High-voltage equipment, e.g. high-voltage transformers, is indeed immersed in insulating fluids, either in special insulating oils or in the gas sulfur hexafluoride, which is more convenient than liquid oil.
This increases several times the breakdown field compared to air but it is not enough to reach similar energy densities like with magnetic fields.
To give some numbers, the air breakdown field is around 3 MV/m, while the maximum magnetic field in a motor might be up to 2 Tesla.
The ratio of the energy densities is the square of the ratio between the product of the magnetic field with the speed of light and the electric field, i.e. the square of (2 x 3 x 10^8) / (3 x 10^6), so 200 squared, i.e. 40 thousands.
Even if you increase the breakdown field 10 times, which is quite hard to achieve with fluids, the magnetic field would give forces much, much higher at a given size.
This increases several times the breakdown field compared to air but it is not enough to reach similar energy densities like with magnetic fields.
To give some numbers, the air breakdown field is around 3 MV/m, while the maximum magnetic field in a motor might be up to 2 Tesla.
The ratio of the energy densities is the square of the ratio between the product of the magnetic field with the speed of light and the electric field, i.e. the square of (2 x 3 x 10^8) / (3 x 10^6), so 200 squared, i.e. 40 thousands.
Even if you increase the breakdown field 10 times, which is quite hard to achieve with fluids, the magnetic field would give forces much, much higher at a given size.