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Inductive loads

Posted By: ANNEMARIE

Inductive loads - 02/01/15 05:29 PM

Hi all I've seen it stated that an inductive load like a fluorescent lamp with conventions ballast will put some power back into the supply at or near the zero of the mains cycle. Is this in the form of a pulse that is there for a fraction of a second or is it actually usable by other loads on the same phase. And does it have an effect on the energy meter. Thanks in advance guys I'm having real trouble getting my head around this
Posted By: gfretwell

Re: Inductive loads - 02/01/15 09:35 PM

This is more just that the current and the voltage are not in phase so you can end up with undesirable current on the neutral. It is most pronounced in 3 phase. At the time when resistive loads are at the peak, the inductive load on another phase is piling on and be adding to the resistive load, burning up the neutral.

It you google up "triplen harmonics", there are plenty of articles about it.
Posted By: ANNEMARIE

Re: Inductive loads - 02/01/15 10:44 PM

Thanks for that gfretwell I think I got it now its that inductive loads create harmonics and it's these that circulate round and make a nuicence of themselves
Posted By: winston_1

Re: Inductive loads - 02/02/15 12:16 AM

Inductive loads don't create harmonics.
Posted By: ANNEMARIE

Re: Inductive loads - 02/02/15 12:30 AM

It looked like that's what the article gfretwell pointed me too said I guess its a combination of fluorescent tube and choke makes all these extra frequency's multiples of 50 cycles otherwise how can you explains the answer to my original question thanks
Posted By: gfretwell

Re: Inductive loads - 02/02/15 12:52 AM

All inductive loads are not created equal but when you are talking about electronic ballasts and PC power supplies (switchers) you are talking about harmonics.
Posted By: ANNEMARIE

Re: Inductive loads - 02/02/15 01:13 AM

Yes I think I got it now its all to do with non linear loads and the fact that current flow stops before the zero volts point this creates a little back EMF but of course it happens 100 times per second therefore there are 100 little pulses per second right?
Posted By: gfretwell

Re: Inductive loads - 02/02/15 05:18 AM

It is not really back EMF, it is a harmonic. These switching power supplies run at very high frequencies and they generate harmonics. The 3d harmonic just ends up adding to the current on a given phase.
Posted By: Trumpy

Re: Inductive loads - 02/02/15 06:03 AM

AnnMarie,
If you're talking about a standard wire-wound ballast and switch type starter, then harmonics don't really come into the equation.
It's really only when you get into the phase-chopping aspects of electronic ballasts that harmonics become a thing.
Posted By: ANNEMARIE

Re: Inductive loads - 02/02/15 03:18 PM

I know that electronic PSU put all sorts of rubbish on their mains input your only got to go near one with a AM radio and you hear all sorts of squeeks and whistles and I appreciate they also create harmonics of the mains 50 or 60 cycle fundamental frequency. I think that inductive loads also put out a small pulse of energy as the waveform gets near zero this is not as you say a harmonic as such just a low level pulse I hope this isright. Sorry to labour the point im just trying to get it in my head
Posted By: winston_1

Re: Inductive loads - 02/02/15 04:17 PM

Originally Posted by ANNEMARIE
I think that inductive loads also put out a small pulse of energy as the waveform gets near zero this is not as you say a harmonic as such just a low level pulse I hope this isright. Sorry to labour the point im just trying to get it in my head


No, you are wrong. A pure inductance ( no such thing) would simply have current lagging 90 degrees from the voltage and consume no power. However all inductors have some resistance so the current is lagging by less than 90 degrees and some power is consumed.

No low level pulses, no harmonics.
Posted By: ANNEMARIE

Re: Inductive loads - 02/02/15 09:59 PM

Surely all inductances create back EMF otherwise how would fluorescent lamp chokes work they ate there to provide a voltage kick to get the tube going. I agree that all inductors have losses that's why they get hot
Posted By: LarryC

Re: Inductive loads - 02/03/15 02:23 AM

Reactive loads store energy and then return it to the mains. Inductive loads store energy in the magnetic fields of the inductors. Capacitive loads store energy in the charged dielectric of the capacitors.

Going back to the definitions of inductance and capacitance. Inductance resists the change in current. Capacitance resists the change in voltage.

Lets ASSUME we have a power source that is creating a sine wave voltage. When you have a fixed resistive load, the higher the voltage goes, the higher the current. When the voltage decreases, the current decreases. The voltage and current track each other.

When you add inductance to the resistive load, the inductor will fight the change in current. It does not want the current to increase, or decrease, or change direction. The way it does that, is to store energy by creating magnetic fields. When the inductor can no longer hold the current constant, the current will start changing. The inductor still wants to keep the current from changing. If the voltage decreases and the current starts to decrease, the inductor will try to maintain the current constant by releasing energy from its magnetic fields.

When you monitor the current and voltage waveforms, you will see the current waveform lagging the voltage waveform. The inductance is effectively delaying the current.

This causes problems in power systems because this stored current does not do any real work. It is not used to make light or turn motors. Because it is current, you have to upsize conductors to handle it but you don't get anything useful out of it. It creates extra losses in the form of heat.

It is analogous to carrying all of your tools with you, even though you only need a screwdriver. It wears you down but you are not doing any useful work by carrying them around.

This extra energy just circulates around, traveling between the power source and the load.

The ratio between the real power (power that does stuff) and reactive power (power that is just going for a ride) is called Power Factor (PF). When the PF = 1.0 the load is resistive. When the PF < 1.0 there is some reactive power in the mix. If you have a load that contains inductance, the PF is "lagging". If the load contains significant capacitance, the PF is "leading".

Remember: Reactive power is not doing any useful work. It is there only for the ride.
Posted By: LarryC

Re: Inductive loads - 02/03/15 02:48 AM

Harmonics are a different kettle of fish. When you have repetitive non sine waves, they are created by adding up a bunch of different sine waves. (Advanced math stuff). What happens when you connect a non-linear load like a switcher power supply, you end up creating a whole bunch non sine waves on the mains. Thru the advanced math stuff, it turns out that all of the different sine waves created are multiples of the mains frequency. It turns out that multiples of three times the fundamental mains frequency can add up and contain a significant amount of energy. These multiple of three harmonics, also know as triplens, cause problems. Anything inductive like a motor or transformer will run hotter because these harmonic currents. Phase currents in neutrals in three phase wye systems don't cancel and in fact the neutral can carry more current than any of the phases.

Typical non linear loads include most items with modern electronics like microwaves, computers, variable frequency drives, Uninterruptible power supplies, electronic lighting ballasts, etc.

Manufacturers are getting much better at producing items that play well with the power systems.

Switching power supplies are much more energy efficient but that comes at the expense of being more complex and requiring care to not create other issues.

The same item can create harmonics and have a non unity power factor at the same time.

Non linear loads are those loads that when you change the voltage by some factor, the current changes by that same factor. Resistors are linear, diodes are not. Reactive loads can be linear. The voltage changes and sometime later in the same cycle, the current will change too.

I hope this explanation helps.
Posted By: ANNEMARIE

Re: Inductive loads - 02/03/15 11:05 AM

Thank you for all the help guys I think I got it now (at last) its a difficult concept at first especially when your not really able to understand the maths. Your explanations have given me what I need so thanks again x Annemarie
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