For fixed devices, electrical energy is generally available directly. Therefore, hydraulic transmission is not the first choice and is the last resort that will be used. But hydraulic technology is still used quite a bit, which means that hydraulic technology cannot be replaced simply because electricity is readily available.

The root cause is high pressure density. In order to obtain the linear motion, hydraulic cylinder structure is simple, which can achieve the thrust. The same volume electric drive is out of reach. You need a little bit of thrust, such as a couple of hundred kilograms of force, and you can have an electric drive. But you need large thrust, not to say tens of thousands of tons, only thousands of tons, you generally do not consider the use of electric drive. This is because the electromagnetic force that can be produced is subject to both the magnetic saturation density of the material and the current strength.

Adding rare earth elements into electromagnetic materials can increase the magnetic saturation density by 4 times. Therefore, the ac servo motor has expanded its application range.

All currently available materials have resistance: the greater the electricity current, the more severe the heat.