Inductance - UK Vintage Radio Repair and Restoration Discussion Forum

hannahs radios · 31st Dec 2015, 2:21 am

As we all know when an inductive load like a motor or flourescent choke ison an AC supply it feeds a little bit of current back into the supply this can in theory last for up to 5 millisecond under worst conditions but will the fact that the next cycle of the mains is going to opose that actually stop some of the energy going back because as mains volts exceeds choke volts it will stop the reverse current flowing?

Herald1360 · 31st Dec 2015, 5:00 am

In steady state (after any switch on transients have settled) an inductor will alternately store energy from the supply and return it twice per cycle. This means that at times in each cycle the current is flowing in the opposite direction to the applied voltage. Is this what you are thinking of?

Radio Wrangler · 31st Dec 2015, 9:54 am

If you used an oscilloscope, it would show you the mains voltage as a sine wave, and if you sampled the current you would see that it was also sinusoidal, but phase shifted by 90 degrees (a quarter of a cycle) The current lags behind the voltage.

David

G8HQP Dave · 31st Dec 2015, 11:50 am

When 'mains voltage exceeds choke voltage' it starts to reduce the reverse current. Choke current doesn't depend on the voltage, but on the accumulated history of voltage. Just like your car's speed does not depend on acceleration, but on the accumulated history of acceleration (and braking)

julie_m · 31st Dec 2015, 12:26 pm

And if you had a hand-cranked alternator connected up to an inductive load, you would be able to feel a stiff spot as you stored up energy in the magnetic field and then an "easy" spot as it was released. Whereas with a resistive load, the force opposing you turning it would be constant all the way around.

(Someone's really got to build a demo version of that. With polished brass, ebonite and mahogany, of course ...)

hannahs radios · 31st Dec 2015, 12:52 pm

Thanks for the quick reply yes I realise the forward current is shifted out of phase I've seen that on a scope I was just trying to make sure I understood what happened with the choke's return current I think I do now. Julie m yes if we could get someone to build a model like that it would be great! Alas my engineering skills are ropey at best HAPPY NEW YEAR and thanks to you all

31st Dec 2015, 2:18 pm

Quote:

(Someone's really got to build a demo version of that. With polished brass, ebonite and mahogany, of course ...)

Out of interest I have just tried my telephone bell magneto on the primary of a transformer, a totally odd feeling when it is turned.

Radio Wrangler · 31st Dec 2015, 3:32 pm

Quote:

Originally Posted by julie_m

(Someone's really got to build a demo version of that. With polished brass, ebonite and mahogany, of course ...)

Careful! With that sort of construction you could accidentally build a time machine and could wind up anywhen.

David

hannahs radios · 31st Dec 2015, 5:44 pm

Radio wrangler that would be good I would go back in time and make sure they didn't close radio Luxembourg on 208

31st Dec 2015, 2:21 am	#1
hannahs radios Hexode Join Date: Jan 2012 Location: Weymouth, Dorset, UK. Posts: 422	Inductance As we all know when an inductive load like a motor or flourescent choke ison an AC supply it feeds a little bit of current back into the supply this can in theory last for up to 5 millisecond under worst conditions but will the fact that the next cycle of the mains is going to opose that actually stop some of the energy going back because as mains volts exceeds choke volts it will stop the reverse current flowing?

31st Dec 2015, 5:00 am	#2
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,535	Re: Inductance In steady state (after any switch on transients have settled) an inductor will alternately store energy from the supply and return it twice per cycle. This means that at times in each cycle the current is flowing in the opposite direction to the applied voltage. Is this what you are thinking of? __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

31st Dec 2015, 9:54 am	#3
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,870	Re: Inductance If you used an oscilloscope, it would show you the mains voltage as a sine wave, and if you sampled the current you would see that it was also sinusoidal, but phase shifted by 90 degrees (a quarter of a cycle) The current lags behind the voltage. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

31st Dec 2015, 11:50 am	#4
G8HQP Dave Rest in Peace Join Date: Sep 2008 Location: Solihull, West Midlands, UK. Posts: 4,872	Re: Inductance When 'mains voltage exceeds choke voltage' it starts to reduce the reverse current. Choke current doesn't depend on the voltage, but on the accumulated history of voltage. Just like your car's speed does not depend on acceleration, but on the accumulated history of acceleration (and braking)

31st Dec 2015, 12:26 pm	#5
julie_m Dekatron Join Date: May 2008 Location: Derby, UK. Posts: 7,735	Re: Inductance And if you had a hand-cranked alternator connected up to an inductive load, you would be able to feel a stiff spot as you stored up energy in the magnetic field and then an "easy" spot as it was released. Whereas with a resistive load, the force opposing you turning it would be constant all the way around. (Someone's really got to build a demo version of that. With polished brass, ebonite and mahogany, of course ...) __________________ If I have seen further than others, it is because I was standing on a pile of failed experiments.

31st Dec 2015, 12:52 pm	#6
hannahs radios Hexode Join Date: Jan 2012 Location: Weymouth, Dorset, UK. Posts: 422	Re: Inductance Thanks for the quick reply yes I realise the forward current is shifted out of phase I've seen that on a scope I was just trying to make sure I understood what happened with the choke's return current I think I do now. Julie m yes if we could get someone to build a model like that it would be great! Alas my engineering skills are ropey at best HAPPY NEW YEAR and thanks to you all

31st Dec 2015, 5:44 pm	#9
hannahs radios Hexode Join Date: Jan 2012 Location: Weymouth, Dorset, UK. Posts: 422	Re: Inductance Radio wrangler that would be good I would go back in time and make sure they didn't close radio Luxembourg on 208