Using a vfd for two machines

[wendy jacksonParticipant @wendyjackson]

[wendy jacksonParticipant @wendyjackson]

Hi chaps. For some reason single to three phase inverters or vfd. Are thin on the ground. I have a mitsubishi vfd in my workshop that’s rated to 1.5kw which is running the lathe. However I had to replace the motor on my milling machine to three phase and this motor is also rated at 1.5kw. So question is there a way that I can run both machines ( not at the same time ) from the single vfd. I don’t however wish to connect and disconnect cables each time. Would some sort of electrical switch work. What are your views.

[Harry WilkesParticipant @harrywilkes58467]

A pair of electrically/mechanically interlocked contactors comes to mind other ways could be a rotorary switch, vfd feeds a 4pin socket into which you could plug machines into as and when. If it was me I would go with the contactors and interlock these with the vfd to make sure the load could not be disconnected whilst running.

H

[Tony RayParticipant @tonyray65007]

Hi Terry,

It could be done but I would challenge your statement regarding the availability of 230V input VFD’s unless you are talking about secondhand units. I buy mine from inverterdrive.com no connection just a satisfied customer they are showing over 130 models currently.

[wendy jacksonParticipant @wendyjackson]

Hi Tony, I would like to use a mitsubishi brand as I have had one for years and never let me down. Apart from the expensive ones they are all on back order. I just don’t want to go through the nightmare of the Chinese ones again with no back up.

[EmgeeParticipant @emgee]

A suitably rated 3 pole + aux contacts changeover switch with centre off would do what you want but you must always isolate the power to the VFD before changing the switch to the 2nd machine.
I would suggest you wire the units direct without using  plugs and sockets.
The aux contacts in the switch would have to suit the STOP circuit requirements (NO or NC) of the VFD so with the switch in an OFF position the stop is activated.

It is not recommended to fit any switching device between the VFD and motor because of possible damage to the VFD if switched off with a running motor but with careful use as above no damage should occur.

With regard settings in the unit you will have to accept common settings for both machines unless of course you re-program certain features at every change-over.

Emgee

Edited By Emgee on 20/02/2021 16:02:29

[wendy jacksonParticipant @wendyjackson]

Thanks for the information chaps.

[Clive FosterParticipant @clivefoster55965]

If you share a VFD with two motors you won't be able to run a self tuning vector drive VFD. Which is a pity because such drives work much better than old style Voltage/Frequency types. Torque holds up much better as speed falls.

There is an intermediate type of vector drive where you simply program in the size, current(?) and rpm of the motor which could be managed if your two motors are the same. Not as good as self tuning but better than simple V/F.

Clive

[SillyOldDufferModerator @sillyoldduffer]

I wouldn't recommend it but something like this might do.

I notice switches sold for 3-phase changeover all come with 'Authorised Personnel Only' guard features like locked doors and padlocks. I suppose absently mindedly switching over a live 3-phase service could be far more exciting than the operator expected! In Terry's case likely to damage the VFD and maybe spiking the motors too, so don't get it wrong!.

I doubt one would be be bought by accident because they're so expensive, but the automatic 3-phase changeover switches are the wrong way round for this purpose. Designed for failover applications, they switch one load between two sources, whereas Terry wants to connect one source to either of two loads.

Saving a bob or two is usually a good thing, but maybe sharing a VFD intended to power a single motor is a risky economy? Some of the better VFD's learn and adapt to whatever motor is connected, and switching one of them to a different motor might cause it to think it had detected a fault. But the real danger is the obvious one – too easy for the operator to switch over before the circuit is completely dead, i.e with both motors stopped and the VFD turned off. The money spent on the changeover switch and wiring might be better invested in another VFD, and imposing a switch means the lathe and mill can't be left idling ready for jobs to be swapped between them. (I do this occasionally.)

I apologise to Terry for spending his money so enthusiastically for reasons that may not apply in his workshop! And how much dosh is available makes a difference too…

Dave

[SteviegtrParticipant @steviegtr]

I don't think the change over switch is a bad idea at all. As long as you expect the ramp up / down times. Min freq. Max freq etc etc can be the same then all is well.

Surely you would not expect the average operator to switch over while the inverter was running on the other machine.

If you thought it could happen then simply use the aux contacts to break the stop circuit. So then it would be idiot proof. Not that there are any on this forum anyway.

If both motors are the same hp & the same poles say 2 or 4, then all should be well.

Steve.

[Clive FosterParticipant @clivefoster55965]

There are dual interlocked contactor sets intended for forward-reverse motor control which will not switch from one setting to the other without going through the stopped position first.

The one I have floating around to (eventually) use on a garage door opener cost about £25. It has external links to do the pole switching needed between forward and reverse connections so there seems no reason why it could not be configured to control a pair of motors rather than forward-reverse on one.

Clive

[Tony RayParticipant @tonyray65007]

I would challenge the statement that 230V input vfds are hard to find unless we are talking about used units. I buy mine new from inverter drive.com No affiliation just a satisfied customer a quick search on their site just now showed 130 odd models in 1.5 kW.

[Martin CargillParticipant @martincargill50290]

All very good but have you considered how you will control the machines? The existing switchgear cant be utilised easily as it will have to interface with the VFD and it won't be swappable between the two machines.

I have a VFD in my workshop that can power various machines. However to do this I have the motors disconnected from the original control gear and each one is fitted with a cable fitted with a 3 phase plug. I then have a control pod with start, stop, emergency stop, speed control and reversing switch that has to be taken to each machine (when it's in use) and this has its own cable.

Its quite messy with having two cables to the machine in use. In addition you have to work out a safe method for holding the control pod so that it's accessible when the machine is in use (and you have to remember that the existing controls don't work!!!! unless you have removed them)

You have to watch to make sure that nobody ever plugs in your 3 phase socket into normal 3 phase mains as your motor will be wired for 240 volts 3 phase and not 415 volts 3 phase, and of course there will be no switchgear feeding the motor…….

Martin

[David JuppParticipant @davidjupp51506]

Potentially, with an extra pole on the changeover switch you can tell the VFD which set of motor parameters to use to match the selected motor. Many VFD's can store at least 2 sets of parameters…

[hushParticipant @hush]

My Boxford lathe is three phase, and is driven by a single phase to three phase box. My miller was also three phase, and was driven from the same box. Fed up with swapping the plugs over. I called the makers of the box to ask about fitting an extra socket on the front panel, this idea was OK'd provided the job was done by a qualified "sparks". This work was done and both machines were connected. It worked very well as I was the only person using the machines.

[old martParticipant @oldmart]

It can be done, but as already advised, the VFD must be isolated from the mains power before switching from one to the other. VFD's do not like switches between the VFD and the motor and if the VFD is off when the switchover is made it will not be adversly affected. The cables between the VFD, the switching and the motors should be screened.

[Robert Atkinson 2Participant @robertatkinson2]

One thing that has not been mentioned is Start Stop controls. You really need to to switch at least the start switch (button) with the motor. You don't want to push the start on the mill and have the lathe start turning!
The Stop and E-Stop circuits for both machines can be wired in series (parallel if normally open) to the VFD so pushing stop on either machine will stop whichever is selected.

EMGEE sugessted a switch with auxillary contacts for the Stop circuit but this educes safety and does not address the start issue.

Personally I'd use a pair of electrically interlocked contactors and a 3 position (Lathe OFF MILL) rotary switch. An indicator light on each machine would be an added safety feature to let you know which is live .

Robert G8RPI.

[ZanParticipant @zan]

StrangeG double post without the edit! See below

Edited By Zan on 21/02/2021 00:04:32

[ZanParticipant @zan]

V d f thin on the ground? There’s thousands for sale Google it. Not worth the risk of disconnecting whithout. Switching the mains off and destroying the vdf Get one for each machine. I have one on each of my 5 machines they all have different perimeters set to suit the individual requirements. They are cheap( ish) compared with the uncertainty of remembering to change the settings n plugeps. What happens when one machine is running very slowly and you want to run another at the same time?

[EmgeeParticipant @emgee]

Posted by Robert Atkinson 2 on 20/02/2021 21:02:19:

One thing that has not been mentioned is Start Stop controls. You really need to to switch at least the start switch (button) with the motor. You don't want to push the start on the mill and have the lathe start turning!
The Stop and E-Stop circuits for both machines can be wired in series (parallel if normally open) to the VFD so pushing stop on either machine will stop whichever is selected.

EMGEE sugessted a switch with auxillary contacts for the Stop circuit but this educes safety and does not address the start issue.

Personally I'd use a pair of electrically interlocked contactors and a 3 position (Lathe OFF MILL) rotary switch. An indicator light on each machine would be an added safety feature to let you know which is live .Robert G8RPI.

I did not mention any starting arrangement as that question wasn't asked, however it would be sensible to have a portable pendant with start/stop/speed pot that could be moved between machines and fitted into a cradle provided on each machine as required.

Auxillary contacts are early break and late make which would ensure if the VFD was running the Stop circuit would be activated before the switch contacts opened, I fail to see how that reduces safety, it is no more than operating the Stop button on the VFD or Pendant control.

Nothing is gained by the additional use of mechanically and electrically interlocked contactors, it just adds complexity and cost to job in hand. With 1 supply into a changeover switch you can only provide power to 1 of the connected motors at any time.

The position of the pendant and the labelling on the changeover switch should be sufficient for Terry to know which machine the VFD is connected to without an indicator light.

Emgee

[not done it yetParticipant @notdoneityet]

The aggravation/cost of fitting all these interlocks or the risk of messing up a perfectly good VFD just does not seem worth it.

I would suggest buying a cheap VFD for the second drive. By all means replace with a mitsu’ when attainable and either keep the cheapie as a spare – or sell it on at less of a loss than messing up an expensive VFD…

[Neil LickfoldParticipant @neillickfold44316]

I was going to look at running two machines off the 1 VFD unit and have a switch over switch. So at any one time, only the one with the switch would be running. After looking into it, gave up and just bought another VFD from China. They are the same brand and have been extremely reliable so far. It is a generic drive and works very well. Mine is are Rich electric from Taiwan but got it through an Aliexpress trader. The 1st one I bought local with the 3phase motor and the vendor set all the parameters for me. That's the one I use on my Myford S7.

[Clive FosterParticipant @clivefoster55965]

If you use an interlocked contactor pair as I suggested earlier most of the control problems go away and everything can be hard wired in with each machine having its own pendant connected to the VFD via relays powered via the interlocked contactors.

Basic idea is that the VFD is turned on via a suitable switch and the appropriate button pushed so that the interlocked contactors connect the output to the appropriate machine. The auxiliary contacts on the activated contactor provide power to close the contacts on a multipole relay connecting the pendant controls on that machine to the VFD. A second relay does duty for the other machine. Hence its impossible to drive the wrong machine. The actual wiring is little more complicated than setting up a pair of VFD boxes. Basically just add some Y splits in the lines.

An advisable refinement is to add an extra relay, controlled via the power on / off buttons on the pendant, in the line to the actuating coils on each of the interlocked contactors ensuring that it cannot be turned off whilst a pendant is live and possibly driving a machine.

Clive

[not done it yetParticipant @notdoneityet]

Question: What would you do IF you then got a third machine with a similar motor? Would you then run three from one VFD?

I think probably not. If that would be the case, why mes about with just one VFD at the moment?

[SillyOldDufferModerator @sillyoldduffer]

Posted by Steviegtr on 20/02/2021 16:55:22:

…

Surely you would not expect the average operator to switch over while the inverter was running on the other machine.

If you thought it could happen then simply use the aux contacts to break the stop circuit. So then it would be idiot proof. …

As human error is the main cause of accidents, I certainly wouldn't trust Mr Average let alone myself! Even highly trained operators like Airline Pilots make amazing mistakes, not because they're stupid, but because something out of the ordinary destroys their situational awareness for just long enough to cause a cock-up.

Though it would reduce the risk, I don't think auxiliary contacts would save the day. 3-phase from a generator is three smooth 50Hz sine waves, changing rather slowly, and well-behaved. A VFD is different: it simulates 3-phase by pulse-width modulating three DC outputs. On an oscilloscope, it's obvious a VFD delivers a hideously spiky wave-form compared with a generator. Normally, the spikes don't matter because the motor's inductive windings choke out the pulses. But, if a winding is disconnected before the others, as when one of three contacts opens a millisecond before the others, then strange things happen. In particular, the floating winding is likely to react to pulse inputs as if it were a car ignition coil, generating spectacularly high voltages.

As the magnitude of the unwanted voltage is related to pulse rise time it can be hundreds of times bigger than when ordinary 3-phase is switched. Easily capable of rising high enough to puncture the motor's insulation or to jump switch contacts. In the worst case the spark creates a low resistance ionised path through the air allowing ordinary low voltage to form a power arc inside the switch or motor.

Although there's potential for spectacular failure it depends on the timing. How high the voltage goes depends on when the contact opens relative to the VFD's pulse waveform. There's an element of luck in it ranging from 'no problem' to 'electrical fire'. More likely than a catastrophe, the volts would spike back the other way into the VFD, and wreck the electronics. It's really not a good idea to switch a live VFD.

It is possible to buy VFDs specifically designed to allow hot switching. Last time I looked, they were all big beasts well beyond anything Joe Public might buy. But who knows, there might be a small version available now – there's certainly demand for them. Off hand, I can't think of a way of making one cheaply: cleaning up spiky power waveforms needs a decent filter and hefty inductors aren't cheap!

Dave

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