About the equivalent series resistance problem

呜呼哀哉

About the equivalent series resistance problem [Copy link]

For a 700mH large coil (similar to the coil of a contactor), the Rs is 270Ω when tested with a benchtop LCR bridge, but only 77Ω when tested directly with a multimeter resistance range. Therefore, how is the DC resistance of an inductor defined? Which one is correct?

maychang

The results measured by the LCR bridge include various losses of the coil (iron loss, copper loss...), while the results measured by the multimeter do not include iron loss, but only the DC resistance.

maychang

It’s hard to say which one is right, it depends on what quantity you want to measure.

gmchen

The question itself is problematic.

The question is about equivalent series resistance, but the content asks about "DC resistance". These two concepts are different. DC resistance is of course correct to measure directly with a multimeter, but equivalent series resistance is related to various losses. Lao Zhang mentioned iron loss earlier. If the test frequency is high, copper loss is not just as simple as DC resistance.

gmchen

The LCR bridge actually measures the amplitude and phase angle of voltage and current, and then calculates the inductance and equivalent series resistance. For a large inductor like the one measured by the OP, in addition to various losses, the distributed capacitance of the coil will not be too small, which will also affect the final measurement result.

呜呼哀哉

gmchen posted on 2021-8-4 17:47 The LCR bridge actually measures the amplitude and phase angle of voltage and current, and then calculates the inductance and equivalent series resistance. Similar to the large current measured by the OP...

I sent you a private message, could you please take a look?

呜呼哀哉

gmchen posted on 2021-8-4 17:47 The LCR bridge actually measures the amplitude and phase angle of voltage and current, and then calculates the inductance and equivalent series resistance. Similar to the large current measured by the OP...

Hello, I want to ask again, it is one thing to test this kind of coil with a bridge. In actual use, the DC voltage is supplied, and copper loss exists. After stabilization, the current remains unchanged, so there is no iron loss. In addition, from the manual, the final current of this kind of DC coil, contactor, and relay is usually the working voltage divided by the coil resistance, so I think when testing the working current of the coil, the resistance should be measured with a multimeter, right?

maychang

Alas, published on 2023-8-5 10:58 Hello, I would like to ask again, this kind of coil is tested with a bridge, but in actual use, it is supplied with a DC voltage, and copper loss exists. After stabilization, the current...

[This type of DC coil, contactor, relay] [In actual use, DC voltage is supplied]

Since these contactor relays use direct current, there is of course no such thing as iron loss, and there is no such thing as "equivalent resistance". Only "DC resistance" exists.

maychang

Alas, published on 2023-8-5 10:58 Hello, I would like to ask again, this kind of coil is tested with a bridge, but in actual use, it is supplied with a DC voltage, and copper loss exists. After stabilization, the current...

[So I think when testing the working current of the coil, you should use a multimeter to measure the resistance, right? ]

You can use a multimeter to measure the DC resistance of the winding, and then calculate the working current through the power supply voltage. There is no problem.

A better method is to directly measure the current through the winding. You can use a multimeter to measure the current through the winding.