Help with wiring up a contactor and stop switch to a VFD

[Rampage]

Morning, I'm building a small control panel for a VFD, The VFD has 24v lines for motor run and stop, these use a momentary switch. The stop button provides a braking function for the motor through the VFD.
I want to connect a contactor between the VFD and motor to safely isolate the motor. I could use a separate switch to disconnect the contactor, but it would be nice to use the same momentary switches as run and stop. There would have to be a delay on the stop button to allow the VFD to brake the motor.
Is this possible?

 

[Darkwood]

Your VFD should have a control terminal connections for you run and stop commands, your Isolating contactor should be possibly via a E-stop system if required or its own power up circuit.

You should not crash the power to the VFD in order to stop the motor as you get an uncontrolled stop and a software crash, strict regulations guide how your control circuit is designed and your VFD controls are subject to manufacturers guidelines and vary.

We would need to know everything from risk assessment to the job the motor is doing to even consider helping you or heading you in the correct direction.

Regarding Braking this is going into specialist area and although some braking options are simplistic, the correct options needs to be made and a variety of braking methods exist - a small motor can use an integral braking option on some VFD's. Larger motors and high inertia motors may require careful design regarding braking but can vary from braking resistors, electro-magnetic slip plates to DC injection and many more, all with their own pro's and con's.

As I've advised you in the past you need to learn to swim before jumping in the deep end, your experience level means this area should be left alone unless your supervised, your enthusiasm is good but just be patient your skipping across alot of ground here which is vital to learn how to be competent in this field.

 

[Bobster]

Just to add although you can use contactors (as in multiple) to isolate a circuit, this will be way above your technical level.

In this instance, a local isolator will be required. Although without a LOT more information I can't advice.

 

[Clive P]

Hi,

Having read one of your previous posts, I installed the same Drive on a Bandsaw a few months ago ( works a treat ). As I have very little experience of these I contacted a friend of mine who does this type of work on a regular basis to layout my thoughts. Funnily enough it was as Darkwood says in his post, but the item I was working on didn't have any braking which did make it easier. What I am saying is best to get some advice from somebody who does this regularly. I've been in the game a long time, still make small mistakes; but never afraid to ask. In the long run you'll extend your skill set, and be able to impart that knowledge to others.

Regards.

 

[SheffSimon]

Just to add although you can use contactors (as in multiple) to isolate a circuit, this will be way above your technical level.

In this instance, a local isolator will be required. Although without a LOT more information I can't advice.

Why multiple contactors? The risk assessment and safety circuit design will dictate the number of contactors, and it may be that only a single contactor is required to reliably perform the emergency stop function. It may also be the case that the VFD has a SIL rated STO input, so no safety contactor is required. None of which should be used as an isolation function for safe maintenance access.

Also, why is a local isolator required? A lockable isolator on the panel door meets the regulatory requirements.

 

[Bobster]

Why multiple contactors? The risk assessment and safety circuit design will dictate the number of contactors, and it may be that only a single contactor is required to reliably perform the emergency stop function. It may also be the case that the VFD has a SIL rated STO input, so no safety contactor is required. None of which should be used as an isolation function for safe maintenance access.

Also, why is a local isolator required? A lockable isolator on the panel door meets the regulatory requirements.

The OP asked for a contactor to safely isolate the motor. I understood this as isolation for maintenance etc... This is what I was saying, it is possible to use contactors to provide this type of isolation, it will always require multiple contactors. It is not cost effective so it's very rarely done. In fact I have only seen it done twice, both on large drives within an EX environment.

Far easier to fit a local isolator, without seeing the panels location to the motor this was the easiest answer.

I didn't mention anything about a safety circuit, as we don't know what PL rating this machine has.

 

[shanky887614]

Why multiple contactors? The risk assessment and safety circuit design will dictate the number of contactors, and it may be that only a single contactor is required to reliably perform the emergency stop function. It may also be the case that the VFD has a SIL rated STO input, so no safety contactor is required. None of which should be used as an isolation function for safe maintenance access.

Also, why is a local isolator required? A lockable isolator on the panel door meets the regulatory requirements.

impractical, its needs local isolation.

ill put it to you this way, will they let yyou lock of isolator on panel when it will kill 10+ motors?

 

[Rampage]

Hi,

To be more specific it's a 1hp motor on the vertical head of a milling machine, connected to the spindle. So it would control a rotating cutter. It doesn't control the table drive or the horizontal head. The purpose of the contactor would be to isolate the motor to change cutting tools. The motor braking is an option in the VFD (Bosch RexRoth) there is an external resistor for this.

 

[Bobster]

Oh dear

 

[Rampage]

Meaning?

 

[Besoeker]

Hi,

The purpose of the contactor would be to isolate the motor to change cutting tools.

Maybe you haven't heard of lock off tag off procedures?

 

[Bobster]

That ^^
If you'd read what has been posted, it's already been said you can't do what your asking. Not without a lot more being involved, making it not cost effective.

 

[Darkwood]

Hi,

To be more specific it's a 1hp motor on the vertical head of a milling machine, connected to the spindle. So it would control a rotating cutter. It doesn't control the table drive or the horizontal head. The purpose of the contactor would be to isolate the motor to change cutting tools. The motor braking is an option in the VFD (Bosch RexRoth) there is an external resistor for this.

I really don't think you have any concept of the depth of responsibility you are getting into taking on such a job, this is not just about having a functional control circuit so I'll re-iterate that strict regulations must be adhered to here and a indepth knowledge of the machine directive helps too as well as risk assessment, your replies are really screaming out you are undertaking work well out of your depth and I say this in a polite way.
Not knowing the existing set-up of your machine and its controls makes it impossible to give the correct advise, this is not like working under the BS7671, altering the existing controls makes you responsible for the whole existing control system and its compliance, any future incident regarding an injury or worse on this machine where an investigation is required then you will be the first in line with the heaviest responsibility to confirm your work complied and you met the requirements needed for both safety and control.

If you ask the original question as a hypothetical we can suggest ways to connect say a simple motor up to operate as you want but when its part of a larger control set-up like you mention then its a whole different ball game.

PS what if the contactor jammed in or the coil remains energised due to a faulty switch or damaged cable- where is your redundancy to ensure accidental start up cannot occur?

Fail safe is key, although nothing can be entirely fail safe steps should be taken to ensure you meet the risk assessment cat' the machine falls under to ensure any such failure or sequence of events is extremely rare.

 

[Rampage]

Ok, fair enough. I thought I posted about using a separate switch in the end to isolate, but obviously not.

The alternative is to use the existing 400v three phase output on the machine which is controlled by the existing safety switches. You said you can't just disconnect a VFD though?

Thanks

EDIT: Just noticed someone's taken away my 'Trusted' badge and it now says trainee! I have no idea how I got the trusted badge anyway!

 

[Darkwood]

Yes you did enquire about using contactor as isolation for a VFD initially but once you expressed the purpose of the isolation was for safe tool change then it totally moves any goal posts established in your opening post and changes the requirements hence you were asked a few questions, many question on here cannot be answered with a simple yes or no so more info is sort, in doing so I have advised you that the original question can no longer be answered in the context it was written due to additional info you supplied and the lack of info we have about the existing controls.

I could broadly help you in that the use of an safety relay could drop out the control voltage for the run and/or enable command and a contactor on the output side of the drive can also be de-energised through a seperate channel of the safety relay, an enable button would reset the relay and energise the contactor again as well as the control circuit for the drive ready for your run command, this would ensure safe isolation for tool change and to note the safety relay is not acting as E-stop so your are not shutting the machine down and/or crashing the drive as you suggested every time you change tool.