# solenoid magnet - turns,volts,amps

I read this book called Practical Electronics for Invetors,and it says that one of the ways to increase magnetic field strenght of solenoid is to increase the current or increase the number of turns.

That got me thinking,why do induction heaters and induction coupled plasma use small number of turns and high current? Why not increase the number of turns and then decrease the current so there is less problem with coil overheating?

I played with online solenoid magnetic field strenght calculator,and as I kept increasing number of turns it kept giving stronger magnetic field without increasing current.So logicaly that extra energy must come by voltage right? I mean energy doesnt just magicaly appear out of nowhere,and since the current stays the same,it must mean that solenoid with more turns needs higher voltage,is that correct?

Can I avoid heat problems by using solenoid that have large number of turns so I can power it with high voltage low current instead of low voltage high current that conventional solenoid with small number of turns would require?

edit : I want to say big thanks for all of you who came here to help me,I didnt expect so much help so fast,this ElectronicsEngineering section seems friendliest of all stackexchange sections I tried,I love you all <3

• More turns = more resistance = more voltage needed = more wasted power, wiping out any gain from reducing current. It's not worse, but it's not better either. Induction heaters have other constraints; they need to run at a high frequency, and that means keeping inductance low. Mar 10 '17 at 18:16
• Induction Plasma uses RF (MHz) to raise impedance with low interwinding capacitance from large spacing, so that parallel resonant frequency does not impede results. we love electronics too. Mar 10 '17 at 19:16

First that's two different questions.

I'll answer the second part first. More turns requires less current to get the same magnetic effect.. Correct. IN your example you used the same current with more turns to get more field... so you need more volts to supply that current to the grater resistance. HEAT comes from power... power = volts times amps. With an ideal device that means for any given Magnetic strength, the power is the same for High V low I vs Low V HIGH I.

Induction heaters work by creating eddy currents in the thing being heated. That requires HIGH currents and frequencies.

• But I was thinking only the current is responsible for heat! I was thinking they use high voltage low current in power lines so they avoid heating the cable. Mar 10 '17 at 18:27
• No, HEAT is WATTS.. and the WATTS = V * I so HEAT = V * I assuming you ignore mechanical work done... humming vibration etc. Mar 10 '17 at 18:29
• They use extremely high V in transmission lines so the voltage drop along the lines is minimized. The cables do get hot. Mar 10 '17 at 18:32

For dc electromagnets, more turns does mean more magnetic field. With ac induction coils, if you double the number of turns, inductance generally quadruples and, for a given ac voltage and frequency applied, the current will quarter.

It's exactly the same problem with industrial metal detection. A single turn coil can be made to resonate and produce big currents very efficiently. This makes the magnetic field proportionately bigger and hence smaller pieces of metal can be detected. If the size of the metal detector is large (such as in timber sawmills), the inductance is quite high and the magnetic field drops. To counter this, multiple parallel coils (separated apart) are used to lower the net inductance and produce more field. On really small metal detectors, two series coils can be used to give more inductance so that the driving oscillator can run flat out.

So, you could say that for induction heating (and metal detection in industry) you design the coil to suit the driving voltage and frequency to maximise the magnetic field.

You have one bad misthinking: "I kept increasing number of turns it kept giving stronger magnetic field without increasing current.So logicaly that extra energy must come by voltage right? I mean energy doesnt just magically appear out of nowhere,and since the current stays the same,it must mean that solenoid with more turns needs higher voltage,is that correct?"

That's not correct. The voltage increase do not go to the magnetic energy. It goes to losses. The magnetic energy in DC coils is 0.5* L* I^2

The coils that produce mechanical force are ignored totally. I return to them soon.