Hi all,

after reading the thread "To import or not?" i got carried away -as usual- while commenting on things with flexible shafts and running 50 Hz brush motors on 400 Hz. I mentioned high frequency tools featuring induction motors and went way off-topic in that particular thread. So i decided to leave it at that and to add the pictures, that i planned on adding there, in this new thread instead.

High frequency AC power tools became popular in the 30's, after the better transportation and induction properties of AC power were grasped and fully put into practice. Around those times insulation materials were not as good and reliable as they are now and it was nearly impossible to wind reliable heat-resistant brush motor armatures with a decent energy density and halfway compact proportions. It would take a few more decades before compact and heat resistant brush motors rating more than 500 Watts were to be seen in industrial power tools. In the mean time, induction motors souped up by increased AC frequencies, were the first choice for heavy applications. These motors only had powered stator windings, which were much easier to manufacture in a robust reliable fashion than armatures were. Induction motors had no powered armatures and needed no brushes and commutators, so they knew no brush commutation problems like arcing because of dirty cooling air and brush bounce during shocks. Shipyards and foundries and construction works already used very heavy and robust grinders and drills fed on HF-AC long before strong brush motor were considered reliable and durable and compact enough for industrial power tools. The only problem was the expensive HF power source itself, which was a rotary converter. What was gained with the highly efficient HF induction motors in the tools themselves, was partly squandered through losses in the rotary gen-sets. Electronics have taken over this field as well, so the rotary sets have seen a successor in the lightweight and highly efficient static frequency converter, with built-in soft-start and overload current limiting features.

Once 200 Hertz was the limit (resulting in a fourfold rpm as compared to the rpm at 50 Hertz) but since 2006 the new record stands at 800 Hertz. Together with dedicated portable converters, this new level may yield some very light and compact yet very powerful tools.

The pics show examples of 200 Hz and 800 Hz machines. The 800 Hz angle grinder is the most compact heavy 230 mm 2.3 kW machine on the market today, and the 200 Hz compacting needle clearly shows how sleek and small HF induction motors can be.


greetings

gerhard