Inverter Control Query

I am looking to replace my single phase motor on my mill to a 3 phase motor with inverter control. That bit doesn't worry me. However, what does puzzle me is how to incorporate an emergency stop without causing damage to the inverter as I understand to shut-off the power supply to the inverter in such a sudden is not desirable and could cause damage to the inverter.

How have other folk done it?

Chris

On my mill when you press the stop button the machine slows down as set by the inverter if i press the emergency stop or open the guard it stops dead ( or very quickly)

Don’t interrupt the supply or output of an inverter instead use the inverter controls to start and stop

Generally you would just wire the e-stop into the start/stop controls and let the inverter do its thing. It will stop more quickly than if you disconnected the motor, even if the VFD didn't get upset.

For a more safety critical application, a proper industrial drive will have a "safe disable" input which is independent of the software ie an external hardware over ride that will kill the drive signal, regardless of what the software thinks. That's one of those features that distinguishes a Huanyang drive from a better quality one.

Murray

This to some extent will depend on the make of the inverter, and/or the model in their range. Enhanced models such as the Mitsubishi E700 SC this can refer to, 720 series single phase or 740 3 phase. The point is the "SC" designation, this is designed to link in to a safety module (some standard ISO thing). For home purposes it is quite simple to use the 2 inputs as a break switch for E stop. (There will be people who wish to argue no doubt) If it is not SC desigantion (relatively recent tech I think) then other options needed.

Yaskawa on their V1000 model also have this, as well as a parameter for "fast stop" programmable to a digital input which also locks out the inverter.

To make them a safety stop they lock out any further possibility of engaging power to the motor, thus you can do without a mains NVR lock out in the home shop and still be in safe practice .

It is also possible to do it with relays that self latch on power start momentary buttons. The safety will cut power to the latched relay and because the safety button phsically latches the relays are unable to reenable. The relays will also give you 3 wire capability with 2 wire programming leaving some inputs free for other purposes like jog.

I would suggest downloading manuals of the inverters you are considering and working through them. Initially it can be a bit daunting until you realise there are only a few key things you need out of all of it.

Hope that helps and not confuses.

Alan

If you wire your inverter via contactor and us it to energise your inverter and then use an extra stop station as an emergency stop, in my experience all will be well. I have three pre owned inverters one about 30 years old and that is how all mine are wired, never had a problem. They also give no volt release.Whilst I haven't any experience about interrupting the supply between inverter and motor I can't think of a reason for wanting to do that. In the booklet containing programming which comes with the inverter there is a diagram for local stop and start. I have wire several Huanyang 2.2 Kw inverters for my friends with bigger lathes than mine, I've not heard of any adverse comments from them so must be happy making swarf.

PS.I made up a basis programming/instruction sheet for my own use, so if you decide to buy a Huanyang inverter I can let you have a copy, please don't get the idea I'm an expert. John

A mains power cut should not destroy the VFD, so an emergency stop on the incoming supply should be OK. Most would likely use the low voltage signal circuit, I would think. No mains wiring is a benefit, for a start...

Many thanks for all the replies - they have steered me in the right direction I am sure!

John, I have sent you a message.

Chris

As Muzzer says, an e-stop with the right programming will stop the motor faster than cutting the power as it will apply back-emf braking or even active braking. With a larger motor you may have to fit an external braking resistor to get the fastest response.

If you want to cut the power it is better to cut it BEFORE the inverter than between inverter and motor, as has been stated by others. Some inverters are designed to cope with having the motor isolated under power, but most normal ones are not.

With my inverter 'coast to stop' acts to cut power promptly and rapidly stops the motor - it is faster than the normal ramp down stop, but if over--used can lead to overheating of the inverter.

Neil

Whilst I appreciate that this thread refers to a mill, anyone considering applying the same answers to a lathe, may wish to consider how quickly they want to stop a spindle with a heavy screw on chuck.

Bill

The purpose of an E-Stop button is to remove all power from the machine. It isn't the normal stop control and shouldn't be used as such.

You won't hurt any inverter I've ever come across by removing the input power, but you could kill someone if the E-Stop is used because of an electrical fault and it doesn't remove the incoming power.

You might not need that if you are the only person to ever use the machine, but it's not a risk worth taking.

PS:- I loath elfin safety bastids, but that's because many of them don't know anything about elfin safety!

Do the risk assessment:

There is a school of thought which holds that a rotating machine must have an emergency stop function, though it doesn't have to be a big red button. It's actually written into the Machinery Directive Essential Requirements for Safety, but why?

In an industrial setting, a machine (pump, press, or whatever) will have to be guarded, and any operator access to rotating parts is prevented. This machine isn't made safer by fitting an E Stop in normal daily use, unless someone defeats the guards or otherwise removes the means provided to make sure that the operator (including casual passers-by) cannot access the bitey bits. That's not normal daily use, but it is a foreseeable hazard and the designer must take account of it. For this class of use the hazard comes from the VSD losing touch with reality, and going out of control. Unlikely, but possible and it's a high danger event however unlikely. So the designer must fit control measures (for example an E Stop) to permit the situation to be brought back under control. In this situation the best way of bringing the machine to a safe stop is to remove the mains supply from the VSD, and it doesn't matter a row of beans whether the VSD survives the experience or no. It's already compromised, switch the damn thing off any how you can.

The same isn't true for a lathe or especially (in spades) for a mill. Here the operator has to have access to the nasty dangerous bit to be able to do the task for which the machine is designed. The possibility of the VSD going doolally still exists, but there is a far higher probability of the operator being the root of all evil, and getting caught in the mangle. In this case the machine must be brought to a safe condition as quickly as possible, so using the VSD dynamic braking effect to slow the machine as quickly as possible is likely to be the proper control measure. That implies using the control stop function of the VSD, and definitely not removing the supply from the drive until the machine has been brought to rest.

There is a remnant hazard of the VSD losing the plot, just as there was with the industrial situation. So one could argue that there should be an additional control measure to cope with this as well. But one couldn't safely have two E Stop buttons between which the operator (with his hand in the mangle) has to choose, that's a silly idea, and furthermore you can't do both - i.e it's not going to work to have both E Stop methods working together, as they are mutually exclusive. So a lathe or a milling m/c (also a pillar drill or its close cousins) should have an E Stop function that invokes a braking effect, and does not isolate the machine. We just have to accept the remnant risk of the VSD going off on one. All the more reason not to skimp on the reliability and specification of the drive as installed.

Of course the braking effect must not of itself introduce another hazard worse that the one it is designed to control - eg unscrewing the chuck off the nose of the mandrel.

I've widely encountered a belief in industry that an emergency stop button is there so it can be "locked off" while someone is working on the machine. The requirements for safe isolation of the machine are NOT met by doing this, only an approved isolator (with the facility for a proper padlock and minimum contact separation, also clear indication of its having reached the safe isolation position) provides for safe isolation. In a home workshop single phase machinery is unlikely to have an isolator, though one would expect to be able to unplug it. This of itself isn't sufficient as it's too easy to plug the thing back in, perhaps unthinkingly or meaning to plug in another appliance. It wouldn't be sufficient in industry, but simply wrapping a few turns of insulation tape around the pins of the pug provides a memory prompt. Of course if the machine is of a stature it is hardwired to the supply it would necessarily have a local isolator, though domestic isolators (eg cooker switches) are usually not lockable and are therefore not isolators within the proper meaning of the term.

To sum up, my belief is that the best safety for a lathe. mill or drill in the home workshop is almost certainly to design a control system that uses the VSD to bring the machine to a controlled stop as quickly as possible. That implies not isolating the drive from the supply, and definitely not isolating the motor from the drive. Invoke whatever braking effect is safe on the way to a standstill.

Hope this helps

Simon

Sorry, but the normal stop button should already achieve the effect of stopping the machine as fast as is practical. E-Stop is not needed for it.

If quicker stopping is needed, DC injection braking can be used, as used on drill presses, activated by pendulum switches.

When the paramedic or the wife comes in, they need that big red button to make the machine safe. That includes the case where the operator is quivering due to the passage of electricity through him/her/it.

As an example, all the CNC machines we had at work, even the PLCs, controller and lube oil pumps got killed if the E-Stop switch was hit. This isn't a normal stop at all.

Edited By Mark Rand on 20/03/2018 22:50:58

Casually looking at equipment aimed at the home workshop they appear to only meet the very minimum standard for electrical safety which may be ok if the risk of injury is small and the likelihood rare. The more the severity of injury increases and the likelihood of an event occurring then the more redundancy will need to be built in to the equipment. Most inverters seem to be tolerant of the supply being removed from the input but not as a method of performing a normal stop. I think i would be inclined to supply the inverter from an NVR even though NVR functionality is built in to some inverters and put any estop buttons in the NVR circuit. Let's be clear that the estop will have no other function than to stop the machine in an emergency so hopefully you will never use it other than an annual check that it is functional. The situations that you would need an estop are most likely that you are entangled with the machine, a drive runaway or a release of magic smoke. The last two probably mean the drive is fubar so killing the power is unlikely to make things worse and if you are tangled in a machine then if you kill the inverter it will be the least of your problems. Serious industrial drives have many options for safety interfaces that monitor speed and have built in redundancy to meet the highest category of safety if required, these will not be needed or met in most home workshops.

Mike

That isn't normal with a VFD, they usually have a default 'soft stop' of around six seconds which mirrors the soft start. Both kinder on machines and nerves for normal stops and starts and less likely to unwind screw-on chucks!

You want quicker than that for e-stop though.

Neil

The stopping of a machine under control can often be quicker than an estop removing power, this is the case with robots but I think an estop had to remove power to protect from a runaway which may not respond to a controlled stop. Robots these days are extremely unlikely to runaway as they have many protection shutdown circuits, they even have a safe working envelope programmed so they cannot approach guarding and solid objects which may cause crush injuries. I have experienced a robot runaway when using a service routine during which many of the shutdown protection circuits were suspended, an estop removing arm power was the only way of stopping that one!

Mike

Hi Mike

If the Robot was running away what difference does switching arm power off, surely you need to switch leg power off ?

Emgee

It was more like thrashing backwards and forwards in one axis but as it was not bolted down at the time It was dancing around aswell, poo nearly came out. Made for an entertaining day though.

Mike

Edited By Mike Poole on 21/03/2018 19:30:14