Small induction heater questions - Page 1
Forum section: electronics / projects

author: charon1985
author_rank: Contributor
timestamp: June 02, 2016, 04:13:08 pm
content: Hi, i have build a small induction heater, and i have some questions. I have attached a photo to understand better the situation.At number1: Primary coil 7 turns2: Secondary Coil 3 turns3: Working Coil around 20 turnsthere is a white capacitor, 0.33uf 450V that is in series with working coilresonating frequency around 62Khz1 stay cool, 2 and 3 get really hot.when 2 and 3 are absent the current consumption is 0,1A at 24V (maybe is the current need to magnetize the core and the loses in the bridge rectifier and transistors...)At: 66Khz:It heats an alloy of Chrome and Cobalt (no iron), relatively fast.It heats stainless steel (that contains Iron, i think !?) very fast.It does not heat aluminum. in fact the current decrease when i insert the aluminum in the coilMy questions are:- how can i decrease the heating of the working coil and secondary coil?- why does aluminum not heat up, and the current decrease? how to fix this?

author: Dago
author_rank: Frequent Contributor
timestamp: June 02, 2016, 04:35:55 pm
content: Quote from: charon1985 on June 02, 2016, 04:13:08 pmProper Litz wire (Litz wire is not just a bundle of smaller wires, if incorrectly dimensioned it can have a larger loss than a solid wire!) will reduce the heating of the coils. In practice for any useful power level the work coil and matching transformer coils are usually water cooled.Aluminum probably draws less current because it is not ferromagnetic. It is also very conductive making heating much more difficult. You need a lot of power to heat any non-ferromagnetic materials.

author: charon1985
author_rank: Contributor
timestamp: June 02, 2016, 04:45:09 pm
content: And why does the current consumption Decrease when i introduce the aluminum in the working coil (compared with the current consumed by the empty coil)? is counter intuitive, if it has a very small electrical resistance (the aluminum piece), the current should increase even if only a few mA...? why does the 1 (primary) does not heat up, is the same wire, the ratio between primary and secondary is 2 to 1 (aprox) if i have 1 amp in the primary there will be 2 amps in secondary, not such a big difference.

author: bktemp
author_rank: Super Contributor
timestamp: June 02, 2016, 04:52:34 pm
content: Quote from: charon1985 on June 02, 2016, 04:45:09 pmAluminum decreases the inductance of the coil, shifting the resonant frequency up. Using a series resonant circuit makes it hard to get the frequency right. If you are a bit off, the current will be much smaller. For small induction heaters I prefer the typical royer converter with the capacitors connected in parallel with the work coil.QuoteBecause of the resonant circuit the current in the work coil is much higher than the current delivered from the oscillator.

author: charon1985
author_rank: Contributor
timestamp: June 02, 2016, 04:54:36 pm
content: If i introduce a huge stainless steel piece in the work coil, it has the same effect, the current DecreaseLater Edit:You are right, the aluminum changes the resonant frequency, i have increased the frequency to 120Khz and came down and the current increased and the aluminum heated, moderately fast.The induction heater is small now (working coil, capacitor, transformer...) but the controling circuit is a microcontroler, a PWM circuit some opamps, and analog comparators and the tranzistors are 50amps 600V IGBT.First i need to understand how it works at low power, and after that i will increase the power up to 4Kw

author: charon1985
author_rank: Contributor
timestamp: June 02, 2016, 05:12:34 pm
content: So for the working coil and secondary coil to consume less power (and heat up less) i should use some high conductivity cooper pipe (to decrease skin effect) and maybe coated with silver? What do you say?All my previous electronics experience consists only of digital systems... no analog

author: bktemp
author_rank: Super Contributor
timestamp: June 02, 2016, 05:22:10 pm
content: Copper pipe works fine, but at 4kW you probably need some water cooling for the work coil.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 03, 2016, 06:55:20 am
content: What is your control circuitry? Fixed frequency?The secondary and coil of course must get much hotter, because the current is stepped up by the transformer ratio.Tim

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: June 03, 2016, 07:46:57 am
content: I think you need a much higher frequency for non-magnetic materials

author: charon1985
author_rank: Contributor
timestamp: June 03, 2016, 05:03:28 pm
content: Tl494 manually variable frequency. But in the future the microcontroler will shift the frequency to maintain a set current flow.@mikeselectricstuff What frequency do you recommend for Chrome Cobalt alloys, or Chrome Nickel alloy?

author: charon1985
author_rank: Contributor
timestamp: June 03, 2016, 06:55:36 pm
content: Why does the heater absorb less power if i introduce a big iron piece? it is not because of the shifting resonating frequency, because i have tried to manually adjust it to be as good as possible. If i introduce a small piece to be heated i can increase the current (by decreasing frequency to approach resonance) up to 8 Amps and more. a big piece will absorb no more than 1,5 Amp...There are some things that i do not understand yet.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 03, 2016, 07:16:51 pm
content: More load in the coil --> lower tank Q --> higher reflected impedance --> less power draw.For a series resonant circuit, supplied by a constant voltage inverter, maximum current draw occurs at maximum coil Q (unloaded).Since the inverter and transformer are normally selected for optimal current draw with a modest work load, the current draw under unloaded conditions is usually so large as to be destructive.Your control circuit must be able to measure this and adjust the frequency (or another process parameter, like supply voltage, using a pre-regulator) to maintain safety and stability.Loads cause the resonant frequency to shift up or down, depending on size, resistivity and permeability. For all of these reasons, fixed frequency operation is a very bad idea...Do you know AC steady state analysis?Tim

author: charon1985
author_rank: Contributor
timestamp: June 03, 2016, 07:33:00 pm
content: "Do you know AC steady state analysis?"NoThe microcontroler will supervise the current consumption and increase or decrease frequency to maintain stability.

author: alsetalokin4017
author_rank: Super Contributor
timestamp: June 03, 2016, 07:38:35 pm
content: Yep, the Royer/ZVS type architecture with parallel resonant tank is much simpler and stays in resonance ...

author: charon1985
author_rank: Contributor
timestamp: June 03, 2016, 07:51:22 pm
content: Can it melt Chrome Cobalt alloy ? 1430C melting pointand to be safe to use, no transistors explosion.

author: charon1985
author_rank: Contributor
timestamp: June 04, 2016, 11:47:57 am
content: I have found almost all the answers herehttp://www.electronics-tutorials.ws/accircuits/series-resonance.htmlBut now i have another question, how can i increase the inductive coupling between the work coil and work piece? i need to be able to use a small crucible 4cm diameter.I have seen a guy on Intructables that had made some king of ferrite material from iron oxide and water glass and made a shield for his coil... did anyone tried that?

author: Kleinstein
author_rank: Super Contributor
timestamp: June 04, 2016, 01:02:25 pm
content: The best frequency depends more on the size of the sample / crucible, not so much on the material. A 4 cm crucible is already not that small.I don't see a special need to form the field. Just have the coil diameter not too much larger than the sample, and keep some distance. In a resonant circuit, the coupling efficiency is not that critical - less coupling just causes more circulating power.The systems I have seen use a coil about as long as the diameter. The coil gets more complicated if levitation of the sample is wanted.Adding some kind of ferrite core might make sense for very small samples (e.g. 1 mm size), as an alternative to really high frequency - but this could than directly be the driven transformer, so no extra secondary coil. I have not seen such a system.

author: charon1985
author_rank: Contributor
timestamp: June 04, 2016, 01:40:41 pm
content: The samples that i need to melt are cylinders 7mm length 12mm diameter approx. But i will melt 2-3 samples minimum and around 10 maximum.

author: chris_leyson
author_rank: Super Contributor
timestamp: June 05, 2016, 02:17:55 pm
content: Hi charon1985, nice projectI think your transformer coil heating is due to copper loss from bulk resistance and skin effect maybe you should wind your transformer in a different way. A while back I wound a transformer to deliver 50A RMS into a piece of 1mm diameter NiChrome wire, transformer core was TDK P50 N27 1mm air gap with a two turn seconary and maybe 25+25 turns push pull primary, 50kHz working frequency. It was a crude and very quick build but it served it's purpose, it even got used as a hot wire plastic cutter for a while. Picture attached.QuoteI assume your are talking about skin effect heating in the context of your statement rather than hysteresis loss heating. Steel is easy. Material does matter as it will change the load impedance so you have to tweak the work coil dimensions to efficiently magnetically couple the work coil and load.

author: charon1985
author_rank: Contributor
timestamp: June 06, 2016, 07:36:21 pm
content: Thank you Chris for appreciations. I have made the work coil from copper pipe and now stay much cooler.But now i have spotted another problem, the signal of the H bridge looks ok when the load is a light bulb.When i install the transformer some spikes appear and i am afraid that those spikes will kill the transistors. Also i don't see the dead time any more on the scope... Even if i increase the dead time, i am unable to see it.I have mounted 4 snubbers (4,7nF capacitor and 22 ohm resistors) between drain and source for each transistor and the situation improved but not much. IF i will put a varistor betwen drain and source will it cut of the spike? will the varistor survive?Thank you.

author: bktemp
author_rank: Super Contributor
timestamp: June 06, 2016, 08:19:35 pm
content: It looks like there is no capacitor across the DC voltage rail near your H bridge. At those power levels the supply rails should be short and wide to have a low inductance. Then the embedded diodes in the mosfets/igbts should clamp the voltage to the supply rails.