求翻译：在现代电力电子电路出现之前，要改变电源的频率是一件非常困难的事，而在技术发展的今天，采用现代功率半导体器件，运用现代电力电子技术，几乎可以随意地改变供电的频率和幅值，以适应各种各样的特殊场合和不同负载的需要。是什么意思？

In before the advent of modern power electronic circuits, the power to change the frequency is a very difficult thing, but in today's technology, the use of modern power semiconductor devices, the use of modern power electronics technology, it is almost free to change the frequency and power amplitu

In the modern power electronic circuits, it is necessary to change before the advent of power frequency is very difficult for the technical development, and today, the use of modern power semiconductor devices, and application of modern power electronics technology, electricity can be almost arbitra

Appears before the modern electric power electronic circuit, must change the power source the frequency is an extremely difficult matter, but in technological development today, uses the modern power semiconductor device, the utilization modern electric power electronic technology, may change the po

Before the advent of modern power electronic circuits, to change the power supply frequency is a very difficult task, and in the development of technology today, the use of modern power semiconductor devices, the use of modern power-electron technology, almost can feel free to change the frequency a

Before the advent of modern power electronic circuits, to change the power supply frequency is a very difficult task, and in the development of technology today, the use of modern power semiconductor devices, the use of modern power-electron technology, almost can feel free to change the frequency a