DC Injection brake modules

[joel89]

hi all, I am looking at a small spinning lathe runs at 1500rpm from a small AC motor (1950s) that is literally it via a dol starter, it has had a PUWER Assesment carried out picked up no guarding, no interlocks, no ESR, no brake on the motor, so i am building a little panel Contactor, ESR, and was looking at these DC injection modules.

1) I am wondering if they are any good at constant Start/ Stop applications every 3 minutes and can run 24/7.


2) my other option is when the M/C stops have a timer on the solenoid interlock so the guard wont release until the motor runs down.

3) E-stop situation do the brake modules still work, or are they power dependant??



Cheers all

 

[Darkwood]

Without any informed info as to the risk assessment it is impossible to answer what is the best measures to take, are you sure their is no braking system on this lathe?.. lathes tend to have electromechanical breaking but if its not been maintined it may be worn and/or require a voltage tweak to increase the grip.

What size motor are we talking?
What load are we turning (high inertia)?
What hazards are you trying to limit are they the standard hazards associated with a lathe?
Does the lathe require manual working on the job or is it partially or fully automated? (CNC)

Just a few to start with here as lathes in one that sits in my ball court

 

[joel89]

it is a spinning lathe... the chuck runs at 1500rpm it has a manual brake on it that you stop with a foot pedal it is around 1930s the machine, the motor is 3kw the motor turns a long shaft thats connected directly to the chuck no gearbox.

The hazards are no guard interlock, the chance of the operator getting caught in the machine, so we need it to stop before he opens the guard.

 

[Darkwood]

The issue with a lathe is they often require close proximity of a user if its a manual tooling of the product, the guard often just covers the spindle head as its a prominent danger, even with guarding of the spindle head you have an inherent risk of user been caught up in the job itself. If the lathe is worked like this then other measures like training, no baggy clothing etc must be brought into the equation.
As this is a very small lathe with probably a small job on it I would recommend a electro-mechanical brake as this will suit most motor duty applications, the issue with using DC injection on a lathe is timing the DC as the inertia of the load is variable so can lead to too short an injection which defeats the object to too long and overheating the motor windings, monitored systems can get around this but are expensive and can be complex as appose to a simple solution I have mentioned also fail safe is an issue too, if you have a power failure or failure of the control circuit you need a system that would still work, this cn be achieved simply by using a powered open brake so it will act if power fails or control to that matter.

It all depends on the risk assessment and level you are designing to, I deal with larger lathes 30 -55kw for steel drums so the risks are higher and inertia is high too making stopping the machine quickly impossible.

 

[netblindpaul]

Obviously the person who did the PUWER assesment is incompetent, so get a refund!

 

[Bobster]

Siemens do a range of VSD's with built in safety functions such as safe stopping. Never used one on a lathe.

 

[Darkwood]

Siemens do a range of VSD's with built in safety functions such as safe stopping. Never used one on a lathe.

These features are in compliance and can be used in the critical safety circuit as they are usually pulse monitored and provide double redundancy to meet 'Sil 3' but having said that, the Safe Stop (SS1) feature is just a controlled ramp down to zero speed and not used in emergency usually but can be if it meets requirements, another feature included on some drives is the Safe Stop Emergency (SSE) which is similar ramp down but a little quicker, then there is Safe Break Control (SBC) which is more suited to a lathe.. having these functions in a drive only give the advantage of not relying on contactors and relays which are more prone to failure as redundancy doesn't exist in these methods so definately an advantage but they are still subject to risk assessment as to whether they are suitable as a method, for the OP the SS1 or SSE may not be fast enough but I would suspect the SBC used with a suitable power open brake would meet the requirements to stop the lathe as quickly as possible without risking machine damage like shaft shearing which could be a bigger hazard in itself.

 

[SheffSimon]

hi all, I am looking at a small spinning lathe runs at 1500rpm from a small AC motor (1950s) that is literally it via a dol starter, it has had a PUWER Assesment carried out picked up no guarding, no interlocks, no ESR, no brake on the motor, so i am building a little panel Contactor, ESR, and was looking at these DC injection modules.

1) I am wondering if they are any good at constant Start/ Stop applications every 3 minutes and can run 24/7.


2) my other option is when the M/C stops have a timer on the solenoid interlock so the guard wont release until the motor runs down.

3) E-stop situation do the brake modules still work, or are they power dependant??



Cheers all

Have you/somebody done all the design required to show compliance to your required SIL/PL targets determined from the risk assessment?

You don't design safety related controls by setting off with a box of red and yellow electrical bits and pieces, that's how YOU end up in court after somebody has got their arm wrapped around the lathe chuck.

 

[joel89]

Have you/somebody done all the design required to show compliance to your required SIL/PL targets determined from the risk assessment?

You don't design safety related controls by setting off with a box of red and yellow electrical bits and pieces, that's how YOU end up in court after somebody has got their arm wrapped around the lathe chuck.

Doesn't answer my question that does it, I am asking about Brake modules

 

[netblindpaul]

It answers your question perfectly.

Is the brake module the correct decision having undertaken your design RA's & does it meet the SIL/PL requirements of the application.

Remember you are designing and implementing a safety related control system, this must, and must in LAW, meet EN 60204-1, EN13849, & EN62061 depending on the application, and a lot of other requirements under STATUTE LAW.

 

[Rigpig]

Aright Joel89

http://www.cromptoncontrols.co.uk/p.../direct-on-line-three-phase.html#OtherProduct

When i worked for a large american drilling contractor i had to fit something similar to these in all our ship workshop machinery. Just match the module to the kW rating of the motor .
As already mentioned, if your lathe has any type of electromechanical braking or clutch gearing dc injection beaking will not work.



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[Rigpig]

The example above complies with puwer98 so might be a decent shout.

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[Darkwood]

The example above complies with puwer98 so might be a decent shout.

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DC injection isn't the best option for a variable inertial loaded machine, like I mentioned earlier, how do you set the timing up when the gear ratio, load etc can vary.

 

[Rigpig]

The instructions for setting up timing came with the module we were told to fit. The lathe was used frequently with no trouble for the 5 years i was on the vessel. Cant really say much more than that......

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[Rigpig]

1 point i do remeber is we received a flash report not to fit them to milling machines. Presumably this was due to the pronlems you mention above @Darkwood

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[Rigpig]

*problems

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[SheffSimon]

Nope, but some of us on here do this design stuff for a living, and we follow the correct process mainly because we want to do it right, but also because we don't want to end up on wrong end of an HSE investigation.

 

[Darkwood]

1 point i do remeber is we received a flash report not to fit them to milling machines. Presumably this was due to the pronlems you mention above @Darkwood

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The problem is more to do with use and size of the lathe and gearing and control and unless you have a full grasp of its use, its motor type and duty, is the lathe used only for a specific task etc (possible in certain industries or workshops)... then possibly DC injection is an option, the downfall of using DC injection is the heating effect of the windings and if you get your timings wrong or start/stop duty is too high then expensive problems can occur, also I have seen DC injection units fail where they continue to inject long after the motor has stopped due to timerboard failures - consequently the motor windings burn out. With an electro-magnetic braking (my personal choice), a timer unit failure at worse would just see the brake on constant (power on units) or no timer board at all to fail (power off units) and dependent on control circuit it should stop a restart of the lathe but if of an older design it would mean motor would trip on start up or when it was put in gear, also some lathes have constant run motors with clutch control for the spindle to run so on the occasion DC injection would be pointless as that is not how the lathe works, another common motor type on lathes are 2 speed motors, again this comes with its own problems when using DC injection.

Im not saying using DC injection on a lathe is a no-go but trying to hi-light the problems and the considerations you need to take into account before you would spec' as a control method, if it was the only option for me then I would possible design a control system that may use a rotation sensor as the timer for injection with a background timeout for redundancy in case the rotation detector falls out of position or fails then at least it won't cook the motor, a warning light would then alert the user to an issue.

When your braking method is been fitted for safety then you really need to know you can rely on it even with component failure, this cannot be achieved if the DC injection module fails but can be if you have a power open electro-mechanical break, if its control fails and it loses power or even on a power cut then you know the brake will operate, this is why for the OP's situe' and because he is been asked to do this as a safety measure that I am advising on said method.

 

[Rigpig]

Thanks for the comprehensive response Darkwood.
In our ships case the dc injection module worked a charm but as you correctly point out it may not suit all situations/lathes.
As an aside i recall now the reason the injection modules were not to be used on milling machines was that on one of the other rigs the sudden stopping of the bit caused it to shatter sending shards flying. Or at least that was the "official "report..... wether that was the actual case is unknown

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[Carl S]

A spinning lathe like this?

https://www.youtube.com/watch?v=XsCFze4zkzk

Some don’t have a positive grip on the workpiece but rely on friction. Be very careful what you do as you could launch the workpiece.

Discus throwing was used in warfare, you may have found a way to automate it.