I am trying to make a room heater. It comprises of a few coils of nichrome wire connected in parallel with each other and in series with a regulator to control the voltage. I am also having a step-down transformer connected in parallel with this setup powering a small dc-motor to spin a rotor and push the air out. I was wondering how I could connect the nichrome wire coils to my regular copper wires which I am using to connect them to rest of the circuit without the copper melting at the junction of the two wires where the hot nichrome will be of a temperature higher than the melting point of copper. How do I join them without this happening? I haven't tried this but I think this is what is going to happen if I do. Any ideas anyone?
Attach both wires to opposite ends of larger strip of metal using screws or welding. The attachment strip can be stainless steel or something that will not corrode or otherwise be damaged by the higher temperature at the nichrome attachment point. The attachment strip needs to be large enough so that it does not get too hot from the current and so that temperature at the copper attachment point is acceptable for the copper wire. The copper wire may need a high temperature insulation near the attachment point.
That kind of depends how hot your plan on driving the ni-chrome.
If you are well under 1,085C, the melting point of copper, a good crimp will do with appropriate insulation on the copper.
If you plan in going closer to the ni-chrome max, 1,400C, you will need an appropriately sized intermediate block of material like stainless steel. The block has to be sized so the TD from the ni-chrome side to the copper side is enough to bring the temperature well below 1,000C.
Here is a general temperature colour chart.
If your wire is at the yellow/white end, you need some intermediate material, if it's down at the dark red end, a direct crimp may be sufficient.
A simple method that seems reliable running a nichrome coil up to ~900°C is to use copper wire as a heat sink and attach to the nichrome with cable sleeves - those thin wall tinned copper tubes with a plastic "funnel" and belled entry at one end, normally used to make decent quality crimped terminations with finely stranded wire. Choose a sleeve that slides over solid copper wire, which in this case is ~1mm OD. Rip the plastic off the sleeves, slip the plain ends over the copper about ½ way, solder in place and also fill the belled end of the sleeves with solder. An advantage is that ordinary soft (electrical) solder can be used and no special flux required.
In my case ~80mm of the copper wire is exposed to ambient air. Nichrome coil ~32 awg and 3mm OD is attached by straightening a section ~10mm long at each end, reheating the soldered sleeves and dipping the nichrome into molten solder.
Strictly, it is not the same as a soldered joint because the nichrome will not be wetted but it is electrically sound (conducts well) and mechanically robust, since the comparatively heavy gauge copper sucks the heat away from the connection far faster than the nichrome can put it back in. Of course if the joint is somehow allowed to get hot the solder will melt and quickly fall out; then the show is over. One can also crimp the sleeves to the copper with nichrome coil in between (ie concentrically) but it is not as elegant and this also introduces a lot more heat to the copper wire and sleeves.
Plain old resin core solder works well, just keep it cool. ! Sleeves with plastic funnels still attached