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Emergencies / Braking

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Lynne30/12/2019 14:29:50
99 forum posts
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There were a number of posts, recently, concentrating on vfd/machine

emergency situations, and various approaches of dealing with them..

Purely in relation to the 'emergency' aspect; which is the better to have

Dynamic braking or DC Injection, built into the vfd?. It appears the choice

is one or the other, probably on the more expensive both. Lynne

Neil Wyatt30/12/2019 14:45:59
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I think DC injection is simpler and normally used on smaller units as dynamic braking needs a big fat resistor to dump the energy.

On my unit (which is an inexpensive jaguar cub) there's an option to add an external resistor for DC braking but this is probably not worth doing as the dc injection works fine with smaller motors and relatively infrequent stops.

Neil

Clive Foster30/12/2019 14:55:07
2885 forum posts
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Lynne

This looks to be a good introduction to the pros & cons **LINK**

Never felt the need for either but I'd be little bit wary of DC injection braking as its likely that the VFD ought to be set-up specifically for the motor and load concerned to get best, or maybe even safe, results. Sounds like something that needs you to have some experience and delve deeper into the manual than you'd ideally like to properly install.

With our sizes of machine and load it probably doesn't matter than much. Even coast down is pretty quick. Odds are that a fast ramp down, especially with a vector drive, will be enough as that can be rather faster than coast down. I did come across a reference to one guy who'd built an add on, solenoid operated, brake unit from bicycle disk brake parts which he reckoned was better than relying on the VFD for his lathe. Some time back tho' before vector drives became more common. American guy with 10" or 12" swing lathe I think.

My Smart & Brown 1024 VSL, albeit not on a VFD, has an electromagnetic brake as standard which kicks in when its coasted down to about half speed which seems fast enough.

Clive

not done it yet30/12/2019 15:00:37
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Neither, unless you are not using a screw-on chuck which could unscrew under braking!

KWIL30/12/2019 16:27:56
3445 forum posts
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What kind or emergency did you have in mind?

Martin Cargill30/12/2019 17:00:25
166 forum posts

A few thoughts and comments about braking on machines.

ElectroMagnetic brakes are normally built into the end of the drive motor. They are normally wired into the power supply for the motor and therefore release and are applied by the motor being turned on or off (the exclusion to this is machines that need to be able to release the brake without the motor running). They work well but they require adjustment from time to time and they do wear out as they use a friction lining (similar to that on a cars brakes)

DC brakes can be stand alone or part of an inverter drive. These are reliable and don't require any adjustment once set up. They don't like differing loads and may struggle with the difference in light and heavy work pieces. Some incorporate standstill detection and these cope better with varying loads. Differing loads may cause a problem if you have one inverter that you use to drive different machines. Inverter ramp down can be as effective as a DC brake.

Regenerative braking is most commonly found incorporated into inverter drives and is not commonly used on machines, as it requires braking resistors.

Solenoid brakes and air operated brakes need the addition of a brake disc or drum to the drive components (and possibly a compressed air supply).

Plugging is also a way of stopping machines (normally industrial 3 phase machines) and involves stopping the machine by briefly reversing the motor. Its not for the faint hearted.....

If you are considering adding braking, as has been mentioned earlier, be careful of chucks, saw blades etc that have a screw thread as they may unscrew themselves during braking.

Some of the comments I have outlined are a bit general as I have tried to keep this post brief. I work in the woodworking industry and have fitted hundreds if not thousands of machines with brakes (to meet the modern braking regulations for ww machines), differing machines require consideration as to the type of brake that they need to operate safely.

One more important point to consider is that many braking systems now fitted to wood machines have to bring them to a halt in 10 seconds. In the case of a wood machine accident the "damage" to an operator has happened before anyone can react - the braking systems are more for the safety of the next operator walking up to a machine that is (unknown to him/her) still spinning despite having been switched off by the previous operator.

Mike Poole30/12/2019 17:37:45
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To be able to use the injection or rapid ramp stops you need to have control of the inverter, an emergency stop must be able to stop all motion regardless of the failure. Some more capable inverters have safe stop which will prevent the output stage of the inverter being driven and will be a properly implemented safety circuit. The dilemma occurs when you want to stop the machine as quickly as possible which usually needs the inverter to be working but if the failure is that the inverter will not shut down because of a control failure then the emergency stop will need to cut the power and let the machine coast to a halt or have a failsafe mechanical brake. Most inverters should be tolerant of having the power removed from the input, after all a power cut when running should not kill your inverter. Disconnecting the motor from the output is the scenario that lots of inverters do not like but I have seen safety relays in the output of an inverter. Emergency stops should not be the routine method of stopping a machine, a control stop should be the normal method and this is the stop where the inverter setup can stop the machine very quickly but this only applies to normal running.

Mike

Lynne31/12/2019 15:25:36
99 forum posts
32 photos

Thanks for all your responses. The project is to convert 1ph mill to 3ph, using a 3ph 0.55kw motor I was given some years ago. (It is a runner) No particular emergency in mind, though many years ago my shirt cuff picked up on the spindle of the pillar drill I was using; fortunately the sleeve ripped off. Some damage to the arm, but not major; so I guess that is in the back of my mind. The VFD I am looking at does have DC Injection built in, If I buy , I will leave it at the default setting, rather than mess things up. I think that configuring the ramp down settings will slow things down fairly rapidly, but I still have the current emergency stop to cut the mains coming in, which seems a reasonable route to go down. Regards, Lynne

John Paton 131/12/2019 16:03:23
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Martin's, thanks for a very clear reply.

On the electromagnetic brake option, am I rigth in thinking that is not compatible with VFD inverters? Certainly I removed mine when changing my VSL to VFD and intend to add a brake resistor if needing to reduce run down time (VSL has L00 chuck mount so no unscrewing problems).

Martin Cargill31/12/2019 20:50:48
166 forum posts

Lynne

A couple of comments on your project

First thing to check is to ensure that the motor you have will run at 220 volts 3 phase. (Inverters that step up the voltage to 415 tend to be expensive).

As you are using the inverter for a single motor it's worth setting up the DC braking. But set it up with the heaviest tool that you have. It will stop the smaller tools a bit quicker so make sure that it doesn't stop them too quickly (as its only 0.55kw I can't see you running huge tooling).

Its best not to have an emergency stop in the incoming power feed. Hitting this will kill the power to the inverter and will prevent the ramp down and the DC injection from working. In an emergency you may extend the stopping time above normal rather than slowing it quicker (we have this conflict when fitting DC brakes in schools etc where they have an emergency stop ring for the whole room). The chances of you having a need to use the emergency stop and an inverter fault happening at the same time are remote but if it makes you feel happier fit an accessible isolator switch into the incoming power feed. I would make sure that the stop circuit and the emergency stop circuit use normally closed contacts to operate - I have seen a couple of inverters that, by default use normally open contacts, I don't like to use these as a broken or loose wire can prevent them from operating.

John

I would normally only have one braking system on a machine. I don't like there being two, some of the machines I work on would become dangerous if they are stopped too quickly. The obvious one being Resaws (big bandsaws with power feeds and 3"-6" wide blades) where the blade could be attempting to stop the top wheel too quickly and it could crack/snap). Electromechanical brakes are normally fed from the motor terminals and they would probably object to the variable frequency being fed into their rectifier.

Martin

Steviegtr31/12/2019 22:17:39
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Hi. My sixpeneth. I spent the last 25 years of my career designing ,building & installing control panels to the food industry. Most of the inverters we fitted were Jaguar & some were Seimens. Some had very elaborate control connections to control rooms etc. Now well retired & the fresh owner of a very nice Myford super 7B. today I have stripped of the motor & pulleys ready for the 2nd hand inverter I just purchased. It is an Omron, which are quite expensive new. I am currently building a control box for it & your topic came to mind. I don't know if the cheaper Chinese inverters have the function but in the configuration you can set a ramp up time, & a ramp down time with the start stop buttons. There are other connections for various other options. One option is that I can configure an E-stop which will stop the inverter instantly if you set it so. Not braking so would not have thought a chuck spin off would occur. I plan to mount the control panel on the wall behind & above the lathe on a arm. The 2nd e-stop I think would be better at the front for obvious reasons.

Martin Cargill31/12/2019 23:00:20
166 forum posts

Steven

I'm sure you are well aware but a couple of points re your installation.

Make sure your control pod mounting is well away from the rotating parts. There have been a number of accidents where people have reached over moving lathes...

You might want to consider a kick stop. If you have a hand or clothing caught you can't reach a normal emergency stop.....

Martin

Steviegtr31/12/2019 23:07:01
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Yes I see your point. I was thinking of the looks rather than the safety aspect. The lathe is going into a left hand corner, with enough room to open the doors for gear access. I could actually mount the control panel on the left wall which would in effect be in front of the machine. Thanks. The lathe is going on the left of that bench. Which I have beefed up to 54mm of hardwood ply bonded & screwed together, did not want any vibration.

My bench

Vic31/12/2019 23:30:24
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Posted by Steven Edwards 1 on 31/12/2019 23:07:01:

The lathe is going into a left hand corner, with enough room to open the doors for gear access.

Are you sure that’s a good idea? Many folks leave lots of room to the left of the machine for passing long stock through the head. In my shop the Lathe is on the right, mill on the left and drill in the middle for example and this has worked out well. Just a thought.

Steviegtr01/01/2020 14:32:44
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I see your point on length of stock. Unfortunately I have spent ages re-enforcing the left side of the bench for the lathe. The right side is set up for welding where I have a steel bed & vice etc. I am only dabbling so cannot think of a situation where I would need room to the left. If I acquired a long length I could cut to size by other means. Thanks.

SillyOldDuffer01/01/2020 17:23:33
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Posted by Steven Edwards 1 on 01/01/2020 14:32:44:

... I am only dabbling so cannot think of a situation where I would need room to the left. If I acquired a long length I could cut to size by other means. Thanks.

Not the end of the world with a relatively light Myford because it can be moved later as necessary but Vic is right: most of the action takes place at the headstock end of lathes, and it pays to have space at that end if it's an option. Handling long stock is one reason (several operations, not just cutting it), but easy physical access to the banjo is very helpful, as is being able to get at pulleys, belts and - on a bad day - the motor and electrics. Almost no advantage in having room at the tailstock.

Sod's Law is liable to strike minutes after a lathe is bolted down into a tight corner! Someone owed a major favour turns up wanting you to thread the ends of several 4 foot long pipes...

Steviegtr01/01/2020 17:43:31
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I think I may have misled some by saying in a corner. Just going out to measure. >>>>>. So the distance from the change wheel cover & the wall of garage is going to be approx. 350mm or 14". I could even push it to 18". Surely that is enough to get to the left side covers etc. I cannot really make it any further away as I have made the top out of 18mm hardwood ply in 3 layers to give strength. That is my limit.

Vic02/01/2020 11:29:30
2954 forum posts
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My little 8 1/2” x 14” lathe only has a through bore of 20mm but I quite regularly pass long stock up to this size through the headstock for machining. Chopping stuff up with a parting blade on the lathe is a lot less tiresome than sawing it to length. Being able to work on the end of a long bar can also be less wasteful than sawing up lengths to fit. I personally would always want at least two or three feet of open space to the left of the head.

Mike Poole10/01/2020 11:06:34
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A hole in the wall would allow long stock or job to be mounted, of course rather depends on what is the other side of the wall.smiley

Mike

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