Zero Crossing Point of ac cycle

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How can equipments work in ac current even it is oscillating from +ve to -ve and to zero?
Alternating current is alternation of positive and negative values.If we consider the sinusoidal case,the current value becomes zero,negative,positive in a cycle.In that case,how can electrical equipment can work without any interruption at zero value.For example,a tungsten bulb should blink continuously in every cycle of current.Please explain and correct if I am incorrect!
Thanks in advance!
 

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  • #2
dlgoff
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For example,a tungsten bulb should blink continuously in every cycle of current.

Well it does blink. But the filament doesn't have a chance to cool off (much). That and the eye has a hard time seeing flickering above about 30Hz.

Here's some animations that might help when it comes to alternating current:

http://www.animations.physics.unsw.edu.au/jw/electricmotors.html" [Broken]
 
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  • #3
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I was once required to document the light dimming effects that different types of motors / loads / start up capacitors had. This is fairly important because if it becomes significant, people object when their appliances come on....

Anyway, the weird part. The sensor not only picked up the dimming of the bulbs, but also, a very discernable 120Hz sine wave (only when exposed to the light). It was a difficult arguement to make with my fellows, but I do believe that there is a discernable temperature induced flicker in many incandescent lamps.

- Mike
 
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Anyway, the weird part. The sensor not only picked up the dimming of the bulbs, but also, a very discernable 120Hz sine wave (only when exposed to the light). It was a difficult arguement to make with my fellows, but I do believe that there is a discernable temperature induced flicker in many incandescent lamps.

Why is this a surprise? It makes perfect sense.
 
  • #5
sophiecentaur
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In workshops where rotating machinery is used, it is common to use low voltage / high current worklamps. These have relatively massive filaments (lots of thermal inertia), for the same wattage and the flicker is a lot less than for mains voltage lamps. Using lamps with less flicker reduces the strobe effect that can lead to people wrongly assuming things are still when they are, in fact, spinning. Fluo tubes are even worse, of course.
 
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What is the difference between manual interruption by switching on and off,and the zero crossing point?.I think the wearing of the equipment occurs when we switch on and off.
Is there any sensitive device which cannot even bear the such a small time interruption of zero crossing point?
 
  • #7
sophiecentaur
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What is the difference between manual interruption by switching on and off,and the zero crossing point?.I think the wearing of the equipment occurs when we switch on and off.
Is there any sensitive device which cannot even bear the such a small time interruption of zero crossing point?
Any change of voltage is the same. As for the "small time of interruption", many radio receivers can detect variations in voltage as short as 1 nanosecond. Is that quick enough for you?
 
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AC can be used to run rotating motors and resistive heaters and such, but for electronic things like TVs, computers, etc, the AC is rectified and smoothed to turn it into constant DC current, so there is no zero crossing.
 
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AC can be used to run rotating motors and resistive heaters and such, but for electronic things like TVs, computers, etc, the AC is rectified and smoothed to turn it into constant DC current, so there is no zero crossing.

Thanks for your reply...

I visited this link http://www.animations.physics.unsw.edu.au/jw/electricmotors.html" [Broken]

and found a graph of DC voltage....though there is no negative cycle...graph touches the x-axis for every cycle which means that it does have zero value for every cycle....
I think it is also a zero crossing, is n't it?
 
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  • #10
sophiecentaur
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That "DC" you refer to (Rectified AC, probably) is better described as 'Unidirectional'. The term DC normally refers to a constant value - as you get from a battery. 'Raw DC' is not much use for anything but battery charging.
Rectified AC will have no zero crossing (if the rectifier is any good) as it will not go actually through zero.
 
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I referred the graph of a DC generator...
Thank you...
 
  • #12
sophiecentaur
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OK. Don't see many of those around these days because they are nothing like as good as alternators with rectifiers. That's why I didn't consider it.
But my remark still applies about it being unidirectional rather than DC. To describe the waveform accurately, you would say that the it has a DC component, a fundamental AC component and a number of harmonics of the fundamental AC frequency.
But it doesn't do to get too bogged down with semantics.
 
  • #13
jim hardy
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There's still some 20 hz power around Niagara Falls. You can plainly see the incandescent lights flickering. That's about the fastest an eye can resolve, and why television is thirty frames a second.

Human eye is sensitive - if you have a neon lighted wall switch or a strobed display on your electronic alarm clock you'll see a series of dots when your eye moves across it in the dark.
Same effect on these darned newfangled automobile taillights - they're a real annoyance when driving at night.

old jim
 
  • #14
Averagesupernova
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There's still some 20 hz power around Niagara Falls. You can plainly see the incandescent lights flickering. That's about the fastest an eye can resolve, and why television is thirty frames a second.

Human eye is sensitive - if you have a neon lighted wall switch or a strobed display on your electronic alarm clock you'll see a series of dots when your eye moves across it in the dark.
Same effect on these darned newfangled automobile taillights - they're a real annoyance when driving at night.
old jim

I couldn't agree more.
 
  • #15
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Thanks for your reply...

I visited this link http://www.animations.physics.unsw.edu.au/jw/electricmotors.html" [Broken]

and found a graph of DC voltage....though there is no negative cycle...graph touches the x-axis for every cycle which means that it does have zero value for every cycle....
I think it is also a zero crossing, is n't it?
That's why I said it's rectified and smoothed. Though "filtered" would be the more accurate term. Those large dips are smoothed out with a capacitor and usually a regulator of some kind to reduce the ripple as much as possible.
 
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