AC and sinusoidal, any difference???

[dudeshan]

Is there any difference between AC and sinusoidal waveform ?
Sine wave is AC, AC means only sine or what ???
Is quassi square waveform AC ?

UPS/inverter genrally give Quassi-square waveform o/p .
To get Sine, we use Ott Filter.
But, can quassi square wave be called as AC.

 

[Nigel Goodwin]

Is there any difference between AC and sinusoidal waveform ?
Sine wave is AC, AC means only sine or what ???
Is quassi square waveform AC ?

UPS/inverter genrally give Quassi-square waveform o/p .
To get Sine, we use Ott Filter.
But, can quassi square wave be called as AC.

AC just means 'Alternating Current', it can be any shape at all, not just sine wave. A square wave is an AC waveform, as is a triangle wave, or a voice pattern - basically anything which will pass through a capacitor or a transformer is AC.

 

[John Sorensen]

basically anything which will pass through a... transformer is AC.

Just for clarity, DC will pass through a transformer. And pulsed DC will even pass across a transformer. The direction of current flow needn't change.

I know what you meant, Nigel, it just struck me as odd :wink: .

j.

 

[Roff]

DC will pass through a transformer.

John, could you clarify that? I'm assuming you mean that a fluctuating DC (equal to AC with a DC offset) will pass through a transformer.

 

[Styx]

basically anything which will pass through a... transformer is AC.

Just for clarity, DC will pass through a transformer. And pulsed DC will even pass across a transformer. The direction of current flow needn't change.

I know what you meant, Nigel, it just struck me as odd :wink: .

j.

pulsed DC is NOT as far as a XFMR is concerned DC anymore IT IS AC!!!

XFMR's are AC components. If you provide it with a step DC the initial step will get through the XFMR since a step has infinite harmonic content. However, if the DC is still provided the XFMR's core will very quickly saturate

 

[Styx]

DC will pass through a transformer.

John, could you clarify that? I'm assuming you mean that a fluctuating DC (equal to AC with a DC offset) will pass through a transformer.

Even if that was the case, then yes the AC content of the DC waveform (ie the ripple) would get through, however the DC content would still saturate the XFMR

 

[Nigel Goodwin]

Just for clarity, DC will pass through a transformer. And pulsed DC will even pass across a transformer. The direction of current flow needn't change.

Pulsed DC is still AC, and as you say will happily pass through a transformer (or a capacitor for that matter). The only difference between pulsed DC and 'AC' is the reference point used. If you feed the pulsed DC through a capacitor it even gives you the 'proper' AC reference (going positive and negative about zero volts.

 

[Roff]

DC will pass through a transformer.

John, could you clarify that? I'm assuming you mean that a fluctuating DC (equal to AC with a DC offset) will pass through a transformer.

Even if that was the case, then yes the AC content of the DC waveform (ie the ripple) would get through, however the DC content would still saturate the XFMR

I thought about that, but it isn't necessarily true. Consider an air core transormer.

 

[samcheetah]

DC can pass through a transformer!

and it doesnt need to be pulsed DC. first lets look at what happens when an AC voltage is applied to a transformer. when the voltage starts to increase in the first half cycle so does the magnetic field intensity. therefore the flux through the core increases. but at some point the flux will stop to change. but in AC that point isnt reached. before that the AC reverses direction and the magnetic field intensity goes negative and so does the flux. thats how the hysteresis loop is formed.

now lets apply a DC voltage to a transformer. the magnetic field intensity H will start to increase and so does the magnetic flux increase. but with DC the transformer gets saturated and the flux ceases to change. this time the field doesnt reverse because DC doesnt change like AC. but if we interchange the input terminals and the output terminals then we could simulate the hysteresis loop of AC with a core without even needing AC. it isnt fluctuating DC niether it is AC but still transformer action is happening.

now about the method to change the terminals. well you could mechanically change the +ive and -ive terminals by a motor or something. but you could also use a better non-mechanical alternative. for more info refer to the book "Switchmode Power Supply Handbook" by Kieth Billings

 

[Styx]

that is a load of crap.

XFMR's are AC devices. DCC converters or SMPS that incorperate XFMR isolation just PWM DC onto the XFMR thus teh XFMR sees and AC signal .

YOU CANNOT PASS A DC LEVEL THROUGH A XFMR>

If that was the case my IGBT's gate drivers would be soo much simpler!!!!

 

[John Sorensen]

Pulsed DC is still AC

Definition of alternating current:

An electric current that periodically reverses its direction.

Pulsed DC does not reverse its direction of flow (the electrons are always going the same way. The don't flow one way, then the other. They flow: then they don't: then they flow: then they don't).

Of course, I wasn't suggesting you could put a DC current through a transformer and get DC out of the secondary.

But, what originally caught my attention was that you said DC won't flow through a transformer, but of course it will, just not across the transformer.

Didn't mean to cause everyone's fur to get ruffled. Geez Styx, settle down :shock:

j.

 

[dudeshan]

come to the point

Everyone is digressing from main point, whats AC and DC.
The discusion is getting centered around Transformer and its properties,
also AC and DC properties/behaviour.

now Lets think logically,
1) AC: Alternating Current
2) DC: Direct Current (Directional/ Unidirectional Current)

DC can be obtained by rectifying AC. Rectifier does not give DC voltage,
it makes Alternating Current to flow in Unidirection to give Direct Current,
to flow thru load , which in turn gives DC voltage ( more precisely: develops DC voltage in load.)

Pulsating DC is just Pulse in one direction and it has to be DC.
Pulsating DC is in the form of pulses and its not continuous, thats all!!!
One can't say pulsating DC to be AC and....lalalala

Whats need of taking Transformer View etc.. ?
Thats confsing, and more confusing coz every1 is talkin at different wavelength, i.e. , every1 has different concepts and definitions.
May be every1 is thinking same, but putting it in different (may be even wrong) manner.

So what i think is that, if u wanna talk abt properties of AC,DC,XFMR, etc.. 1'st state ur definitions precisely, it will not only help u, but also the ones who r reading it.

regards.

 

[Styx]

Pulsed DC is still AC

Definition of alternating current:

An electric current that periodically reverses its direction.

Pulsed DC does not reverse its direction of flow (the electrons are always going the same way. The don't flow one way, then the other. They flow: then they don't: then they flow: then they don't).
j.

except the harmonic content of a square wave will have a fundemental AC component as well as other harmonics (to produce the sharp leading/falling edge and flat periods)

 

[John Sorensen]

True 'nuf.

Peace?

j.

 

[samcheetah]

that is a load of crap.

XFMR's are AC devices. DCC converters or SMPS that incorperate XFMR isolation just PWM DC onto the XFMR thus teh XFMR sees and AC signal .

YOU CANNOT PASS A DC LEVEL THROUGH A XFMR>

If that was the case my IGBT's gate drivers would be soo much simpler!!!!

:lol: lolzzz

i think i mentioned that this was from a book. so i guess its not crap. when i first read this i was also skeptic about this but this is true.

the only thing that limits DC from being passed through a transformer is the fact that the core saturates and the flux stops to change. but we can change that as i mentioned. now its up to you to call it anything u want !!!!

 

[Juglenaut]

Pulsed DC does not flow to and from the supply as the current flows in one direction, only true AC flows to and from.

Basically pulsed DC is a logic level high or floating if not grounded on the off pulse, tied low on the off pulse it would yield an astable multivibrator like that of a 555.

I agree that DC will travel though a transformer, but once the flux is at it max acording to certain math the transduction of the flux stops as the flux must move across the poles to activate current flow. Using pulsed DC will achieve this necessary induction to happen as the flux raises to max out on the on pulse, then drops which will then cause more flux change as it collapses, causing in a sence AC current on the secondary side of the transformer as the current will leave the transformer on the on pulse swing then come back on the off pulse swing.


Pulsed DC is not AC as it still only travels out from the source, where as current follows voltage. from 0-5 volts then from 5-0 volts in a sence this creates a 180 degree phase two more changes in logic yield a 360 rotation yes it would seem as though, but pulsed DC doesn't go negative on the axis itself as the current goes up and down to zero at only 180 degrees this does not make a full 360 even though it may seem to, simply because the voltage on pulsed DC never goes negative on the axis. This is by itself, other means can make DC act more like AC.

Think of pulsed singular DC as having a offset to a point where the wave form never drops below the negative center axis.

 

[samcheetah]

let me explain this a little bit. an ideal transformer would pass all frequencies from DC upwards in both directions with no power loss. but why do we see that when we connect a DC battery to one winding of a transformer nothing happens at the other winding (except for a little spark maybe). the reason is that no material in the world is infinitely permeable! there would always be a limit where the material will magnetically saturate and the flux will stop to change across a material. thats why when DC is connected to a transformer the core material saturates and B (flux) becomes constant. and according to faraday's law if there isnt any change in flux, there wouldnt be any voltage induced.

but when we connect an AC source to a transformer what happens is that the current reverses its direction before the core becomes saturated. and therefore throughout the whole AC cycle the flux never stops to change. although at one time it is decreasing and at one time it is increasing, the change will not cease to exist.

but you could reverse the current even if u have DC. just reverse the +ive and -ive terminals of both the secondary and the primary. it will be like AC although you have connected a DC source to the transformer. the same thing will happen, flux continues to change, the core doesnt saturate and DC is passed through the transformer.

 

[Nigel Goodwin]

Pulsed DC does not flow to and from the supply as the current flows in one direction, only true AC flows to and from.

But this is a 'distinction, without a difference' - if you pass the 'pulsed DC' through a capacitor it removes the DC offset and produces a signal which swings positive and negative about zero volts.

I can't think of any application where pulsed DC has any disavantages over AC, can you?.

 

[Styx]

Pulsed DC does not flow to and from the supply as the current flows in one direction, only true AC flows to and from.

But this is a 'distinction, without a difference' - if you pass the 'pulsed DC' through a capacitor it removes the DC offset and produces a signal which swings positive and negative about zero volts.

I can't think of any application where pulsed DC has any disavantages over AC, can you?.

well I can think of only 1 when the harmonic content is of concern (but this would only be for power and you would want to use a pure sinewave to ensure hte XFMR is at its most efficient)

@Juglenaut and all that saw square wave is NOT AC.
I am a power engineer and deal with analogue signals. When I talk about a square wave it is a signal that goes +v to -V THUS IT IS AC. Also a square wave is AC

@samcheetah
A XFMR needs changing flux to operate. The way this is done is to provide a changing voltage to its terminals to ensure that the core does not saturate. As far as the XFMR is concerned when you swap over the terminals it is no longer being subjusted to a DC voltage BUT an AC voltage THUS you cannot pass DC through a XFMR. What you are suggesting is using a H-bridge to invert the voltage to the primary terminals. This is what I do with my isolation trasformers providing it with a sqaure wave voltage.

please do not say that you can pass DC or supply DC to a XFMR because that is not true.

Also your aguement also falls apart because in doing so the output is not DC naymore it will be AC (not sinusoidal) thus DC cannot pass through a XFMR.

 

[Nigel Goodwin]

I can't think of any application where pulsed DC has any disavantages over AC, can you?.

well I can think of only 1 when the harmonic content is of concern (but this would only be for power and you would want to use a pure sinewave to ensure hte XFMR is at its most efficient)

But that's comparing a square wave with a sine wave, not pulsed DC and AC - where the AC could quite happily be a square wave, and the pulsed DC could always be a sinewave (just a different type of 'pulsing').