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### Author Topic: Electromagnet impedance vs transformer impedance  (Read 3712 times)

#### nix85

• Hero Member
• Posts: 1149
##### Electromagnet impedance vs transformer impedance
« on: May 15, 2019, 08:30:18 PM »
I want to clear something up and i am sure there are people here who will instantly know what i am asking. So...

For a speaker coil impedance is said to be mostly due to dc resistance and in small part due to reactance. In other words we can measure speaker coil's dc resistance and we know it's impedance is little bit above that, like so. > https://www.youtube.com/watch?v=4Ocgo1p8Z3Y

On the other hand, a transformer, which is basically also an electromagnet is said to mostly consume energy due to reactive power and dc resistance being just a small part of it's overall impedance.

If secondary of a transformer is open, no load connected, it looks like near perfect inductor to the driving circuit. This means current lags the voltage by 90° and average power through the primary is 0. In reality there is little power consumed due to resistance and core losses.

If load is connected ti the circuit driving the primary it appears as if it got more resistive, thus voltage and current which are almost 90° off phase in no load condition become more and more in phase and more power is consumed.

This is explained in more detail here. Don't read the confused question, just the answer below. https://engineering.stackexchange.com/questions/7485/transformers-with-no-load

My question is how come speaker coil mostly consumes power due to dc resistance, why does it not behave like primary of a transfomer with no load - making current lag the voltage by 90° and overall power ~0?

Few related formulas:

XL= 2πfL
XC= 1/2πfC
Z = sqrt(R² + (Xc - Xl)²)
F = 1/6.28(LC)
F = 1/2π√LC
power per second: P = IV and P = V²/R and P = I²R
true power P=VIcosφ

#### nix85

• Hero Member
• Posts: 1149
##### Re: Electromagnet impedance vs transformer impedance
« Reply #1 on: May 17, 2019, 11:45:29 PM »
BUMP

#### citfta

• Hero Member
• Posts: 1006
##### Re: Electromagnet impedance vs transformer impedance
« Reply #2 on: May 18, 2019, 01:33:28 AM »

The simple answer is that the speaker is not a transformer.  It is in a sense a motor.  When signal is applied to the coil the magnetic field of the coil interacts with the permanent magnet of the speaker and causes the coil to move which then moves the cone of the speaker.  The cone has to move air when it moves and that is the load on the coil which causes it to draw current.

Carroll

#### nix85

• Hero Member
• Posts: 1149
##### Re: Electromagnet impedance vs transformer impedance
« Reply #3 on: May 18, 2019, 11:33:57 AM »
The simple answer is that the speaker is not a transformer.  It is in a sense a motor.  When signal is applied to the coil the magnetic field of the coil interacts with the permanent magnet of the speaker and causes the coil to move which then moves the cone of the speaker.  The cone has to move air when it moves and that is the load on the coil which causes it to draw current.

Carroll

Simple but good answer, exactly what i assumed.

#### gyulasun

• Hero Member
• Posts: 4137
##### Re: Electromagnet impedance vs transformer impedance
« Reply #4 on: May 18, 2019, 08:39:34 PM »

...
My question is how come speaker coil mostly consumes power due to dc resistance, why does it not behave like primary of a transfomer with no load - making current lag the voltage by 90° and overall power ~0?

...
Hi,
There would be one more thing to consider as an addition to the explanation:  a speaker coil has much smaller self inductance than that of a primary coil of a transformer.  You can consider speaker coil inductances in the order of some hundred microHenry to some milliHenry, while a primary coil in a transformer starts from at least several hundred milliHenry to over a few Henries,
And the actual audio frequency a speaker coil receives does count of course: at the some kHz and over 10 kHz the impedance may increase much higher of course.  With an L meter, you can estimate this better for speaker coils.

Gyula

#### nix85

• Hero Member
• Posts: 1149
##### Re: Electromagnet impedance vs transformer impedance
« Reply #5 on: May 18, 2019, 09:21:34 PM »
at the some kHz and over 10 kHz the impedance may increase much higher of course.

Do you think this is the main reason for amp power drop of 1.5-2 decibels at ~20KHz?

#### gyulasun

• Hero Member
• Posts: 4137
##### Re: Electromagnet impedance vs transformer impedance
« Reply #6 on: May 18, 2019, 09:30:29 PM »
I am not an expert on loudspeakers,  so cannot give you a correct answer for your question. If you google for    loudspeaker coil inductance   or   voice coil inductance,   you will find some good papers on the behaviour of speaker coils at the higher audio frequency ranges.  It seems a complex problem.
Consider the effect of cross over networks (if used such) too.
Amplifier power drop : if you use a dummy load instead of loudspeaker(s),  then you can check the amplifier behaviour first, you surely know this.

#### nix85

• Hero Member
• Posts: 1149
##### Re: Electromagnet impedance vs transformer impedance
« Reply #7 on: May 18, 2019, 10:39:16 PM »
Consider the effect of cross over networks (if used such) too.
Amplifier power drop : if you use a dummy load instead of loudspeaker(s),  then you can check the amplifier behaviour first, you surely know this.

Sure, there are other factors too why amp power drops, some mentioned here. https://answers.yahoo.com/question/index?qid=20190510203041AAZbp3b

Coil Inductance Calculator