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Harvesting energy from 3-phase

A customer has a three-phase supply drawing from 50A to 100A at nominally 60Hz. He wants to tap some power from this without any electrical contact, so probably using say three current transformers with each phase going through one toroid. Probably the wire is too thick to make multiple turns.

The power needed will have to end up as a 30V 300mA supply, so 10 watts or so which is a lot for a normal CT.

This is not something I've looked at before and sounds quite difficult - a quick Google shows that this can be done for very low power devices.

Any ideas? Multiple toroids with secondaries in parallel? Long sausage toroid? The energy has to come from the wire, obviously, so the impedance of the wire will have to increase a little which implies that some length will be needed, but now I'm rambling.

Is the following even in the same county as the ballpark?..

30V @ 300mA is 100R 50A -> 300mA is a turns ratio of 1:167. Inductance goes with the square of the turns, so the wire must see 3.6mR 3.6mR at 60Hz is 10uH 10uH/N^2 cores are doable, maybe multiple 3 watts per core isn't too big

Cheers

--
Clive
Reply to
Clive Arthur
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On a sunny day (Thu, 8 Aug 2019 09:55:47 +0100) it happened Clive Arthur wrote in :

OK, 50 A, to get 300 mA (AC) you need 50 / .3 = 167 turns secondary, to get 30V @ 300 mA your load needs to be 100 Ohm. What is the problem?

You will need a decent iron core current transformer.

If you use all 3 phases 1/3 per phase, 3 phase rectifier bridge makes things simpler and transformers smaller?

Clamp on cores?

Reply to
Jan Panteltje

If this is a one off project, locate some current transformers rated for 100 A. Standard current transformers have either 1 A or 5 A nominal secondary current,

Then use some ordinary voltage up-transformers to lift the voltage to

20-30V before rectification. Depending of the connection (wye/delta) select the connection for correct voltage.

If all three phases are used, use three 5 or 10 VA current transformers. If the voltage transformers do not load the current transformer sufficiently for 1 or 5 A secondary current, you can always use some burden resistors in parallel with voltage up-transformer primary.

Reply to
upsidedown

Do you need the second transformer to increase the voltage? Wouldn't the current transformer output voltage rise to a sufficient level if the burden resistance is higher than the specified value. A bridge rectifier and big zener or shunt regulator should work.

John

Reply to
jrwalliker

You could buy a standard 100:5 CT, and then drive a standard transformer used as a step-up, then a bridge rectifier and shunt regulator.

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics
Reply to
John Larkin

Well, consider the cases where the motor is off -- no output. And, when the motor starts, you get maybe 6X the current for several seconds. Your circuit needs to handle this, as the output of the current transformer will be 6X normal running output. Your current transformer will be outputting 300 mA X 6 = 1.8 Amps!

Why not just power the device from an ordinary single-phase transformer from a convenient outlet?

Jon

Reply to
Jon Elson

.

Hoping I do not miss it, but:

One wire of the mains, with a solenoid winding would around it. You can get whatever current you like, in this case 300mA. But the voltage developed a cross the single wire will be extremely small, even if you have 167 ratio i t will not be 30V. To have the power you are talking about, you need to bre ak the loop

Cheers

Klaus

Reply to
klaus.kragelund

k

es.

e

R

et whatever current you like, in this case 300mA. But the voltage developed across the single wire will be extremely small, even if you have 167 ratio it will not be 30V. To have the power you are talking about, you need to b reak the loop

To develop a voltage across the magnetising inductance, you need a very big inductance, enough that you develop voltage a 50Hz. So the ferrite you nee d to use for a 30W transformer would be very very big

Cheers

Klaus

Reply to
klaus.kragelund

No, a current transformer is not a solenoid around the mains wire, it is a toroid magnetic core around the mains wire, with a multi-turn winding around the toroid.

You can design it so it will saturate during motor starting, so you don't have to dissipate a great excess amount of energy during the start.

For a 50/60 Hz motor, one would not use a ferrite, but a laminated iron core of some design.

Jon

Reply to
Jon Elson

clandestine power abstration? I'm guessing one could light a house & listen to the radio on 10w :)

I assume it's more a rules & regs thing, or letting it be installed by a non-sparky.

NT

Reply to
tabbypurr

While a starting induction motor can take up to 6 times nominal current, you are not allowed a start a 40-80 kW motor by directly power it on. You need a soft starter or a classical wye/delta starter (with a short break when going from we to delta).

Reply to
upsidedown

[intended for 10W of output power]

Why? You can (six diodes) rectify the three-phase, and filter it, and make a switchmode isolated power supply, where does the 'electrical contact' become a problem?

A 'current transformer' gives very low voltages, into a short circuit, it's a terrible choice of power-delivery component. It's useful for METERING, where power transfer (and impedance matching) is not required.

Reply to
whit3rd

There are current transformers from 1.5 VA to 60 VA and standard secondary currents of 1 A and 5 A. Thus the 1.5 VA version could deliver 1.5 V resp. 300 mV. The 60 VA models could deliver 60 V resp.

12 V.

The high power and hence high voltage is useful if the distance between the current transformer to the actual measurement device is large due to the voltage losses due to the wiring.

Anyway sturdy wires must be used to handle any short current peaks due to short circuit in the high power line. Long wires are typically encountered when th CT is outside and the instrument is indoors.

Reply to
upsidedown

yeah get three current transformers and three bridge rectifiers connect the three DC supplies made by that in series and feed that to a high-power zener, or to some sort of current-fed switcher, to get your target voltage. (or target current whichever one he really needs)

--
  When I tried casting out nines I made a hash of it.
Reply to
Jasen Betts

Power off doesn't matter, and the max current is limited to 100A.

Thanks for all the input, this is probably easier than I'd thought albeit completely outside my experience.

[The nearest outlet isn't convenient, being possibly a few miles away at the top of a very deep hole!]

Cheers

--
Clive
Reply to
Clive Arthur

Toroid or solenoid, I do not care, it's a transformer

Please tell me how you will get 10W out, with out a VERY big transformer?

The voltage on the primary will be arbitrarily low....

Regards

Klaus

Reply to
klaus.kragelund

Please note that the short circuit current (and hence CT secondary current) can be much larger, so hopefully your DC load survives this.

On Europe in mines often 400 V delta only (or 690 or even 1000 V) cabling is used. To power low power devices and providing lighting at each site a 400/230 V transformer primacy is connected between two phases and the cold side of the secondary is grounded to a local ground electrode. Control system voltages (e.g. 24 Vdc) are then derived from the local 230 V network using standard components..

Apparently this is in the Americas (60 Hz). If the power feed is several kilometers away, this has to be a medium voltage (MV) motor (with 4 kV ? delta only). Thus a small transformer with 4 kV primary and 120 V secondary would be needed. If this is an underwater site, I can understand why galvanic connection to the MV lines are avoided.

But anyway, the cable outer shield of 3 phase cble must be broken, so that only a single phase wire is running through the CT (or 3 phase wires through 3 separate CTs). There is no point of running a complete

3 phase cable through a single CT, since the phase currents and hence magnetic fields will cancel.
Reply to
upsidedown

Not if you're using a shunt regulator!

Tim

--
Seven Transistor Labs, LLC 
Electrical Engineering Consultation and Design 
Website: https://www.seventransistorlabs.com/
Reply to
Tim Williams

How about this (just thinking out loud)...

Wind a solenoid on a bundle of insulated iron wires, like a Rogowski coil but with a 'laminated' core, and wrap this around the current carrying conductor, maybe several turns.

One for each phase, of course.

Cheers

--
Clive
Reply to
Clive Arthur

On a sunny day (Fri, 9 Aug 2019 12:24:32 +0100) it happened Clive Arthur wrote in :

Interesting, had to look that up

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Or perhaps UI cores, coil on U leg and shift across current carrying inductor, then close with the I part.

Personally I would just cut a mains transformer ring core in 2 halves. Has to be real iron, no ferrites. Winding on one side.

?
Reply to
Jan Panteltje

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