By continuing to use this site, you agree to our use of cookies. Find out more
Forum sponsored by:
Forum sponsored by Forum House Ad Zone

Using a vfd for two machines

All Topics | Latest Posts

Search for:  in Thread Title in  
Neil Lickfold21/02/2021 09:18:18
858 forum posts
195 photos

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 Foster21/02/2021 10:07:31
3135 forum posts
109 photos

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 yet21/02/2021 10:32:26
6806 forum posts
20 photos

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?

SillyOldDuffer21/02/2021 12:02:06
Moderator
8664 forum posts
1960 photos
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

SillyOldDuffer21/02/2021 12:03:18
Moderator
8664 forum posts
1960 photos
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

All Topics | Latest Posts

Please login to post a reply.

Magazine Locator

Want the latest issue of Model Engineer or Model Engineers' Workshop? Use our magazine locator links to find your nearest stockist!

Find Model Engineer & Model Engineers' Workshop

Latest Forum Posts
Support Our Partners
Dreweatts
cowells
Rapid RC
Eccentric Engineering
Eccentric July 5 2018
Subscription Offer

Latest "For Sale" Ads
Latest "Wanted" Ads
Get In Touch!

Do you want to contact the Model Engineer and Model Engineers' Workshop team?

You can contact us by phone, mail or email about the magazines including becoming a contributor, submitting reader's letters or making queries about articles. You can also get in touch about this website, advertising or other general issues.

Click THIS LINK for full contact details.

For subscription issues please see THIS LINK.

Digital Back Issues

Social Media online

'Like' us on Facebook
Follow us on Facebook

Follow us on Twitter
 Twitter Logo

Pin us on Pinterest