mains tech trying to understand L in LCR - Page 1
Forum section: electronics / beginners

author: rjardina
author_rank: Contributor
timestamp: May 15, 2017, 04:18:20 am
content: i understand this is and electronics forum and dealing with mains is a little obscure.But I have found very few resources online and the level of knowledge.I've been working with mains when it comes to troubleshooting equipment andcontrols over ten years, and found no body in my field that could ever explainimpedance to me. I've spoke with master electricians, teachers and this seems tobe a cloudy subject.I have two LCR meters; Der DD DE-5000 and Extech LCR200. And I'm trying tounderstand why I should set it at a certain frequency, to get the correct inductionsreading.I have a contractor coil I'm trying to measure.I apply 122.1 volts and get an amp reading of .0552 at 60 hertz122.1voltss/.0552amps = 2.211KohmsSo if I plug in my numbers to 2,221 = 2 pi F L or L = Z / (2 pi F)L= 2,211 / (2 pi 60)L = 5.864 Henrys is my math correct??? I understand there is resistance in the wire of the coil,but at the moment I just trying to get a ball park figure.So, I connect my LCR meters and manually push in the yoke of the contactor and on bothmeters at 10kHz I get about what the math comes out to. Reading the manual of the DE-5000I should also be able to read at 100Hz/120Hz/1kHz. But I don't get anything near 5.8H onother frequenciesAm I doing something completely wrong or am I completely not understanding this?

author: agehall
author_rank: Frequent Contributor
timestamp: May 15, 2017, 07:09:02 am
content: I'm no expert and it was a long time since I took these courses, but I don't think you can measure the way you did.At low frequencies, the impedance will probably be small compared to the resistance of the coil. Therefore I'd say you need to measure the resistance and account for it when calculating the value of the coil.

author: tatus1969
author_rank: Super Contributor
timestamp: May 15, 2017, 07:32:33 am
content: Each and every electrical thing does always have resistance, inductance, and capacitance. Resistance and inductance are clear in your case. Capacitance is always created between two metallic things, in this case between the windings.To make things worse, a capacitance and an inductance always create a resonant circuit. For an inductor like yours, this is called self resonant frequency. Inductors with that huge amount of turns can have a surprisingly low self resonant frequency.This is the reason why an LCR meter normally has several test frequencies to choose from. The meter needs a certain minimum frequency in order to be able to measure. Consider an inductor in microhenry order, that will basically look like a short unless tested with >= 100kHz.On the other hand, you want to keep the test frequency as low as possible to keep away the capacitive effects, and not to run into or even test above self resonance.You need to take the DC resistance into account in your calculation. You may as well have a component with 2211 ohms resistance and .000001H inductance. Your LCR meters should have that function. Then you do the trigonometric math and combine R and XL into Z.

author: Neilm
author_rank: Super Contributor
timestamp: May 15, 2017, 06:55:07 pm
content: Inductance can vary with frequency. An inductor rated for mains could easily change when testing at 120Hz and bare no relation to anything tested at 1kHz

author: rstofer
author_rank: Super Contributor
timestamp: May 16, 2017, 01:03:13 am
content: You also need to test near rated current so, all in, trying to determine the inductance is more trouble that it is worth.However, it you have a wattmeter, you can measure voltage, current (combined to give apparent power, VA) and real power (W) and then use a little trig to reactive power. Then, give the reactance (from reactive power) and the coil resistance, you can calculate the inductance.Far more work than it's worth...http://www.te.com/commerce/DocumentDelivery/DDEController?Action=srchrtrv&DocNm=13C3344_AppNote&DocType=CS&DocLang=ENhttps://www.allaboutcircuits.com/textbook/alternating-current/chpt-11/true-reactive-and-apparent-power/http://www.p3international.com/products/p4400.html

author: rjardina
author_rank: Contributor
timestamp: May 22, 2017, 11:03:53 pm
content: Quote from: rstofer on May 16, 2017, 01:03:13 amFirst, thank you and everyone one else for replying. I'm still trying to juggle this concept in my mind.I have a better understanding of it. Thanks to you I've revisited the idea of power factor, something Itouch on very little years ago and never gave it much thought after that. Also never really understoodwhy Dave in his videos and other people in this forum got so crazy over having Watts on a meter. Butnow it blows my mind.I've been able to solve one problem at my current job, three at my past jobs and have a betterunderstanding. Still trying to FULLY understand the effects of inductors and capacitors on AC.Most people I work with only regurgitate information and not understand it, in my opinion no onefully understands because it always a on going process.

author: Ratch
author_rank: Regular Contributor
timestamp: May 23, 2017, 06:12:51 pm
content: I am not going to explain how to operate your LCR meter. That is the manufacturer's job. I will try to explain electrical impedance. Impedance does not make sense unless you are dealing with sinusoidal functions. A sinusoid is the the only continuous periodic function that retains its shape after passing through a linear RCL network. Impedance is a complex number, specifically a duplex number containing arealpart and anorthogonalcomponent usually misnamed the "imaginary" part.The components of impedance are resistance and reactance. Both those impedance constituents impede current, but they do it in different ways. Perfect resistors impede current by dissipating the electrical energy as heat, thereby reducing the voltage after the charge flow passes through the resistor. Remember, voltage is the energy density of the unit charge,so if energy is wasted as heat, the energy density of the charges will be less and the voltage will drop. A perfect inductor produces a back voltage that lowers the applied voltage so that less current is present in the inductor. This does not dissipate energy and produces no heat. Instead the energy is stored in a magnetic field during part of the sinusoidal period and released back into the circuit later in the period. A perfect capacitor also produces a back voltage and stores energy is a electric, not a magnetic field. This, too, produces no heat or energy loss. In a perfect inductor, the current lags the voltage by 90°, and in a perfect capacitor, the current leads the voltage by 90°. Therefore, inductive and capacitive reactance cancel each other out. The impedance is the absolute value of the complex total of the resistance and the reactance in an orthogonal (90°) orientation.To find the impedance of a circuit containing more that one component, you have to be cognizant as to how the components are hooked up (series or parallel). That will make a big difference on what the impedance value will be.Time for you to ask some questions.Ratch