Thread: 415v three phase from 240v single phase a home made digital phase converter?

I don't have three phase at my place and getting it would be expensive. With that in mind I set up an experiment that has been a few months in the making (I had to wait for Kryn to bring me a crucial part from Melbourne). The picture shows a 240V VFD (only 1.5kw here) connected to an 8Kva three phase autotransformer.
IMG_20180831_151936.jpg
The autotransformer was originally designed to take three phase 415v and output 230v three phase but I run it in reverse. Setting the VFD to ramp up to 50hz at 240v over a few seconds avoids an over current situation when switching the thing on . I connected my unmodified Colchester Bantam to the output of the transformer and was happy to see it run with direct online starting without any issues. As expected the inductance of the transformer avoids any over current situation at the VFD when the motor starts.

As mentioned this was using only a 1.5kw VFD I intend getting something around the 4kw mark so as to more fully utilise the capacity of the transformer any suggestions here would be welcome.

The voltage out of the transformer even in reverse is around 400v so close enough.

Anybody see any problems using this as a phase converter?

Cool. Interesting concept.
So the VFD is already running (at 50hz) when you switch on the motor?

For interest, have a look at Dynoguy on YouTube, l think he does a similar setup
https://youtu.be/UA9-jXlw4r4

Yes it is running at 50hz before the motor gets turned on. It has to run at 50hz so that the output is 400v so that the contactors in the lathe will work as normal. The plan is not to have to modify machines before they can be used.

Looks like a nice idea to me. I've seen those 3 phase step-down transformers around either cheap or free and it looks like it will give you the advantages of a phase converter that you just just plug 3 phase 400v gear into without the issues of a rotary converter.

Steve

Originally Posted by Petip

For interest, have a look at Dynoguy on YouTube, l think he does a similar setup
https://youtu.be/UA9-jXlw4r4

He puts the transformer before the VFD. I put the transformer after the VFD for two reasons. I have a machine that needs a neutral which I can get from a three phase autotransformer connected in Y. The other reason is the dampening effect the transformer has on the noise from contactors.

Hi,

If you vary the VFD frequency a little, you should be able to find the sweet spot for the transformer output.

Originally Posted by BaronJ

Hi,

If you vary the VFD frequency a little, you should be able to find the sweet spot for the transformer output.

Hi BaronJ,

Sweet spot in what way?

Hi,

If you measure the output voltage, you will find that there is a point where the voltage increases or falls with changing frequency. The sweet spot for that transformer will be where the voltage peaks. The better the quality of the transformer the shallower the voltage peak will be.

So for instance if at 50hz you get 400 volts and at 60hz you get 405 volts, but at 70hz you get 395 volts, then 60hz is closest to the sweet spot. The reverse is the same. If the voltage rises at 40hz then the transformer favours the lower frequency.

You may not see much of a change in which case you have a good transformer. Do it under load as well the change will be more marked. The difficulty making measurements under load using the motor is that the load will change slightly with motor speed.

Let me know what you find.

Cool idea but I'd like to see what happens when the motor is switched on or off under load. OK this is is unlikely to happen except in an emergency but I don't think the VFD will like it much as the transformer may not be able to protect the VFD.

The way to get around this would be to wire emergency switches to turn off the VFD and I do mean cut the mains to the VFD.

Are you using any of the other features of the VFD?, eg speed and braking controls?

I can see it being useful on 415V delta machines with built in motors.

As mentioned this was using only a 1.5kw VFD I intend getting something around the 4kw mark so as to more fully utilise the capacity of the transformer any suggestions here would be welcome.

Does this mean you intend running more than one machine from the VFD? I would be be interested to see how that goes.

I like Baron J's idea of tweaking the frequency to improve the transformer output but bear in mind that using V'F mode VFDs motors develop less power below 50Hz so there m ay not be any gain. I'm not sure if vector drive characteristics will transfer though a transformer. BTW Vector drives have to be tuned to a specific motor so this feature can't be used in a multi motor situation anyway.

I have thought of this setup in the past. I assume the VFD supplies 3 phases at 240 volts with no neutral? If I have that wrong please correct me.

Your created "neutral" has no relationship to earth. A consequence is that any out of balance loading may cause a voltage rise on your "neutral". The VFD complicates things a bit because I don't know enough to know how it will affect the circuit, however that rise in voltage on your neutral may translate to a voltage between your "neutral" and any structure that is bonded to the general mass of the earth e.g. the structural steel of your shed or an earthed machine frame. Thus there is a risk of shocks and tingles. How much the voltage on the "neutral" could rise is determined by the magnitude of the out of balance load and the circuit impedance.

A second issues arises if you are intending to use this to create a 415v 3 phase supply to several machines. How will you protect the individual circuits or will you rely on the VFD. The VFD can't see a fault to "neutral/earth" as anything other than an overload for faults past the autotransformer. Thus if you put yourself between a phase and neutral the result is likely a rather grim llama. I would check the fault loop impedance, after checking that your fault loop impedance meter will not place damaging loads onto the VFD in the process of testing.

The transformer inductive impedance is a two edged sword. If load suddenly disappears from the secondary side of a transformer while the core is energised, the collapsing magnetic field will very likely induce high voltage spikes, not at all good for your VFD.

I am actually very keen to hear how you go with this. I'm in a similar situation with the cost of 3 phase supply and this is an option I have been looking at.

Cheers

The Beryl Bloke

Originally Posted by Theberylbloke

I have thought of this setup in the past. I assume the VFD supplies 3 phases at 240 volts with no neutral? If I have that wrong please correct me.

Correct.

The transformer inductive impedance is a two edged sword. If load suddenly disappears from the secondary side of a transformer while the core is energised, the collapsing magnetic field will very likely induce high voltage spikes, not at all good for your VFD.

That's what I was referring to in an emergency situation. Now I'm not sure if even turning the VFD off at the mains is a good idea as an emergency either.

Any reason why you couldn't tie the neutral from the Y to the mains neutral?

Originally Posted by BobL

OK this is is unlikely to happen except in an emergency but I don't think the VFD will like it much as the transformer may not be able to protect the VFD.

I am thinking of adding some circuitry to get around this possibility.

Originally Posted by BobL

Are you using any of the other features of the VFD?, eg speed and braking controls?

No just simple V/F mode with acceleration at startup at a rate that stops the initial inrush from causing an over current fault.

Originally Posted by BobL

I can see it being useful on 415V delta machines with built in motors.

I intend using it on a geared head drill with two speed motor and on my colchester bantam which also has a two speed motor.

Originally Posted by BobL

Does this mean you intend running more than one machine from the VFD? I would be be interested to see how that goes.

I certainly do and like you I am interested to see what happens.

I am currently working on a low pass filter between the VFD and the autotransformer.

Originally Posted by Theberylbloke

I have thought of this setup in the past. I assume the VFD supplies 3 phases at 240 volts with no neutral? If I have that wrong please correct me.

Correct.

Originally Posted by Theberylbloke

Your created "neutral" has no relationship to earth. A consequence is that any out of balance loading may cause a voltage rise on your "neutral". The VFD complicates things a bit because I don't know enough to know how it will affect the circuit, however that rise in voltage on your neutral may translate to a voltage between your "neutral" and any structure that is bonded to the general mass of the earth e.g. the structural steel of your shed or an earthed machine frame. Thus there is a risk of shocks and tingles. How much the voltage on the "neutral" could rise is determined by the magnitude of the out of balance load and the circuit impedance.

Again correct.

Originally Posted by Theberylbloke

A second issues arises if you are intending to use this to create a 415v 3 phase supply to several machines. How will you protect the individual circuits or will you rely on the VFD. The VFD can't see a fault to "neutral/earth" as anything other than an overload for faults past the autotransformer.

I will probably use an RCD downstream of the transformer.

Originally Posted by Theberylbloke

Thus if you put yourself between a phase and neutral the result is likely a rather grim llama. I would check the fault loop impedance, after checking that your fault loop impedance meter will not place damaging loads onto the VFD in the process of testing.

Not the only way of becoming grim during this experiment. But I do thank you for your input. I have thought about the issue of voltage between earth and neutral and if I can't resolve it safely then I will simply do away with the feature and modify the one machine I have that utilises it.

Originally Posted by Theberylbloke

The transformer inductive impedance is a two edged sword. If load suddenly disappears from the secondary side of a transformer while the core is energised, the collapsing magnetic field will very likely induce high voltage spikes, not at all good for your VFD.

The VFD should have back emf protection but I am not against using some judicially placed metal oxide varistors.

Originally Posted by Theberylbloke

I am actually very keen to hear how you go with this. I'm in a similar situation with the cost of 3 phase supply and this is an option I have been looking at.

Thank you for the input it is good to have ideas kicked around a bit.