Motor connection

Assuming that Michael doesn't want to change his inverter, I'd suggest using the built in brake function would be the best approach in the circumstances:

• He has no form of emergency stop at the moment.
• Assuming the motor to be driving the lathe at speed when the emergency occurs:
  • Disconnecting the single phase mains will cause the motor to coast to a stop. While coasting it is still dangerous.
  • Disconnecting the 3-phase output is somewhat complicated, risks damaging the inverter, and the motor will coast to a stop. While coasting it is still dangerous.
  • Using an inverter function to apply a braked stop is the quickest way to stop the lathe. If the brake part fails, the power is still disconnected and we are no worse off in that the lathe will coast to a stop,

Bob's comment about software failing is very true. "Failsafe systems fail by failing to fail safely". However, is there any evidence that Inverters suddenly go mad and refuse to stop? Although affordable inverters are a recent innovation, the technology has been around for about 40 years and works well. So Michael's risk assessment might legitimately conclude "In the event of a unlikely 'hand in the works' emergency, the fastest way of stopping is to use the inverter and risk of my inverter failing to stop and brake is low.

Bob has highlighted one of the main advantages of doing a Risk Assessment, which is peer review. He put his thoughts in writing and we were able to review them. I have put my thoughts in writing too. I'm absolutely not saying Bob is wrong, rather I'm suggesting an alternative for consideraton. Michael and others now have the opportunity to spot logical flaws, mistakes and misunderstandings etc. That should lead to a more balanced risk assessment, perhaps inspiring Michael to change his inverter.

One good thing for me is that I've realised that I don't know how the emergency stop on my lathe is implemented. I assumed hitting the big red button would do the very best thing; now I'm off to study the wiring diagram.

Dave

Thanks Dave, now I'll have to let you into a secret - I have risk assessed my lathe and reverse engineered the circuit diagram only to find out that the 'so called Emergency Stop' in no way conforms to the legislation.

The stop side of the inverter, which included the guard safety switches, are open contacts which close when the Emergency Stop is pressed - a serious no-no in control systems. Needless to say, it's on the list of jobs to address - but I haven't as yet come up with a solution as the inverter is Chinese and I'm not understanding their manual too well !

Just a little explanation of the no-no :

Safety contacts should be normally closed and open when the device is actuated - Why ? - well several reasons :

• Firstly, should the wire fall off, the circuit will shut down,
• Secondly, should the switch fall apart, failing in the open mode, the circuit will also shut down and
• Thirdly, should the switch contacts weld closed, the actuation of the mechanical part of the switch will forcibly disconnect the circuit ( albeit leaving the device permanently damaged, but better than an injury - eh ).

Using standard European Approved Emergency Stop push buttons will provide these safeguards, but, limit switches for guard interlocks are not so easy to specify correctly as the safe operation of the switch depends upon the correct mechanical application of the switch as well as the electrical aspects. Just take a moment to think about a limit switch applied to a safety guard - on my milling machine, there is a belt guard safety switch which is depressed when the belt guard is closed, relying on the spring inside the switch to operate the mechanism when the guard is opened - what happens if the spring fails or the switch jams ?

Bob

Hi Bob,

So far so good with my WM280. The emergency off and safety interlocks are all in series and they are Normally Closed types.

They break a live mains connection to the Contactor which is as far as I've got. After that I'm not sure if the contactor disconnects the Inverters power input directly , or if it works a control input. (The inverter stays powered up even if the emergency stop is pressed.)

All would become clear if I knew what the abbreviations meant. Guess who can't find the Inverter Manual even though I clearly remember putting it in a safe place. No idea which one though.

Dave

Before! we drift into the world of pendantry may I remind you this isn't my image, this is transwave converters or power capacitors' image, not mine, yes mine are similar to this, no, nothing has gone wrong because of it.

They manufacture these units commercially, and have done so for a very long time, if they can't competently do it, then maybe you should tell them, because it isn't bothering me or my inverter(s).

In fact, I don't think any of those connections are clamped in the holder, they are simply presenting them to show you where they go.

Michael W

Edited By Michael-w on 02/04/2017 14:53:40

Please see the link I posted for a way to do this.

The other is to fit an E-stop between the supply and the inverter (you need a specially designed inverter if you want to fit one between it and the motor).

If no braking resistor is fitted to the VFD, coast to stop should be OK as this is the normal situation with non-VFD machines unless they are big enough to need a brake in which case a mechanical one is fitted (although separate to the estop).

There is the DC injection option, on operating an E stop, DC is injected to the motor bring it to a rather abrupt stop...

Ok, there has been a lot of responses to this, in one case i'm glad but in another sense, my apologies to the OP for diverting the topic of conversation to this, I just didn't think it would generate this much talk at the time.

I'm thankful to all the sensible posters who have come up with what needs to be considered with regards to the inverter, I feel i'm left with a couple of options.

1. Provide an emergency stop directly to the source of the power, namedly an emergency "type" stop to bridge the single phase live wire from the wall socket to the inverter,

providing it does what is intended, it shall provide an almost guaranteed cut-off of power, that much we can be sure of, However, what is not certain, is if the inverter will actually shut off instantly, it actually takes 3-4 seconds to respond to the wall socket being switched off, this would not suffice as an emergency stop if that is the case.

2. Use either the existing connections between GND/24V on the controls, OR to use the free ports to program/remap a new function that interrupts the motor with no coasting time, I have used this inverter control before to setup new frequency from 50hz to 60hz parameters so I am fairly familiar with the jargon and how it's laid out.

I am aware that if I am successful it may not come under the technical definition of an "emergency" stop, but i'm not perfect, the worlds not perfect, and no matter how stupid anyone thinks I am, I did at least try to pre-empt some kind of risk here, as well as an opportunity to learn more about the system, this might lead me to knowing how to rig some L.E.D's in parallel to the system to have functioning lights.

For the ambiguity that I still have in my mind with regards to what route I shall take, I have made sure that the red button twist switch I have chosen, will be able to sustain a thermal current load (I-th) of 10A and 660V, well within the remit of a 7A maximum 2.2KW inverter.

I can't really do much until then but I will trawl through the manual to see what I can gather as to whether a programmable function is doable.

Thankyou,

Michael W

Edited By Michael-w on 02/04/2017 18:37:04

The problem is that you can't have both a gentle ramped down stop AND a braked stop programmed at the same time.