:: Combatting Transformer Hum ::

[suby786]

Ive created a current transformer using an iron core with a primary winding of 4 and a secondary winding of 90.

The primary current varies from 100mA to 13A, but at the moment im doing testing at 100mA-1A in the load (so 4 times this amount)

I get a buzzing sound, quite like an 'angry insect' and the sound gets LOUDER when i increase the load current.

When i remove the secondary from the circuit, the humming dissapears.

This is making me think it is the DC voltage I am producing going back onto the secondary causing the HUM?

The output is passed across a load (4k7) and to a bridge rectifier circuit to produce an analogue voltage connected to my PIC which works fine.

Its just this voltage is causing the humming...

any1 know what I can do or if im even on the right track

 

[k7elp60]

Several things come to mind. Is the transformer in a steel box? Are the laminations large enough to handle the power. It sounds like you may be using the transformer as a current sense.

 

[suby786]

I do want to measure/sense current. The voltage i get at the output circuitry is proportional to the current through the load.

But the voltage i am producing, is going back onto the iron core, i forgot the equation E = 4.44 something or rather????

anyway what do you mean about the laminations? I was given an iron core by my lecturer, i then wound 90 turns (myself) using breadboard wire (with insulation)

the core seems to be made up of two cores stuck together, i.e two donuts on top of each other.

Any ideas????

 

[dch222]

Iron cores are usually made of lots of thin plates (laminations) stuck together rather than a solid lump of iron. This is to stop the generation of rotating eddy currents which would lead to heat losses.

Sometimes a loose lamination will vibrate at the frequency of the ac current due to the reversing magnetic field.

 

[RODALCO]

I have used a discarded mains transformer (230/8 Volts) quite succesfully.

Sometimes there is enough space to add a couple of turns of 2.5 mm² wire for the "primary" current wire. The 8 volts winding can be used for the output.
The 230 volts winding may be removed or is not used. ensure the end wires are properly insulated, otherwise you could get a shock from them.

By doing accurate measurement you can work out the turns ratio, between the couple of turns you wound and the 8 volts winding.

In your case i think the 4k7 resistor is to high, hence the hum. try a lower value, 10 ohms and see if the hum dissappears.
You may have to alter the values of your measuring cct to the lower imput voltage

In the utility industry metering CT's are used on high current electrical circuits, but the CT secondaries need to have as low resistance wiring path to the metering terminals as possible to avoid overloading the CT's with the associated hum.

 

[suby786]

i understand all that, i have used 4k7 as i need at least 1.4v at the output as I am passing the signal thru a bridge rectifier to get a DC output (as the line current is AC). If i reduce this value, my output voltage quickly falls to zero.

What about if i scrap the bridge and go for a precises ac-dc converter using two op-amps? i have a circuit that will convert whatever the ac voltage, but to a dc, so no need for the 1.4v offset?

But will this rectifiy my problem? I may well still get the hum?

 

[Optikon]

i understand all that, i have used 4k7 as i need at least 1.4v at the output as I am passing the signal thru a bridge rectifier to get a DC output (as the line current is AC). If i reduce this value, my output voltage quickly falls to zero.

What about if i scrap the bridge and go for a precises ac-dc converter using two op-amps? i have a circuit that will convert whatever the ac voltage, but to a dc, so no need for the 1.4v offset?

But will this rectifiy my problem? I may well still get the hum?

Does your hum sound like it is 50Hz ? Sometimes, large currents flowing though transformer windings make them vibrate due to the force of attraction between two current carrying conductors. This force operates at 50Hz thus vibrating the windings and creating a hum. I'm not sure if this is your problem but it is normal. Transformer manufacturers help eliminate this problem by coating/dipping the wound core in a varnish that when solid, helps keep the windings from vibrating.

Or were you talking about 50Hz electrical noise hum? As in you are picking up 50Hz components in your measurement?

 

[suby786]

its hard to say it sounds like 50hz, ive read if it sounds like an angry insect (which it does) its a 100Hz...

im not picking up stray frequencies, if i disconnect the secondary from the rest of the measuring circuit, the humming dissappears so this suggests that the cause of the hum is the voltage going back across to the secondary winding,

Let me iterate, the AC current i get from the CT is fed to circuitry that converts this current to a DC voltage to be read by a PIC. The voltage generated (thus far is 0-3v say) and the more voltage i create, the louder the HUM gets.

Remove 1of the wires of the secondary from the circuit, and the HUM goes.

I hope that has made this issue clearer, i hope..

Also bare in mind this is NOT an off the shelf CT. i have constructed it myself using an OFFTHE SHELF CORE and wound it myself. pretty simple circuit but i have no idea how to go about removing it...

what about a series diode in line with 1 of tyhe secondary windings, actually that will rectify the signal BEFORE my load. what implications would that bring? i do eventually want to rectify the ac signal.

 

[Zed]

The humming noise at higher currents will most likely be due to the coils vibrating on the core due to the alternating magnetic flux.
To minimise humming the windings need to be as tight as possible on the core and potted to some degree. Potting can be done by applying either electrical lacquer or epoxy resin as you are winding tightly. When it has hardened the humming noise should be much quieter or even no humming at all.

 

[Optikon]

The humming noise at higher currents will most likely be due to the coils vibrating on the core due to the alternating magnetic flux.
To minimise humming the windings need to be as tight as possible on the core and potted to some degree. Potting can be done by applying either electrical lacquer or epoxy resin as you are winding tightly. When it has hardened the humming noise should be much quieter or even no humming at all.

See my post above