# Using resonant effect to maintain high current at high frequency (Series RLC resonant circuit)

#### Kimtien

Joined Sep 2, 2016
18
Hello guys,
I have built a coil with the following parameters: inductance L= 288mH, parasitic resistance R=15.5 Ohm, self resonance frequency Fsr=50KHz
The AMETEK 3001iX programmable power supplier is used to supply the power for this coil.
At high frequency, the coil's impedance is dramatically increased, thus the current through the coil become very small with the same applied voltage.
In order to maintain the high current at desired frequency, I use the series RLC resonant effect at desired frequency, therefore some capacitors to connect to the coil in series, which values are chosen from this eq.
C=1/(L*(2*pi*f)^2)

The point is when I connect the capacitors to the coil, the current did not increase as my calculation! It seem did not work with DC sine wave power.
I did try with AC power, however, I seem to be not as expected!
Thank you so much.

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#### crutschow

Joined Mar 14, 2008
34,720
What frequencies are you trying to resonate?
What is "DC sine wave power"?
Sine wave is AC.

#### Kimtien

Joined Sep 2, 2016
18
Hi crutschow,
The coil strongly decrease the current with frequency over 30 Hz. Therefore, I try with frequencies range from 30Hz to 300Hz.
DC sine wave can be generate by using high frequency Pulse Width Modulator.

#### crutschow

Joined Mar 14, 2008
34,720
Here's an LTspice simulation of your circuit with a 1μF capacitor which resonates at about 297Hz.
Is that what you expected?
What did you see in your circuit?

#### Kimtien

Joined Sep 2, 2016
18
I did the simulation using PSPICE with various of capacitors. The simulation results is good as my calculation.
However the real experiment is not as I expected.
That is why I wonder.

#### #12

Joined Nov 30, 2010
18,224
the real experiment is not as I expected.
What did the real experiment do that you didn't expect?

#### Kimtien

Joined Sep 2, 2016
18
As I mention in my first post, when I apply the power (150 VAC) and desired frequency to the coil and capacitor in series.
The current through the coil should go up to around 9A as in simulation.
However, it do not go that high. It is just around 4A.

#### #12

Joined Nov 30, 2010
18,224
At what frequency is this 150 VAC?
What capacitance did you use?
What kind of capacitor did you use?
What size wires did you use to connect the parts?
What kind of amp meter did you use?
Is your amplifier still producing 150 volts AC RMS when it is connected to this 9 amp load?

#### Kimtien

Joined Sep 2, 2016
18
I can switch the variable capacitors to change the resonance frequency as I want to.
The capacitor I use is
Film Capacitors 760vac 1.0uF
Film Capacitors 10uF 875volts
Film Capacitors PEC MKP 100 UF 480 V
I use the cooper wire, 1.5 mm in diameter to connect the part
I use the Lecroy 204MXi-A 2GHz Oscilloscope with the current probe to measure the current through the coil
The amplifier I use is AMETEK 3001iX which can produce up to 22A at 150V mode

#### #12

Joined Nov 30, 2010
18,224
Again you have not stated what frequency and what capacitor you are using in this test that did not allow as much current as you expected.
However, if this is any help, 50Hz would need 35.2 uf to arrive at resonance.

#### kubeek

Joined Sep 20, 2005
5,796
In your post you say 50kHz, for which a 288mH coil is nonsense. Please state your goal without such errors, otherwise we have no idea what you are actually trying to achieve.

#### Kimtien

Joined Sep 2, 2016
18
Dear #12,
I may not explain well what I did.
I use 9 1uF, 9 10uF , and 3 100uF capacitors which are connected parallel to each others. And I can switch among them to create the capacitance I want to make the circuit resonant at desired frequency.
Please refer the following link to see the detail of each capacitor that I use
http://kr.mouser.com/Search/Product...tualkey64600000virtualkey80-PHE845VW7100MR6L2
http://kr.mouser.com/ProductDetail/...=sGAEpiMZZMv1cc3ydrPrF0gALJPP0xBjM93KLz0mYcc=
http://kr.mouser.com/ProductDetail/...GAEpiMZZMv1cc3ydrPrF1u7m5EY228LBvuHlRf%2bVwM=

As you said "However, if this is any help, 50Hz would need 35.2 uf to arrive at resonance." Yes, thank you so much.
If you can see in my previous post, I did simulate with 35uF capacitor to make the circuit arrive resonant at nearly 50Hz.

#### crutschow

Joined Mar 14, 2008
34,720
Did you change the frequency in your setup to find the resonant (maximum current) point for each value of capacitor?

If your current is that much lower at resonance then there would appear to be more resistance in the circuit than you say.

#### Kimtien

Joined Sep 2, 2016
18
Yes, I did try to adjust the frequency around the calculated value. The current is true highest at that point, but still much smaller than I expected.
I did try to measure each parameter of the coil itself using LCR meter HIOKI 3532 , like parasitic resistance, inductance, as well as self resonance frequency of the coil.

#### DGElder

Joined Apr 3, 2016
351
Try putting a 150 ohm resistor in series and then see if you get results matching that simulation.

I believe the supply has a programmable output impedance. What is it set at?

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#### crutschow

Joined Mar 14, 2008
34,720
Try using the LCR meter to measure the parasitic resistance of the complete circuit including all wiring (I'm assuming the meter can measure resistance with a capacitor in series).

#### Kimtien

Joined Sep 2, 2016
18
@DGElder: why should we putting a resistor in series with this circuit?? In my thinking that will increase the total impedance, therefore reduce the current, isn't it?

@crutschow: Yes I did try to measure it as well. The results are almost the same

#### DGElder

Joined Apr 3, 2016
351
@DGElder: why should we putting a resistor in series with this circuit?? In my thinking that will increase the total impedance, therefore reduce the current, isn't it?
Of course, it is just a test to try and learn what is going on. You need to measure more things, change more variables and see how the system behaves to help you figure out the source of the problem. Did you even measure the phase between source voltage and current and how that changed with frequency?

For example, you simulated your circuit with an ideal voltage source. Given the low impedance of your circuit near resonance maybe you are seeing loading of the real world supply. What is the actual output impedance? Try the unit driving a simple resistive circuit. Check to make sure your current probe is giving expected results by measuring voltage across the resistor to calculate current and see if the current probe tracks. And so forth and so on. You need to explore different ideas.

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#### crutschow

Joined Mar 14, 2008
34,720
DGElder has a good point.
Measure the output voltage of the generator when it is driving the load at resonance.
You load is only about 15Ω at that point which could well be reducing the output voltage.

#### DGElder

Joined Apr 3, 2016
351
If loading of the power supply is the issue, then a stepdown transformer on the output of the power supply to give a better impedance match could be the fix..