# Help with KEPCO power supply used as AC amplifier

#### Shotky

##### Newbie level 4
Hello everyone!

I am trying to obtain the phase relation between the output voltage of a pick up coil (wound coil of 150 turns), when inserted into a variable magnetic field, and the filed excitation current.

This field is obtained from an alternating excitation current that flows into a wire loop, the pick up coil is placed at the center of this current loop.
The excitation current is obtained from a KEPCO 72-6m power supply/amplifier, used in current mode and controlled with a sinusoidal waveform at fixed amplitude and with frequency in the range 100Hz- 100kHz, obtained from a waveform generator.

Current excitation and coil output voltage signals are acquired with a DAQ board; the coil is connected to the DAQ analog input in floating source configuration with a 20 m long twisted pair cable, the excitation current is obatained from the voltage drop on a shunt resistor (50m ohm).

what I obtain is in the attached files: the difference between coil voltage phase and excitation current phase is not in a range of 180 degrees, anche it seems that the peak resonance in the amplitude plot ( ration of V amplitude anche I amplitude) is not at the same frequency of the resonance in phase plot.

#### Attachments

• Measured_bode_plot.png
68.1 KB · Views: 33
• circuit-3.png
51.4 KB · Views: 35

##### Super Moderator
Staff member
Strange as it seems the blue plot is correct (in purely mathematical terms) to have that abrupt jag at the point where it changes between +180 and -180 degrees. Even though it disturbs our sense of continuity.

It's not clear from your schematic whether the pickup coil is: (a) a continuous loop (zero or low ohms), or (b) a coiled length of wire (infinite ohms). The wiring also is unclear how your reading is taken from it.
It could make a difference in the phase relationship. In case A the time constant is long, in case B it's very short.
Inductive time constant formula: L/R

#### Shotky

##### Newbie level 4
HI,

thanks for the help.

the pick up coil is made of 150 turns, R=767 ohm, L=76 mH, it has twisted pair cables that I connect to the analog input of my DAQ as floating source

#### Attachments

• pick_up.pdf
15.6 MB · Views: 1

##### Super Moderator
Staff member
From the looks of your setup you're taking a reading of voltage across the pickup coil itself. Yet little or no current flows through it.

To see a phase change, current must flow in an inductor. But you can't read current by taking a direct volt reading across the leads of the inductor. The usual method is to measure voltage across a resistor installed in the path. (Current through a resistor automatically generates a voltage across the resistor, proportional to Ampere level.)

Therefore I believe you'll get success by installing a resistor across the ends of the pickup sensor. The amount of phase change is greater with less resistance. Try a low-ohm value although high enough ohms so that you can take a usable reading.

Also try tilting the pickup coil in different directions, to see how its orientation affects how much it's influenced by the sending coil.

Shotky

### Shotky

Points: 2

#### Shotky

##### Newbie level 4
Hi,
thanks for the help.

Actually I want to obtain the phase difference between the coil output voltage and the excitation current in the outer loop (coming from Kepco amplifier in current mode).
For this reason I used a shunt resistor in series with the current loop (source) to obtain a voltage drop and from this the excitation current

#### FvM

##### Super Moderator
Staff member
At 100 kHz both measurement channels have probably phase errors, there's nothing strange about the measurements provided in post #1. You would need to calibrate the setup before making conclusions.

What is causing the sharp resonance in the excitation current?