Three-phase power for workshop

[Sonic EscapeParticipant @sonicescape38234]

All my machines need three-phase power but unfortunately I have only a single-phase connection. My lathe had an inverter that I used to power other stuff but this was inconvenient. I had to reconnect the wires every time. So I sold it and I bought two Delta VFDs, VFD022EL21A and VFD007EL21A . I need two not because I plan to use two machines in the same time, but the mill has two motors.

This is the electrical wiring that I’m planning to do. Nothing complex, just the bare minimum. There will be two sockets where I’ll plug the machine I want to use in that moment. I’m not decided yet about the controls (start/stop and speed). I think I’ll make one small mobile box with buttons and a long cable for each VFD and I’ll attached it somehow to the machine that is in use. Or one fixed box with a connector for each machine.

I hope the long control wires will not be susceptible to EMI. I’ll add some decoupling capacitors. Also I’ll add a resistor between the analog speed control input and ground to stop the motor in case the cable is disconnected somehow.

 

[Robert Atkinson 2Participant @robertatkinson2]

Not a bad start. I’d suggest a couple of changes:
1/ Use a second relay in the 24V circuit with two contacts one connected to each of the VFD E-Stop inputs Don’t use extra contacts on the existing relay, keep the mains and low voltage control circuits separated for safety.
2/ Unless the VFDs have input filters you need to add a filter at each VFD input.
3/ Use screened / armoured cable e.g. CY or SY between VFD and sockets and plugs and motors

Ideally use outlets with interlocked switches BUT wire the switches to the INPUT the respective VFD (keep your control circuit). This has the advantage that you can’t unplug the motor with VFD on.

Robert.

[Ian PParticipant @ianp]

Your emergency stop will not be instantaneous. Cutting mains supply to the VFD can leave the motor running for several seconds until the DC bus capacitors discharge. Better to use the VFD’s E-Stop feature or at least use the deceleration parameter you have set.

I’m not conversant with the VFD you linked to but ones I have come across have setable options to suit the motor it is driving so ideally the motors you are switching between should have similar ratings.

Ian P

 

 

[Robert Atkinson 2]

On Robert Atkinson 2 Said:

Not a bad start. I’d suggest a couple of changes:
1/ Use a second relay in the 24V circuit with two contacts one connected to each of the VFD E-Stop inputs Don’t use extra contacts on the existing relay, keep the mains and low voltage control circuits separated for safety.
2/ Unless the VFDs have input filters you need to add a filter at each VFD input.
3/ Use screened / armoured cable e.g. CY or SY between VFD and sockets and plugs and motors

Ideally use outlets with interlocked switches BUT wire the switches to the INPUT the respective VFD (keep your control circuit). This has the advantage that you can’t unplug the motor with VFD on.

Robert.

I’m not sure I understood what you mean at point 1. In my understanding this VFD has a N.C. relay output (RB) that by default is signaling a malfunction. I want to add this N.C. contact in series with the 24V circuit. So any fault will cut the supply of the contactor coil.

The input relay is actually a Siemens 3TB41 contactor. It has 3 N.O. and one N.C. contacts. You mean that is not safe to rely on insulation between contacts?

 

This VFD claims to have filters at both input and output.

 

 

On Ian P Said:

Your emergency stop will not be instantaneous. Cutting mains supply to the VFD can leave the motor running for several seconds until the DC bus capacitors discharge. Better to use the VFD’s E-Stop feature or at least use the deceleration parameter you have set.

I’m not conversant with the VFD you linked to but ones I have come across have setable options to suit the motor it is driving so ideally the motors you are switching between should have similar ratings.

Ian P

 

 

I don’t have much experience with VFD’s but I think only when you are using vector control you need to match the motor parameters with VFD settings. In V/f control mode I suppose the VFD doesn’t care about the motor unless it doesn’t exceed its output current.

This VFD doesn’t have an emergency stop input. I think is safer to just cut the power and not to rely on other features. There is an optional breaking module. Is not very expensive but is not clear for me the benefit of a quick stop. So far I never had to quickly shut down a machine.

 

 

 

 

 

 

[Robert Atkinson 2Participant @robertatkinson2]

Hi Sonic,
The arrangement with the NC fault outputs on the VFD is OK. The additional relay is to cover the fact that disconnecting the input supply of a VFD does not result in a rapid stop of the motor (This was also raised by Ian P). Whenever the E-stop is pushed the Stop inputs on both VFDs must be activated. This ensures the motor stops quickly (power is provided by the “DC Link” capacitors inside the VFD and the inertia of the motor and load). The braking is DC for this so you need to set the 8.x parameters.
It is OK to use the auxillary contacts on a contactor for low voltage control circuits as they are adequately isolated. However you need two contacts, one for each VFD Stop input. So an additional relay is required or a contactor with a second auxillary contact.

There is one oddity in your circuit which could catch the unwary. The ON button is at mains potential while the OFF and E-STOP are at 24V. You would not normally expect one to be on mains. This is not good practice.
If you add an isolator switch on the supply (which you should have anyway) You can leave the 24V transformer primary permanently connected and put all of the control switches at 24V.  If you are using the mains plug as an isolator then you would have to switch off at the socket.

Robert.

[Sonic Escape]

On Sonic Escape Said:

On Robert Atkinson 2 Said:

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On Ian P Said:

Your emergency stop will not be instantaneous. …

 

 

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This VFD doesn’t have an emergency stop input. I think is safer to just cut the power and not to rely on other features. There is an optional breaking module. Is not very expensive but is not clear for me the benefit of a quick stop. So far I never had to quickly shut down a machine.

 

 

 

 

 

 

The benefit of a quick stop is because ‘so far I never had to quickly shut down a machine.’   People who catch clothing or hair in a machine, or who have a whirling rod incident, appreciate the feature.   I make do with a big red button, but foot or belly operated rails are common on new industrial kit and amateurs do fit them.

Safety apart, a machine that stops quickly speeds workflow.  Also braking by heating some big resistors is less wearing than dissipating the energy in the drive train.   Neither of these seem particularly advantageous in a  hobby workshop.

Dave

[EmgeeParticipant @emgee]

You may want to consider fitting a higher fault current RCD if at all 1 is necessary, you no doubt already have RCD protection on your workshop installation.

Even with a 30ma operating current RCD on a VFD supply tripping can be problematic, you will have 2 operating with 1 machine so makes it even more likely you will experience tripping issues.

Bond the protective braid of the SY cable to the incoming cpc (earth conductor) as advised by the VFD manufacturer, some advise connection to cpc at both ends, others only at the VFD end.

Ensure you have an adequately sized cpc conductor connected at the motor casing, even if the braid is connected both ends don’t rely on that as the resistance will be too high.

Emgee

 

[Robert Atkinson 2]

On Robert Atkinson 2 Said:

Hi Sonic,
The arrangement with the NC fault outputs on the VFD is OK. The additional relay is to cover the fact that disconnecting the input supply of a VFD does not result in a rapid stop of the motor (This was also raised by Ian P). Whenever the E-stop is pushed the Stop inputs on both VFDs must be activated. This ensures the motor stops quickly (power is provided by the “DC Link” capacitors inside the VFD and the inertia of the motor and load). The braking is DC for this so you need to set the 8.x parameters.
It is OK to use the auxillary contacts on a contactor for low voltage control circuits as they are adequately isolated. However you need two contacts, one for each VFD Stop input. So an additional relay is required or a contactor with a second auxillary contact.

There is one oddity in your circuit which could catch the unwary. The ON button is at mains potential while the OFF and E-STOP are at 24V. You would not normally expect one to be on mains. This is not good practice.
If you add an isolator switch on the supply (which you should have anyway) You can leave the 24V transformer primary permanently connected and put all of the control switches at 24V.  If you are using the mains plug as an isolator then you would have to switch off at the socket.

Robert.

I don’t want to add an isolator switch on the supply and indeed that is why ON button is at mains potential. But it is safe. It is a plastic button in a plastic box. Everything is connected to a wall socket so I have a way to disconnect the circuit in a more convincing manner if needed.

About braking, I think the best is first to make some tests to see how the motors stop in different cases. Then it will be more easy to decide what to do. For example if it make sense or not to buy the braking unit.

 

On Emgee Said:

You may want to consider fitting a higher fault current RCD if at all 1 is necessary, you no doubt already have RCD protection on your workshop installation.

Even with a 30ma operating current RCD on a VFD supply tripping can be problematic, you will have 2 operating with 1 machine so makes it even more likely you will experience tripping issues.

Bond the protective braid of the SY cable to the incoming cpc (earth conductor) as advised by the VFD manufacturer, some advise connection to cpc at both ends, others only at the VFD end.

Ensure you have an adequately sized cpc conductor connected at the motor casing, even if the braid is connected both ends don’t rely on that as the resistance will be too high.

Emgee

 

Yes, I have a 30mA RCD for the whole house. But I want to try also a 10mA one only for the workshop. I’m aware that this old machines might have some issues with residual currents. We will see.

I start to add all components on a wood plank. It is 200x50cm and 27mm thick. Normally it is used for concrete forms. I will fix it on the wall close to the celling so nobody could reach it by mistake. And there is plenty of space left for future electrical stuff and the VFDs ventilation is perfect. There are still a lot of wires to connect.

 

I’m also thinking if not to make another earthing system only for the workshop. Or at least to bring a separate cable from the grounding rods. I don’t like the ground connection in the wall socket. It is a chain of cables connected together with some screwless connectors.

By chance I made also a small discovery. The 10mA RCD had a C25 marking so I assumed that is has also a 25A circuit breaker. But I noticed a tiny warning “to be protected by and circuit breaker” or something like this. So it has no overcurrent protection, it only monitors the residual current.

[Nigel Graham 2Participant @nigelgraham2]

This may depend on the make of inverter but on mine at least the instructions specifically say the emergency stop button is for emergency use only.

Using it instead of the normal stop button can lead to damaging the electronics. I don’t know how but I don’t intend to find out the hard way!

[Robert Atkinson 2Participant @robertatkinson2]

Sonic said:
“I don’t want to add an isolator switch on the supply and indeed that is why ON button is at mains potential. But it is safe. It is a plastic button in a plastic box. Everything is connected to a wall socket so I have a way to disconnect the circuit in a more convincing manner if needed.”

Using a socket outlet to provide isolation is an acceptable alternative to an isolation switich. However it is still bad practice to combine mains and low voltage (technically mains is low voltage and 24V is extra low voltage) signals in the same control circuit. Particuarly having two similar switches, start & stop, next to each other in the same box with one on 240V and the other on 24V. Even you might make a mistake when working on the system in the future. A third party who might have to work on it is at significant risk.

You ansl appaer to be using individual circuit breakers in live and neutral supplies to the VFDs. This is very poor practice. Generally the neutral should not be switched other than at the master switch / isolator. f you do want to use neutral switching then double pole devices should be used. These are not readially avialable. You also appear to be using light blue single conductor wire for all connections. This again is very poor practice and likely breaks local regulations in Romania.
The VFDs should be mounted on a metallic panel for EMC reduction. The VFDs and other components neeed to be enclosed.

If the square plastic box with two push buttons and a red indicator at the lower left is your Start/Stop control then the switches are NOT suitable given that you have mains on the switch. The metallic bezel of the push buttons could become live. Even if the contacts are rated for 240V it does not mean the switch is suitable. Ideally the ON button should be green and shrouded (so it can’t b activated accidentallly) and the OFF should be RED and proud of the panel. Can’t you find an old Start/Stop control switch at that Bazzar?

Yes this is critical but we don’t want you to kill yourself or someone else. It’s not hard to o it properly.

Robert.

[Sonic EscapeParticipant @sonicescape38234]

Actually there is a guy who brings every Sunday some Start/Stop switches. He has a few of these brand new. Inside there is an NO and NC contact, among other things I don’t need. I wanted to buy one last Sunday but my buttons make a more satisfying click when pressed 🙂 That buried green button feels like the Enter key from a bad keyboard. But I think I’ll buy it next Sunday. I bought from him also a few emergency stop buttons, also new. He has a lot of Schneider stuff.

I used single color wires to avoid having a false sense of security. Since I plug the circuit into a wall socket the live phase can be anywhere. Better to assume there is no safe place to touch. I’ll use green/yellow for PE, of course.

But double pole circuit breakers are available everywhere here. I’ll replace the single pole ones.

Yes, ideally everything should be enclosed, preferably in a metal enclosure. But I’ll stay with this simpler method. The lowest part of the circuit will be high enough so nobody could touch it by accident. As for EMI … we will see.

[Robert Atkinson 2Participant @robertatkinson2]

Ahh, I forgot you use non-polarised plugs so you need double pole switching.

Robert

[Sonic Escape]

On Sonic Escape Said:

On Robert Atkinson 2 Said:

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Yes, I have a 30mA RCD for the whole house. But I want to try also a 10mA one only for the workshop. I’m aware that this old machines might have some issues with residual currents.

 

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I’m also thinking if not to make another earthing system only for the workshop.

There’s no particular reason I can think of for tolerating an old machine leaking current.  It almost certainly indicates an insulation fault,  worth investigating at least.   Perished rubber, fraying, dirt bridging terminals,  burnt motor windings etc.

Conversely, fitting a VFD, and especially an EMC filter, will legitimately increase leakage current.  Both contain capacitors that short electrical muck to earth, so unwise to install a too low current RCB because it will pop unnecessarily.

Nothing sensible I can say about adding an extra workshop earth other than check local rules.   In the UK, our most common wiring scheme makes adding an extra earth dangerous, because a certain supply side fault might cause a big bang by dumping an enormous current into it.  Older UK systems and I believe many of the grounding systems used abroad, can have extra earth rods added galore, but not all.    I’ve no idea of what’s OK, or not, in Romania.

Dave

[John Doe 2Participant @johndoe2]

Yikes !

That electrical board really must be enclosed in a box, and access to the breaker switches needs to be engineered while preventing fingers from touching the breaker terminals and wires.

As it is, it would be too easy to touch the live wires where they enter the MCBs – the insulation stops and the wire conductor can clearly be seen. You know this and you would not deliberately touch one, but a moment’s inattention, or someone else touching the panel to switch off the power because they found you unresponsive on the workshop floor one day, might not.

You can buy ‘consumer units’ which are purpose built metal boxes fitted with a DIN rail to mount the MCBs and which physically cover all the terminals while allowing access to just the switch of the MCBs. Consumer units also come with proper busbars and kits to connect the MCBs in an approved and safe manner.

Then you would need to put the VFDs in another metal box.

Please re-think your installation. Having a bare open board like that is not safe, even if it is mounted out of reach, because when someone needs to switch it – to rescue you perhaps – they will have to get a step ladder and will then be able to touch bare wires.

 

[John Doe 2]

I continued to read about RCD because I felt that there were some subtleties that escaped me. And I was right! I discovered that there are at least 3 types of RCDs. Here is a nice description of each. In short, for inverters is best to use type A, or better type B. The worst are AC. AC ones are actually banned in Germany since the ’80s when SMPS started to be more common. But in Romania you find them everywhere. And unfortunately I bought an AC type.
The problem seems to be related to bandwidth. AC have such a low bandwidth that the high frequency components of an poorly filtered inverter are phase shifted enough to no longer cancel each other and so they trigger the RCD. There is also an issue with the DC component that I didn’t understand it very well. Good to know.

On John Doe 2 Said:

Yikes !

That electrical board really must be enclosed in a box, and access to the breaker switches needs to be engineered while preventing fingers from touching the breaker terminals and wires.

As it is, it would be too easy to touch the live wires where they enter the MCBs – the insulation stops and the wire conductor can clearly be seen. You know this and you would not deliberately touch one, but a moment’s inattention, or someone else touching the panel to switch off the power because they found you unresponsive on the workshop floor one day, might not.

You can buy ‘consumer units’ which are purpose built metal boxes fitted with a DIN rail to mount the MCBs and which physically cover all the terminals while allowing access to just the switch of the MCBs. Consumer units also come with proper busbars and kits to connect the MCBs in an approved and safe manner.

Then you would need to put the VFDs in another metal box.

Please re-think your installation. Having a bare open board like that is not safe, even if it is mounted out of reach, because when someone needs to switch it – to rescue you perhaps – they will have to get a step ladder and will then be able to touch bare wires.

 

Here a box large enough to fit everything costs almost as much as the inverters. And still I will have to drill holes for ventilation. But you gave me an idea. I can put the circuit breakers, contactors and other small stuff in a box. A smaller one is much cheaper. And leave only the inverters and the 3-phase sockets outside. Anyway their terminals are deep enough so you can touch them. Why I didn’t think on this from the beginning?!

[SillyOldDuffer]

On SillyOldDuffer Said:

In the UK, our most common wiring scheme makes adding an extra earth dangerous, because a certain supply side fault might cause a big bang by dumping an enormous current into it.  Older UK systems and I believe many of the grounding systems used abroad, can have extra earth rods added galore, but not all.    I’ve no idea of what’s OK, or not, in Romania.

Dave

I did some research and in Romania the rule is to make your own earthing and to connect it to N. Then you split this in PE and N and use both in your house. Also according to local regulations the N is grounded at the last transformer and also at every pole. So in theory the benefit would be a low impedance N. In practice, with poor building quality specific to this country you can easily be the only one with a good earthing system in the area. And if N is disconnected by accident for the whole street then everybody will draw current trough your earthing 🙂

[Sonic EscapeParticipant @sonicescape38234]

And one more thing about RCDs. You have to push the test button from time to time. For example this low quality RCD requires a monthly test. Otherwise over time the residual current threshold will increase. But what is worst is that AC type RCDs can be desensitized by a small DC component. That would be really bad.

[SillyOldDuffer]

On SillyOldDuffer Said:

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Safety apart, a machine that stops quickly speeds workflow.  Also braking by heating some big resistors is less wearing than dissipating the energy in the drive train.   Neither of these seem particularly advantageous in a  hobby workshop.

Dave

My Myford ML7 has a very efficient brake. When you release the belt tension lever while the shafts are still spinning, the large countershaft pulley drops down and the rubber belt rubs on the heavy cast belt cover, bringing the whole lot to a rapid halt. Very handy when doing fiddly stop-start jobs.

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