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Inverters and stop switches

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Brian H06/11/2019 19:42:40
1637 forum posts
108 photos

Now that some kind members have advised on wiring an inverter, I have another query!.

I know that the inverter should be connected directly to the machine motor with no switches inbetween so, could an emergency stop switch be wired into the single phase supply to the inverter?


Alan Waddington 206/11/2019 19:56:30
501 forum posts
87 photos


Both of my inverters have the E Stop wired into the low voltage ‘Control’ side of the inverter. The lathe has a pendant with Forward, Reverse, Jog and E stop switches, wired to the inverter using alarm type multicore cable.

Mike Poole06/11/2019 20:04:26
2572 forum posts
60 photos

An emergency stop will generally need to cut the power to the machine, inverters are not too happy about having their power cut but as it is an emergency stop it should only be used in an emergency. The debate has long raged that a controlled stop can be faster than the coast down you will get from killing the power but if the drive has failed in some outrageous way then cutting the power to it may be the only effective solution. More and more drives are getting a safe stop function built in which can guarantee to remove any drive signals to the output stage and I doubt that a failure of the output stage exists that could keep the motor turning. Industrial drives are featuring safe stop but they have control systems that integrate with these features.


Simon Williams 306/11/2019 20:06:30
507 forum posts
80 photos

Simple factual answer to the question as stated - "could the e stop be wired into the single phase supply" is "Yes!"

But the question deserves some more thought than that, and the pro's and con's of putting an E Stop in the supply need a bit more consideration.

Firstly, my apologies if I've missed the relevant post, but I don't think I've seen what sort of machine this is. This arises because I think the proper answer to your question needs a rudimentary risk analysis to inform the design decisions. If the machine is something nasty and dangerous, then the risk of injury is offset (controlled) by being able to stop the machine quickly, and switching off the mains supply won't necessarily give you the optimum control. If your arm is in the mangle then having an active machine braking function activated by the emergency stop would probably be a good idea.

If the hazard is of the machine going berserk because of the inverter losing control then shutting the mains off is your only option. For the milling machines and lathes I have in my shed I think this is a very unlikely that a modern good quality inverter is going to go doolally, so I have the E Stop function as a stop signal to bring the inverter to zero frequency and with DC braking if appropriate. The inverter stays energised so it can do what it needs to do to effect a rapid stop pf the offending machine.

So the short and simple answer to your question is "Yes" - you can connect some sort of E Stop functionality to remove the mains supply from the machine motor, but my preference is to use the extra control facilities of the inverter to give me a quick stop function and for this I leave the supply on the inverter but tell the inverter to stop the machine. There's a big red latching button on the machine within reach, and if I bang it the machine comes to a stop whichever way it is configured.

Horses, as they say, for courses.

Hope this helps Simon

old mart06/11/2019 20:11:40
1735 forum posts
138 photos

I bought an inverter from a firm which also had quick start guides in pdf form which showed exactly how to wire in remote switches and potentiometers. I added two emergency stop switches in series with the regular stop switch. Anyone who just goes out and buys this sort of device without doing research first to find out whether the instructions are understandable for anyone who doesn't have a degree in electrical engineering is in for a shock.

If you post the exact model of inverter you have, I will see if a quick start guide is available for it. Every make has its own exclusive connections and programming, so a general guide is of very little use.

 Just adding a switch in the power supply is better than nothing, but the motor will stop quicker if the inverter is set up correctly, with remote switches. Never put a switch between the inverter and the motor.

Edited By old mart on 06/11/2019 20:15:39

Edited By old mart on 06/11/2019 20:19:46

Pete Rimmer06/11/2019 20:35:49
707 forum posts
49 photos

An Estop you wire into the control circuit, otherwise it's not and Estop it's a power disconnect, and not even a good one as it'll invariably leave neutral connected. if you desperately want the VFD to be instantly disconnect-able then feed it via a NVR.

Stopping in an emergency you want maximum braking and you won't get that by killing the VFD power.

Robert Atkinson 206/11/2019 20:42:26
641 forum posts
16 photos

You should use the E-Stop function of the inverter if it has one. Almost all of them do. This will switch off the invertor output instantly and with most units provide at least some dynamic braking where inverter "shorts" the motor. On simple inveters this is limited due but some have positive braking either by dumping the energy or injecting DC into the motor.
There is one big problem with putting the E-Stop in the input supply - stored energy. The inverters convert the incoming AC to DC before inverting it back to AC. As part of the AC to DC conversion there is a filter capacitor that stores energy. Sometimes this is called a DC bus or link capacitor. If you just cut the supply the motor will continue to run on the energy stored in the capacitor. If the inverter is lightly loaded compared to it's maximum rating the machine can continue to run for a significant time on this stored energy. It will certainly run longer than if you use the E-Stop function.

Robert G8RPI.

Edited By Robert Atkinson 2 on 06/11/2019 20:43:32

Michael Briggs06/11/2019 20:50:21
187 forum posts
9 photos

An "emergency stop button" wired in series with the regular stop button is just another stop button. I normally stop my machines with the stop button and the VFD ramps down to standstill. I have emergency stops on my machines, they isolate power from the VFD. Fortunately I have not had to use them, but if I do I know the machine will stop even if the VFD control has failed.

old mart06/11/2019 21:24:28
1735 forum posts
138 photos

The machine will stop quicker ramping down than if it just freewheels if the power is isolated from the VFD. The emergency stop buttons I have fitted are in positions where the operator can reach them easily, I cringe when I see somebody's machine with nothing but the tiny buttons on the VFD controlling it.

Edited By old mart on 06/11/2019 21:31:22

not done it yet06/11/2019 21:29:18
4629 forum posts
16 photos

Grid power failures must be accommodated by these VFDs, so switching them off while powering a motor is clearly permissible. Not necessarily the recommended route, of course. But a brake on a lathe with screwed chuck might not be such a clever idea, either.

Pete and the NVR is likely the better alternative, if the VFD control circuitry is not empowered.

Solar power inverters are very similar - they disconnect and reconnect automatically with no problems, but they will not tolerate switching the DC input while powered up.

XD 35107/11/2019 00:26:53
1425 forum posts
1 photos

I know that my inverter will continue to run for a second or two after the power has been switched off , Presumably because of the large capacitors inside that are there to account for sudden load changes .

With my unit the difference between E - stop and stop is with the stop switch i can alter the ramp down setting / braking if i want it to stop faster and the E - stop applies full braking to stop the chuck instantly .

I have the mains power switch mounted right next to the lathe head and within easy reach if something does go wrong with the inverter i can cut the power just as quickly as using the E- stop .

I can see what NDIY is saying about the screw on chuck , my last lathe had one and it also had a keeper plate that screwed to the backplate with one 5mm screw and it had a tang or hook that fitted into an annular groove on the spindle to stop the chuck coming off completely should it come loose. I know one thing for sure is if i was being pulled into a rotating chuck the last thing i would be worrying about is the chuck unscrewing , i would want it to stop dead !

One thing to remember is some of those emergency shut off buttons are not rated to carry the amperage that your mains can supply or your machine may draw , not a problem if you are going to fit a contactor as an NVR as you simply fit the emergency cut off switch in series with the contactor coil .

The ultimate would be a mains isolation switch /emergency stop switch /NVR contactor combo mounted on the wall and the start/stop /speed and E - stop mounted conveniently on the machine and other than being able to predict the future I don’t think you could do anything more to protect yourself !

Mark Rand07/11/2019 01:39:09
887 forum posts
5 photos

This is from the 16th edition of the Wiring Regulations, since I don't intend to upgrade my house further (Feel free to purchase and work to later versions). Subject to the effects of beer it is an exact copy (Oxford commas included).

This actually constitutes the law in the United kingdom. It is, however, up to you to decide if it applies to your shed. In my case an NVR contactor operated by a button by the door of the shed would allow SWMBO to rescue me in the case of any electrically related emergency.

Turning of a lathe or bandsaw in normal use does not constitute an emergency stop, so NVR switches with silly clip on red buttons over the main controls get modified with extreme prejudice.

537-04 Devices for emergency switching

537-04-01 A means of interrupting the supply for the purpose of emergency switching shall be capable of cutting off the full load current of the relevant part of the installation. Where appropriate, due account shall be taken of stalled motor conditions.

537-04-02 Means for emergency stitching shall consist of:-

(i) a single switching device directly cutting off the incoming supply, or

(ii) a combination of several items of equipment operated by a single action and resulting in the removal of the hazard by cutting off the appropriative supply: emergency stopping may include the retention of supply for electric braking facilities.

A plug and socket-outlet or similar device shall not be selected as a device for emergency switching.

537-04-03 Where practicable a device for emergency switching shall be manually operated directly interrupting the main circuit. A device such as a circuit breaker or a contactor operated by remote control shall open on de-energisation of the coil, r another technique of suitable reliability shall be employed.

537-04-04 The operating means (such as a handle or pushbutton) for a device for emergency switchig shall be clearly identifiable and preferable coloured red. It shall be installed in a readily accessible position where the hazard might occur and, where appropriate, further devices shall be provided where additional emergency switching may be needed.

537-04-05 The operating means of the device for emergency switching shall be of th latching type or capable of being restrained in the OFF or STOP position. The resetting of the emergency switching device shall not re-energise the equipment concerned. A device in which the operating means automatically resets is permitted where both that operating means and the means of re-energising are under the control of one and the same person.

Ian McVickers07/11/2019 06:40:07
180 forum posts
88 photos

Just as an additional note to this, BS7671 is not a statutory document and does not apply to machinery wiring.

Robert Atkinson 207/11/2019 07:39:31
641 forum posts
16 photos

The wiring regulation (BS7671) are not applicable to this (or the law, you can in theory show alternative means of compliance, but good luck trying). The quoted text applies to electrical isolation, not machine E-Stops. The mention of machines is to allow the input to motor drive to NOT be isolated from the supply so it can stop the machine more quickly.

If using a VFD (inverter) with a separate E-Stop input (and you should be if the machine needs an E-Stop) that is what you should use for the E-Stop as it stops the machine in the shortest possible time (loose chucks are a separate hazard and should be dealt with accordingly). Use the Stop / Start inputs of the VFD for just that and connect a proper isolator switch (Rotary red and yellow with provision for a padlock in the OFF position) at the VFD supply input.

A No Volt Release (NVR) should not be required with a VFD because you should set it up not to power the motor on power up or when the E-Stop is released. (This is normally the default but can be bypassed when the drive is used under automatic control)

E-stops should not be used for routine stopping of the machine but should be tested regularly.

If you want to get legal, then there are a host of assessments to do and all safety components must be designed and approved for the application. Most Hobby machine tool safety provisions, though CE marked, would not pass a through assessment. The NVRs with flip-over E-Stops are a particular example.

Any E-stop is better than none and you should NEVER EVER use the mains plug as the only means to start or stop any machine.

Robert G8RPI.
(CEng and have done this stuff for a living on machines to CE / UL & C-Tick).

J Hancock07/11/2019 08:50:50
400 forum posts

A 'proper switch' definitely, placed somewhere that your last uninjured limb can reach, quickly.

Pete Rimmer07/11/2019 11:27:03
707 forum posts
49 photos

At work we have numerous items of high-powered portable/pedestrian operated plant, probably a couple of hundred different machines from 1kw to 35kw nearly all 'high risk'. Many of them have three Estops but not one single one of them is fitted with an Estop that disconnects the incoming supply. All of them stop the motor and serve as a control interlock so that no control can be used until the Estop is released.

BTW I wasn't suggesting than someone SHOULD power a VFD with a NVR. It was just a suggestion for someone who was so paranoid about the above that they find it a distraction. I wouldn't find it necessary myself, at all.

SillyOldDuffer07/11/2019 12:49:43
5741 forum posts
1207 photos

I vote for both!

+1 to Robert's explanation. It means the normal way of stopping a motor should be by using whatever 'stop' facility the VFD itself provides. This is usually a low-voltage system that can be wired to as many interlocks as wanted, perhaps a chuck guard, a foot bar, and a red-button. The VFD is probably smart, not just switching the power off but able to positively brake the motor as well.

But there's no objection to installing a second Big Red Button that disconnects power to a machine or even all the machines in a workshop. Then when the neighbours break in to stop the screaming, its obvious to them how to turn the power off so the shredded victim can be disentangled from his tool and rushed to A&E. The same button also deals with unlikely electrical faults such as when a machine gradually vibrates it's way through a mains cable (missing grommet) and cuts the earth before connecting the operator to Live.


Brian H07/11/2019 13:20:13
1637 forum posts
108 photos

Just to clarify what I'm up to; the machine is a Tom Senior Major Mk2 milling machine that was once owned by the Boots company. It was fitted with a number of safety switches on every door, all connected to a huge box of tricks to control them. All of which have now been removed and I have bought an inverter rated at 2hp (1.5kw).

A previous query on this excellent site will enable me to wire the inverter to the 3ph motor but I wondered if a remote emergency stop button might be a good idea, especially as I have a box with start and emergency stop buttons.

This box has 2 switches marked Telemechanique 240V under the stop button which is the type that locks down and has to be rotated to release, and a single identical switch under the start button. Being a miser it would be nice if this could be reused but I would need guidence on the wiring.


Emgee07/11/2019 13:36:41
1483 forum posts
217 photos

Brian, check your VFD manual for the remote Stop/Start circuit.
if you have a separate drive for table traverse that should also be connected to stop if the Em stop is operated.
You may be able to add a second NC (normally closed) contact block on the Stop function.


Edited By Emgee on 07/11/2019 13:39:22

Brian H07/11/2019 16:52:10
1637 forum posts
108 photos

Back again, this time with pictures!

I have now rebuilt the electric motor (a Crompton Parkinson 1.5 kw one) and have relaced both bearings after finding one siezed due to rust.

I am now ready to test the motor before fitting it back in place in the base of the mill.

I think that I have followed the instructions given previously but would be grateful for confirmation that I've got it right.

I left the U,V, W wires in place and then placed vV2, W2 and U2 over them. I then added the connections to the inverter.

proposed wiring to motor.jpg

The inverter instructions follow;

inverter instructions.jpg

inverter controls.jpg

The rear of the proposed start/stop controller is next and if someone can advise where to wire this in then I would be most grateful.

proposed start-stop switch.jpg


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