if you can't switch the negative? A sensor or other I/O device is nothing but a switch to the plc.

Stationmaster:

Quote:
At least the plant I was referring to requires a bell to be rung 3 times before starting. I guess if no one yells during the startup bell they figure it's ok to start the system. I know they have power lockouts near the motors and conveyors....if they use them. I try to keep my head buried in a panel and stay out of the machine area if I can. It's out of my control......I just get called when needed. The owners/managers make those decisions, I just try to make the control system do what they want it to.
That used to be the way the conveyors were controlled here. I changed them to sound the alarm for 4 seconds and then require another push of the start (after the 4 seconds but before 8 seconds) before they start. My reasoning is if someone yells, then the conveyors won't start automatically so someone would have to hit the stop button. We also have estop ropes (with slack detection) and a red light on each switch so when it is activated, it can be seen for hundreds of feet away.


I'm also not sure what prompted this but my general warning is to take what is said with a grain of salt. I don't follow Doug's "rules" in many cases and think that some of his rules would violate modern safety practices. As Ron point out, Risk Assesment wtih the proper applicaiton of safety categories is the "proper" way to design a system.
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Old October 22nd, 2007, 09:47 AM   #17
rsdoran
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When I first came here I thought I knew something and was quick to throw out things. I found out real quick I did not know sheet and many of the things I did know there were other and sometimes better ways to do.

The thing is people learn in different ways, in different industries, and under different conditions. Codes, Standards, and Laws can vary from locale to locale as can availability of components.

People that work with plc's usually end up with a specific brand they either "have to" or "want to" work with. Over time they develop code they can use and reuse over and over. Is it better then someone else's method? Maybe, maybe not, what is relative is the fact that it works. The problem, in some cases, is many may think their method is the only way to do it.

I think the original poster has fallen into the idea that Mitsi should be the plc of choice, his methods are the best, and things can only be done one way.

There are others that think that way but the one thing I have learned is everything changes, you have to keep your mind open.
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Old October 22nd, 2007, 09:56 AM   #18
mordred
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Sinking or sourcing
I personally like sinking the inputs and sourcing the outputs. I think that sinking inputs are less susceptible to noise. A sourcing input is sitting there low all the time waiting for a high and it could get the high off of a wire laying next to it from a solenoid valve when the field is collapsing. A sinking input is sitting there high and its waiting for a low. I would source the outputs just to make it less confusing. The Japanese like every thing sinking and you see it more in the electronics field.

I never design a 0 volt activate on an input that triggers a device. active high inputs are safer as a short to ground after the switch can fool a 0 volt activated input, but that same short to ground will cause the fuse to pop in the active high input scheme.

a good article on the subject is at

http://www.mrplc.com/kb/index.php?pa..._v2&id=44&c=38

Last edited by mordred; October 22nd, 2007 at 09:58 AM.
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Old October 22nd, 2007, 11:52 AM   #19
PeterW
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Quote:
Originally Posted by brucechase
Completely disagree on this!


Quote:
Originally Posted by brucechase
Several drive manufacturers have instructions on putting contactors AFTER the inverter and how to control them.
can't say I've read any reacently, most drive books I've read tend to show fuses before the drives.


Quote:
Originally Posted by brucechase
I do this when safety application demand it.
Recently I worked on sorter applications which run at 10m/s and a zone 3 guarded area. If it was to stop under emergency conditions the PILZ would send a signal approx 5 seconds before removing power (contactors before the inverters) on receiveing the PLC would instruct the inverter(s) to imediate stop. The inverters would then stop the sorter under power, after 5 seconds the PILZ would remove all power.

To have done this before the inverters had stopped would have mean't about 20 seconds of free running.

I know different applications will require different things, some things will stop almost as fast if you just remove power.


Quote:
Originally Posted by brucechase
In some situations, I've installed 2 contactors after an inverter. I have NEVER had an inverter fail because of this.
Why 2? you can only stop it once!

Quote:
Originally Posted by brucechase
All overloads go between the inverter and the motor - not before the inverter.
why have an overload for a single motor after the inverter?

Quote:
Originally Posted by brucechase
(I'm not sure why you would use one if you are only controlling 1 motor since all drives I've seen have an overload function built in.)
Oop there! exactlty as I said! I only mentioned protection where you have multiple motors.

Quote:
Originally Posted by brucechase
As a matter of fact, most overloads won't work properly before an inverter since it has a dc bus that can charge and discharge. If there are multiple motors, each motor get an overload and that is between the drive and motor.
OK, being pedantic, it would be fuses or circuit breakers.

Quote:
Originally Posted by brucechase
A circuit breaker or fuse goes before the inverter to protect the inverter in case there is a SHORT CIRCUIT. The circuit breaker protects the wiring and components in the drive and motor.
Exactly.


If one motor - no protection between the inverter and motor, if multiple then each have its own (as I said). Even this is difficult as the current rating on the motor is for the voltage at the frequency on the name plate.

Why put a contactor between the inverter and the motor??? What is the run/stop digital input for?

The ONLY contactor I would have would be a safety contactor, that whilst the machine is switched on and in a safe mode, the contactor is in.

The inverter has a RUN/STOP input, normally switched from a 24VDC relay from a PLC output, this is used to run and stop the motor.

For single motors the inverter protects the motor.

For multiple motors each motor should have protection.

Suddenly switching the load on the inverter can be detremental to the inverter, the damage I wold imagine would correspond to the size of the loads and the inverter.


It appears really, the contactor location is all we disagree on.
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Old October 22nd, 2007, 12:26 PM   #20
Stationmaster
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Quote:
Originally Posted by rsdoran
When I first came here I thought I knew something and was quick to throw out things. I found out real quick I did not know sheet and many of the things I did know there were other and sometimes better ways to do.

The thing is people learn in different ways, in different industries, and under different conditions. Codes, Standards, and Laws can vary from locale to locale as can availability of components.

People that work with plc's usually end up with a specific brand they either "have to" or "want to" work with. Over time they develop code they can use and reuse over and over. Is it better then someone else's method? Maybe, maybe not, what is relative is the fact that it works. The problem, in some cases, is many may think their method is the only way to do it.

There are others that think that way but the one thing I have learned is everything changes, you have to keep your mind open.

A- G R E A T- B I G- D I T T O ! ! ! from me.

I would add: Reading, imagining, and researching ideas and theories and practices proposed here, on this forum, give one the opportunity (not the obligation) to expand or change ones "bag of tricks" or "style" or "tool box". Select from the ideas that can help you, debate things that don't make sense, but do it ALL with an "open mind". You will gain, we will all gain.

Stationmaster

P.S. to PeterW: IMHO, Where the contactor can BE is not the issue. The issue is how it is MANAGED.

Stationmaster

Last edited by Stationmaster; October 22nd, 2007 at 12:29 PM. Reason: spacing
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Old October 22nd, 2007, 12:30 PM   #21
seppoalanen
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Talking

I wonder.. There are (in VFD) some stone with semiconductors whoes switching thousends time / sec them on and off. PLC controls VFD's via fieldbus. If run-command goes off, inverter will stop the motor, I'm quite sure. For what we need contactors before the VFD?

Ps. VFD's are so inexpensive that I put own for every motor.
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Old October 22nd, 2007, 12:36 PM   #22
Stationmaster
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Quote:
Originally Posted by brucechase



Stationmaster:

That used to be the way the conveyors were controlled here. I changed them to sound the alarm for 4 seconds and then require another push of the start (after the 4 seconds but before 8 seconds) before they start. My reasoning is if someone yells, then the conveyors won't start automatically so someone would have to hit the stop button. We also have estop ropes (with slack detection) and a red light on each switch so when it is activated, it can be seen for hundreds of feet away.
Similar setup at this plant. PLC detects and counts 3 rings, start button must be pressed within 10 seconds of end of last ring......

Havn't seen "estop ropes". Any more info?

Stationmaster
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Old October 22nd, 2007, 12:46 PM   #23
PeterW
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Quote:
Originally Posted by Stationmaster
Similar setup at this plant. PLC detects and counts 3 rings, start button must be pressed within 10 seconds of end of last ring......

Havn't seen "estop ropes". Any more info?

Stationmaster

Never come across a 2 button start. Normally a 5-10 second alarm to warn people the line is about to start and to clear the line followed by the line starting, no second push.

If someone is working on the line, by local rules, they should ensure the equipment is safe and that means only 1 thing, lock off.



E-Stop ropes, we call lanyards, used to be common but I more often see E-Stops placed at regular intervals nowadys, they still pop up now and again.
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Old October 22nd, 2007, 12:53 PM   #24
Stationmaster
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Quote:
Originally Posted by seppoalanen
I wonder.. There are (in VFD) some stone with semiconductors whoes switching thousends time / sec them on and off. PLC controls VFD's via fieldbus. If run-command goes off, inverter will stop the motor, I'm quite sure. For what we need contactors before the VFD?

Ps. VFD's are so inexpensive that I put own for every motor.
The only manufacturer "I" know of to use a contactor before the VFD claims to do so in order to protect the VFD from severe electrical spikes when not in use. The systems are typically used about 8 hours a night to irrigate the golf course, and the other 16 hours the VFD is not exposed to line power. I don't agree with this practice, its just something I have seen and had explained to me at the manufacturer's training seminar. With the great surge suppressors I have discovered, I hardly see the need to open the line whenever possible. I will say that one of these systems has been in service for about 15 years, has been moved from one site to another, and the VFD is still running fine. We DO have frequent and severe lightning here in Florida.

I also, in panels that I build, put one VFD per motor. I think you save enough in switchgear, I/O, cable, and labor to help offset the cost of the VFDs. You then have full redundancy in case of a VFD failure.

Stationmaster
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Old October 22nd, 2007, 01:02 PM   #25
PeterW
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My two shillings worth


Quote:
Originally Posted by seppoalanen
If run-command goes off, inverter will stop the motor, I'm quite sure. For what we need contactors before the VFD?
SAFETY.

Quote:
Originally Posted by seppoalanen
Ps. VFD's are so inexpensive that I put own for every motor.
Small ones are inexpensive.

When you have 22 motors driving the same thing, then its cost effective to have a single inverter (or grouped motors, 8 per inverter for example).

Also sometimes if you have two things that run at the same time and you want same speed that variable, then again you may have just one.
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Old October 22nd, 2007, 01:09 PM   #26
Stationmaster
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Quote:
Originally Posted by PeterW
Never come across a 2 button start. Normally a 5-10 second alarm to warn people the line is about to start and to clear the line followed by the line starting, no second push.
No, I didn't elaborate enough....it isn't a 2-push start exactly. The alarm bell is rung manually, the PLC detects the 3 rings and makes the "start" button active for 10 seconds and only 10 seconds. The rings have to be 3 seconds long each and all 3 have to be completed within 15 seconds. The PLC lights a pilot light for each qualifying ring. This is the only way to activate the "start button".

Ring-Ring-Ring-Start

After the 10 second window, the lights all go out, signifying that the start button is not active anymore. I've spent longer typing this than I spent writing the code.

: )

Stationmaster
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Old October 22nd, 2007, 01:24 PM   #27
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Interesting thread.

Funnily enough I'm writing a project currently using 2 button functions. Its a split-mold press, a pretty big one.
I've also seen it used on printing & binding systems where they use big guillotines.

The only other comment I have is on safety systems.

I've used regular PLC such as Compactlogix and Modicon Quantum on customers burner management systems (at their request) and it can be done.
You have to lock down the CPU pretty tight, use an external watchdog, monitor critical IO etc etc. Turns it into a bit of a PITA.

On a plant-wide safety system, I would ALWAYS reccommend going with the real deal, a Triconex, FSC, ProSafe or similar.
Something designed for the job so we dont have to spend days trying to make the thing come up to standard.

I guess in the machine world thats why the Pilz is so popular.
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Old October 22nd, 2007, 01:28 PM   #28
brucechase
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Quote:
Originally Posted by PeterW

Why 2? you can only stop it once!
Because the risk assessment mandated a redundant monitored safety contactor. Not for stopping, for safety.

Quote:
Originally Posted by PeterW

why have an overload for a single motor after the inverter?



Oop there! exactlty as I said! I only mentioned protection where you have multiple motors.
Only because I was answering you here.




Quote:
from post #10 -


The only contactor required for an inverter, would be the power on contactor, which could be the safety contactor, which would be BEFORE the inverter.





Doug is quite correct, you should not put a contactor between the inverter and the motor. You use the control signals to RUN/STOP the motor, not the contactor.

Overloads also go before the inverter as you are protecting the inverter, the inverter protects the motor.

Where the inverter supplies multiple motors, each motor should have its own protection after the inverter (in addition to the inverter protection), but if one trips the inverter should be stopped.


Quote:
OK, being pedantic, it would be fuses or circuit breakers.
Just clarifying what I believe, not being pedantic.

Quote:

If one motor - no protection between the inverter and motor, if multiple then each have its own (as I said). Even this is difficult as the current rating on the motor is for the voltage at the frequency on the name plate.

....

The ONLY contactor I would have would be a safety contactor, that whilst the machine is switched on and in a safe mode, the contactor is in.
....

For single motors the inverter protects the motor.

For multiple motors each motor should have protection.

Suddenly switching the load on the inverter can be detremental to the inverter, the damage I wold imagine would correspond to the size of the loads and the inverter.


It appears really, the contactor location is all we disagree on.
When I read your post(#10), I did not get this. I must have misunderstood or misread it. I am glad that we are at least close for safety systems. As for location, I feel that (like most everything else) the different applications require different designs. I always put the safety contactor between the drive and motor but I don't have systems that run on after power off.

As a side note, I do have a machine from a foreign country that has 1 VFD (a 5HP) controlling 4 motors (all around 1 hp) and 3 of those motors have contactors that switch constantly. Funny thing is we haven't had to replace the drives in 10 years. Not what I would think as a good design, but it works.
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Old October 22nd, 2007, 01:31 PM   #29
mordred
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We use a lot of safety lanyards here as its the easiest method although not the most reliable (tension problems from the lanyard) method they do provide a safety device that anyone can activate on the conveyer line itself which is where we typically use them
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Old October 22nd, 2007, 01:46 PM   #30
Stationmaster
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Quote:
Originally Posted by brucechase

As a side note, I do have a machine from a foreign country that has 1 VFD (a 5HP) controlling 4 motors (all around 1 hp) and 3 of those motors have contactors that switch constantly. Funny thing is we haven't had to replace the drives in 10 years. Not what I would think as a good design, but it works.
Hmmmm.......Maybe you should try SOFT STARTERS between the VFD and the 3 motors. (I'm just kidding, please don't spaz)

: )

Stationmaster

Last edited by Stationmaster; October 22nd, 2007 at 01:47 PM. Reason: : )
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