Forums

Rogowski transformer query

Started by Clive Arthur March 8, 2017
If I pass a single wire through a wound air-cored toroid with, say, 1000 
turns, then that's a 1000:1 transformer.

If, for convenience, I make it a 1000 turn Rogowski coil and wrap the 
coil around the single wire, then that's the same.

What happens If I wrap the 1000 turn Rogowski coil around the wire 
twice?  My feeling is it that it could be considered as two 500 turn 
coils in series, or perhaps as 1000 turns in two layers, so no difference.

Is that right? I'm not 100% sure, so confirmation welcome.

Cheers
-- 
Clive
>If I pass a single wire through a wound air-cored toroid with, say, 1000 >turns, then that's a 1000:1 transformer.
Not if there's no ferromagnetic core in the toroid, it isn't. The coupling coefficient in that geometry wil be way less than 1. With no core, there's a strong tendency for the contributions of the inner and outer sides of the toroid to cancel each other. Cheers Phil Hobbs
On 08/03/2017 12:47, pcdhobbs@gmail.com wrote:
>> If I pass a single wire through a wound air-cored toroid with, say, 1000 >> turns, then that's a 1000:1 transformer. > > Not if there's no ferromagnetic core in the toroid, it isn't. The coupling coefficient in that geometry wil be way less than 1. > > With no core, there's a strong tendency for the contributions of the inner and outer sides of the toroid to cancel each other. > > Cheers > > Phil Hobbs >
OK, point taken, thanks. So, not a Rogowski coil, but a long 1000 turn solenoid wound on some flexible (or hinged) core material up to the end and back so the two connecting wires are at the same end. Bend it into a circle to form a toroid around a single wire and you have a transformer with a 1000:1 turns ratio. What if you bend it into two turns of a circle with a single wire through it? Or three? Does it matter, as far as first-order effects are concerned? Cheers -- Clive
On Wed, 8 Mar 2017 16:29:45 +0000, Clive Arthur
<cliveta@nowaytoday.co.uk> wrote:

>On 08/03/2017 12:47, pcdhobbs@gmail.com wrote: >>> If I pass a single wire through a wound air-cored toroid with, say, 1000 >>> turns, then that's a 1000:1 transformer. >> >> Not if there's no ferromagnetic core in the toroid, it isn't. The coupling coefficient in that geometry wil be way less than 1. >> >> With no core, there's a strong tendency for the contributions of the inner and outer sides of the toroid to cancel each other. >> >> Cheers >> >> Phil Hobbs >> > >OK, point taken, thanks. > >So, not a Rogowski coil, but a long 1000 turn solenoid wound on some >flexible (or hinged) core material up to the end and back so the two >connecting wires are at the same end. > >Bend it into a circle to form a toroid around a single wire and you have >a transformer with a 1000:1 turns ratio.
It certainly has a 1000:1 turns ratio. But it doesn't act much like a 1000:1 transformer. -- John Larkin Highland Technology, Inc lunatic fringe electronics
On Wednesday, March 8, 2017 at 11:29:46 AM UTC-5, Clive Arthur wrote:
> On 08/03/2017 12:47, pcdhobbs@gmail.com wrote: > >> If I pass a single wire through a wound air-cored toroid with, say, 1000 > >> turns, then that's a 1000:1 transformer. > > > > Not if there's no ferromagnetic core in the toroid, it isn't. The coupling coefficient in that geometry wil be way less than 1. > > > > With no core, there's a strong tendency for the contributions of the inner and outer sides of the toroid to cancel each other. > > > > Cheers > > > > Phil Hobbs > > > > OK, point taken, thanks. > > So, not a Rogowski coil, but a long 1000 turn solenoid wound on some > flexible (or hinged) core material up to the end and back so the two > connecting wires are at the same end.
If the core material is not magnetic, then there is not much coupling from the turns on the end to the single turn (presumed) in the middle. Is there some reason not to use a ferrite? George H.
> > Bend it into a circle to form a toroid around a single wire and you have > a transformer with a 1000:1 turns ratio. What if you bend it into two > turns of a circle with a single wire through it? Or three? Does it > matter, as far as first-order effects are concerned? > > Cheers > -- > Clive
On 08/03/2017 17:17, George Herold wrote:
> On Wednesday, March 8, 2017 at 11:29:46 AM UTC-5, Clive Arthur wrote: >> On 08/03/2017 12:47, pcdhobbs@gmail.com wrote: >>>> If I pass a single wire through a wound air-cored toroid with, say, 1000 >>>> turns, then that's a 1000:1 transformer. >>> >>> Not if there's no ferromagnetic core in the toroid, it isn't. The coupling coefficient in that geometry wil be way less than 1. >>> >>> With no core, there's a strong tendency for the contributions of the inner and outer sides of the toroid to cancel each other. >>> >>> Cheers >>> >>> Phil Hobbs >>> >> >> OK, point taken, thanks. >> >> So, not a Rogowski coil, but a long 1000 turn solenoid wound on some >> flexible (or hinged) core material up to the end and back so the two >> connecting wires are at the same end. > If the core material is not magnetic, then there is not much coupling > from the turns on the end to the single turn (presumed) in the middle. > > Is there some reason not to use a ferrite?
No, I'll use ferrite, I had thought I could use air, but I was wrong.
> George H. >> >> Bend it into a circle to form a toroid around a single wire and you have >> a transformer with a 1000:1 turns ratio. What if you bend it into two >> turns of a circle with a single wire through it? Or three? Does it >> matter, as far as first-order effects are concerned?
What of the question about how the number of wraps of the solenoid would affect the turns ratio? Cheers -- Clive
On 08/03/2017 16:49, John Larkin wrote:
> On Wed, 8 Mar 2017 16:29:45 +0000, Clive Arthur > <cliveta@nowaytoday.co.uk> wrote: > >> On 08/03/2017 12:47, pcdhobbs@gmail.com wrote: >>>> If I pass a single wire through a wound air-cored toroid with, say, 1000 >>>> turns, then that's a 1000:1 transformer. >>> >>> Not if there's no ferromagnetic core in the toroid, it isn't. The coupling coefficient in that geometry wil be way less than 1. >>> >>> With no core, there's a strong tendency for the contributions of the inner and outer sides of the toroid to cancel each other. >>> >>> Cheers >>> >>> Phil Hobbs >>> >> >> OK, point taken, thanks. >> >> So, not a Rogowski coil, but a long 1000 turn solenoid wound on some >> flexible (or hinged) core material up to the end and back so the two >> connecting wires are at the same end. >> >> Bend it into a circle to form a toroid around a single wire and you have >> a transformer with a 1000:1 turns ratio. > > It certainly has a 1000:1 turns ratio. But it doesn't act much like a > 1000:1 transformer.
OK, pretend I have a flexible ferrite core. What difference is there between bending my solenoid into a ring (toroid) and passing a wire through as opposed to bending my solenoid into a double (or triple) ring and passing a wire through? I think none, to a first order, but need a sanity check. Cheers -- Clive
Clive Arthur wrote:
> If I pass a single wire through a wound air-cored toroid with, say, 1000 > turns, then that's a 1000:1 transformer. > > If, for convenience, I make it a 1000 turn Rogowski coil and wrap the > coil around the single wire, then that's the same. > > What happens If I wrap the 1000 turn Rogowski coil around the wire > twice? My feeling is it that it could be considered as two 500 turn > coils in series, or perhaps as 1000 turns in two layers, so no difference. > > Is that right? I'm not 100% sure, so confirmation welcome. > > Cheers
Clive, The result is correct, but the reasoning is not quite right. The output voltage of a Rogowski coil is not a direct function of turns ratio. Instead, the output is proportional to the total number of turns in each major loop around the current-carrying wire, the area of each turn, and the di/dt of the central conductor. So, if your Rogowski coil has a total of N turns, a single loop of N turns around the central wire develops the same output voltage as two loops, where each loop has N/2 turns. Bert -- Bert Hickman Stoneridge Engineering, LLC http://capturedlightning.com
Clive Arthur wrote:
> On 08/03/2017 17:17, George Herold wrote: >> On Wednesday, March 8, 2017 at 11:29:46 AM UTC-5, Clive Arthur wrote: >>> On 08/03/2017 12:47, pcdhobbs@gmail.com wrote: >>>>> If I pass a single wire through a wound air-cored toroid with, say, >>>>> 1000 >>>>> turns, then that's a 1000:1 transformer. >>>> >>>> Not if there's no ferromagnetic core in the toroid, it isn't. The >>>> coupling coefficient in that geometry wil be way less than 1. >>>> >>>> With no core, there's a strong tendency for the contributions of the >>>> inner and outer sides of the toroid to cancel each other. >>>> >>>> Cheers >>>> >>>> Phil Hobbs >>>> >>> >>> OK, point taken, thanks. >>> >>> So, not a Rogowski coil, but a long 1000 turn solenoid wound on some >>> flexible (or hinged) core material up to the end and back so the two >>> connecting wires are at the same end. >> If the core material is not magnetic, then there is not much coupling >> from the turns on the end to the single turn (presumed) in the middle. >> >> Is there some reason not to use a ferrite? > > No, I'll use ferrite, I had thought I could use air, but I was wrong. > >> George H. >>> >>> Bend it into a circle to form a toroid around a single wire and you have >>> a transformer with a 1000:1 turns ratio. What if you bend it into two >>> turns of a circle with a single wire through it? Or three? Does it >>> matter, as far as first-order effects are concerned? > > What of the question about how the number of wraps of the solenoid would > affect the turns ratio? > > Cheers
A true Rogowski coil does not use ferromagnetic core materials. These materials will create saturation problems for very high peak currents for pulsed power measurements. Air-core Rogowski coils provide virtually linear response, even for currents in the meg-ampere range. Adding ferromagnetic material would simply degrade accuracy and limit dynamic range. Bert -- Bert Hickman, Owner Stoneridge Engineering, LLC http://capturedlightning.com
On 03/08/2017 11:29 AM, Clive Arthur wrote:
> On 08/03/2017 12:47, pcdhobbs@gmail.com wrote: >>> If I pass a single wire through a wound air-cored toroid with, say, 1000 >>> turns, then that's a 1000:1 transformer. >> >> Not if there's no ferromagnetic core in the toroid, it isn't. The >> coupling coefficient in that geometry wil be way less than 1. >> >> With no core, there's a strong tendency for the contributions of the >> inner and outer sides of the toroid to cancel each other. >> >> Cheers >> >> Phil Hobbs >> > > OK, point taken, thanks. > > So, not a Rogowski coil, but a long 1000 turn solenoid wound on some > flexible (or hinged) core material up to the end and back so the two > connecting wires are at the same end. > > Bend it into a circle to form a toroid around a single wire and you have > a transformer with a 1000:1 turns ratio. What if you bend it into two > turns of a circle with a single wire through it? Or three? Does it > matter, as far as first-order effects are concerned?
You need a continuous path in ferrite, so either you need to take 2 turns of the high-current wire through a normal annular toroid, or you need the core to be like a figure-8 except with a full twist instead of a half-twist between loops. ;) With an air-core toroid, the signal will depend somewhat on the exact position of the high current wire inside the coil, so if you make the loops really symmetrical it should work fine. You have to have an even number of twists in the loop though, or the two will cancel out instead of adding. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net