Inverters and stop switches

[old martParticipant @oldmart]

 

You would have to be the unluckiest person to have the inverter fail to respond to the stop switch at exactly the same time as you caught your arm in the works. I will not be worrying about having the two extra latching stop switches in series with the regular remote stop switch, especially as they are in places which are easy to reach. When one of these is pushed, the mill cannot be started until it is reset.

Edited By old mart on 07/11/2019 21:21:41

[Chris HembryParticipant @chrishembry84309]

Posted by old mart on 07/11/2019 21:11:20:

How would an EStop work between the inverter and the motor, there are three wires to disconnect.

Which is why I specified a three phase EStop so all three wires are broken together. Or use a contactor, with a 24V coil fed from the inverter control circuit to give another protection level.

Of course you are free to do what you want on your machinery, but I prefer an EStop to actually guarantee to make a machine safe rather than just in theory.

This stems from an instance when a machine had a dangerous but rare tendency to start on it's own because the inverter picked up a stray signal from the spindle encoder when the chuck was moved by hand. I lived with it for a short while, but there was no way I was going to let anyone else anywhere near it !

[old martParticipant @oldmart]

That's why I deleted that line before you posted.

I cannot identify Brian's VFD from the pictures and recon it would be better to bin it and get one with decent instructions, here is an example of the guide in pdf printable form for the VFD I bought:https://inverterdrive.com/file/Schneider-ATV12-Easy-Start-Guide

Edited By old mart on 07/11/2019 22:17:10

[EmgeeParticipant @emgee]

If you do feel it best to have an isolator between the VFD and motor I would advise use a 4 pole unit so you can connect the stop circuit through the isolator, possibly less chance then of damage to the VFD.

Emgee

[I.M. OUTAHEREParticipant @i-m-outahere]

The manual for the Huanyang inverter i have clearly states “ Do not place any switching gear between the inverter and load or disconnect the load while the inverter is running or serious damage will occur “ .
Disconnecting a Mosfet or IGBT while it is operating is a good way of killing it , the sudden cessation of current flow will cause a voltage spike which can destroy the gate . That is why it is best to let the electronics do the job they were designed to do and install the unit as per manufacturers recommendations along with the electrical standards for your country ( yes they are all different )
Looking at the diagram in the photos I would say the 6 switched inputs on the left are the terminals that can be configured to handle the run/stop , E- stop , jog fwd , jog rev and possibly high and low speed operations and your manual should tell you how to configure these .
The relay terminal on the right side is used for a water cooled spindle or can be used to turn a coolant pump on if you run one .

[Mark RandParticipant @markrand96270]

Note, I have not tested this, but:-

If an inverter/VFD is dynamically braking a motor, it will be absorbing power from the motor (Hence the reason for auxiliary braking resistors). In this case, the VFD's DC bus voltage will be maintained for as long as is necessary even without an input supply. If the VFD is one which can be fitted with a braking resistor, the E-stop switch could release the supply contactor and trigger emergency braking from the VFD to obtain rapid braking combined with an electrically safe machine in the minimum time. The VFD would need programming to not trip on an input phase loss, which tends to be the default behaviour.

[old martParticipant @oldmart]

When the mill is stopped with the normal stop switch (normally on) the VFD display goes to "rdy" (ready) and pressing the start switch (normally off) restarts the motor. When one of the emergency buttons is pressed, it works exactly the same as the normal stop switch, but latches off. The VFD display goes to "nst" (no start) and pressing the start switch will not start the motor until after the emergency stop switch is reset.

All of the VFD manufacturers specifically prohibit switches between the VFD and the motor, they would never say that if it was contrary to legislation.

[Robert Atkinson 2Participant @robertatkinson2]

As stated putting any switch between the VFD and motor risks damaging the drive. Apart from this most E-Stops are not adequately rated for this.

Havig a faulty system that starts when it should not is not a got reason for fitting a switch in the motor. The root cause should be identified and fixed.

Robert G8RPI.

[Michael BriggsParticipant @michaelbriggs82422]

A switch between a VFD and the motor does not risk damaging the drive provided that it is only opened or closed when the drive is at standstill though there are exceptions. In industrial installations is not uncommon for a drive to be ramped down to standstill and then disconnected from the VFD output.

[Robert Atkinson 2Participant @robertatkinson2]

I've never seen a switch between a VFD and motor on an industrial installation. Can you give an example?

[Michael BriggsParticipant @michaelbriggs82422]

I can Robert :

Bread dough mixer with a 90 Kw motor. The risk assessment required a category 4 safety system, a VFD powered up and stopped is not safe enough to meet HSE requirements. You can of course isolate the supply making the system safe but the VFD was not rated for the required 10 power cycles per hour. The solution was disconnection of the motor from the VFD. The machine was guarded (mandatory) but locked until the VFD brought the motor to a standstill and then was isolated from the VFD so the VFD could remain powered up and the system made safe.

Reverse operation on startup, lock access gates engage VFD connection to drive then start the VFD.

Outcries from the forum "what has that got to do with my machines", perhaps not a lot, a VFD at rest and powered up is unlikely to start by its own accord but it is possible. So when I am setting up on the faceplate for example I isolate the lathe.

For some time there have been VFDs available with a safe off facility that can make the VFD safe while powered up but no need for that in my workshop.

Hope that makes sense,

Regards

Michael

Edited By Michael Briggs on 08/11/2019 21:31:37

[Robert Atkinson 2Participant @robertatkinson2]

That application makes some sense, being a secondary interlock after the motor has stopped and VFD output is off. It's not putting an E-Stop in series with the motor. Probably more cost effictive than using a safety rated VFD.

Robert G8RPI.

[Michael BriggsParticipant @michaelbriggs82422]

Robert,

I am pleased it makes some sense to you, I will do my best to respond to any questions you have to anything that doesn't make sense.

Regards,

Michael

[SillyOldDufferModerator @sillyoldduffer]

Posted by Robert Atkinson 2 on 08/11/2019 18:19:22:

As stated putting any switch between the VFD and motor risks damaging the drive. Apart from this most E-Stops are not adequately rated for this.

…

Michael has spotted that not all VFDs are equal!

The small simple type we buy are usually designed to be connected to a single motor and don't take kindly to being switched on the output side, nor do they like powering more than one motor. Check the Manual! Big units are another kettle of fish; quite likely designed to power several machines switched on the output side, and used – for example – up into the megawatts to provide emergency power to a hospital.

Switching an unsuitable VFD risks more than damaging the drive, it could also damage the motor, or fail to stop it! Robert rightly mentions inadequate e-stop ratings for this service. The problem is VFDs simulate the output waveform by pulse modulation. The motor is fed high frequency spikes rather than a clean sinusoid, which isn't a problem provided all three coils are connected. Disconnecting is dodgy because sharp pulses fed into either end of an open winding cause it to behave like a car ignition coil generating several thousand volts. The spikes are well capable of puncturing wiring insulation and leaving an ionised trail across switch contacts perfectly capable of starting a power arc, sustaining the motor and possibly welding the contacts together.

Unless the VFD specifically allows it don't switch the output. My advice – don't guess, don't generalise, read the instructions.

Dave

[Mike PooleParticipant @mikepoole82104]

Around the year 2000 dual safety contactors were fitted between the inverter and motor, these were monitored and controlled by a Pilz safety PLC, the next standard positioned the dual contactors before the drive, the third generation has gone to safe off controlled by Siemens S7 with safety program. All the inverters were SEW Eurodrive which are definately at the high end of inverter technology so unlikely to be in most peoples home workshop.

Mike

[not done it yetParticipant @notdoneityet]

Re the switching between motor and VFD. It is possible without damaging the VFD. They are sold for this very purpose but the VFD needs to be more powerful than the individual drive, such that several drives can bne run from a single inverter.

Most certainly follow the VFD instructions, BUT they are out there. I don’t know if the last motor has to be stopped by the inverter stop sequence, but they are available for multiple drives from the one inverter switched separately between motor and VFD.

Dave is right (and see my post much earlier as well).

Try reading down to, and including, the plug and play section on this item.

**LINK**

That should make it clear to the naysayers.

[Michael BriggsParticipant @michaelbriggs82422]

SOD with due respect, I haven't spotted that VFD's are not all equal, I have worked with them for the last 40 years.

Regards, Michael

[Brian HParticipant @brianh50089]

I decided not to fit the stop button that I have, it's easier that way.

Brian

[SillyOldDufferModerator @sillyoldduffer]

Posted by Michael Briggs on 09/11/2019 00:41:48:

SOD with due respect, I haven't spotted that VFD's are not all equal, I have worked with them for the last 40 years.

Regards, Michael

Michael, the comment I'm picking up on is:

'A switch between a VFD and the motor does not risk damaging the drive provided that it is only opened or closed when the drive is at standstill though there are exceptions.'

Given that most readers of this forum buy VFD's from the "though there are exceptions" group, I thought it worth highlighting the value of reading the manual to anyone thinking of switching VFD outputs!

I'd have been happy if you'd shifted the emphasis to suit an audience buying small VFDs: 'Though there are exceptions a switch between a VFD and the motor risks damaging the drive .'

Dave

[not done it yetParticipant @notdoneityet]

Posted by Michael Briggs on 09/11/2019 00:41:48:

SOD with due respect, I haven't spotted that VFD's are not all equal, I have worked with them for the last 40 years.

Regards, Michael

Well, you should have now! Things have changed a lot in 40 years!

That said, I for one will not be going plug and play – ever. I’m not prepared to pay perhaps ten or twenty times the cost (of a cheap chinese inverter) for that luxury.

The sentence of text in the ad makes that clear: This type of Plug and Play system when correctly sized can be used to power a whole workshop via a ring main,

There may be some on the forum that this could apply to, but I doubt there are many – and likely none would even consider this option as a practical proposition at this time. Another 40 years and things might have changed again.

For a start all the programming opportunities and protection, afforded by discrete VFDs, is non-existant. That would not be acceptable to me.

Most certainly any failure to follow the inverter manufacturer’s instruction (just looking at the wiring diagram is enough) would void any warranty in this respect. This is the first time (within the last year?) I have seen this type of system on the market, btw.

But the fact remains. They are available.

[Michael BriggsParticipant @michaelbriggs82422]

Hello Dave, by "the exceptions" I meant that there are VFDs that are capable of having the load switched while the drive is running, large and small.

The point I would like to make clear to the forum is that the type of VFDs we generally have in our workshops when at rest and powered are unlikely to start due to failure of internal control circuit but are not safe. I recommend that when you are setting up a job on the faceplate for example, you isolate the power. I don't isolate for everything, that is my choice.

Michael

[old martParticipant @oldmart]

It seems patently obvious to me that disconnecting the motor from the VFD when the motor is not running will not harm the VFD. There is no current flowing between them at the time.

As for the pastry mixer, it seems the switching between the VFD and the motor is only used when the motor has already been stopped by the VFD and is nothing to do with emergency stopping, but only an extra means of isolating the mixer.

 My use of latching emergency switches works quite well as when the stop circuit is broken, the start switch is inoperative. If I was changing the tooling and someone pressed the start switch, nothing would happen.

 

Edited By old mart on 09/11/2019 22:10:09

[I.M. OUTAHEREParticipant @i-m-outahere]

I think the important thing to remember is that we are talking about a domestic installation and not an industrial one !

Brian,

For the simplest installation all you need is :

A latching stop switch for an emergency stop – IMHO all machines should have one of these, whether it is cutting off the mains power or telling an inverter to stop is irrelevent as long as the machine has one that works .

A momentary pushbutton switch for a start switch and the same for a stop switch , you can use a toggle switch for forward / reverse switching as it only changes which terminal the start switch is connected to ( you will have start fwd and start rev on the inverter ) the stop is connected to reset and you can usually configure e stop in the programme to use a terminal suit your needs .

My mini lathe is running a 3 ph motor and inverter ( see my photo library ) and the only other thing i have is a mains power switch mounted next to the lathe head to turn it all on and if i was pedantic enough i might even turn that off while i fiddle around mounting something on the faceplate ! . What is the make and model of your inverter Brian ? Maybe i can download a copy and try to make some sense of it for you .

Back to the discussion –

In all seriousness once the E-stop is active no input signal of any kind should turn the inverter output on – it is in the programming ! , even if the IGBT'S failed and went short they would only supply DC and fry the motor coils .

If you have an inverter that is activating randomly you have issues with shielding and / or transient voltages .

[Mark RandParticipant @markrand96270]

I guess that I shouldn't rattle on yet again about my VFD that feeds the entire shed, with three machines and a combined 6 three phase motors all direct on line started from it then…

[old martParticipant @oldmart]

I have also asked Brian for the details of his inverter, with no success.

Mark, do you run more than one machine at once, and if so, how many?

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