But there are generators that have straight stators. These types of generators are called linear induction generators.
A linear induction generator is basically an electric motor with its stator unrolled and laid out in a line. Opposite of achieving torque by rotation, it causes a linear force along the length of the stator.
There are two design categories of linear induction generator, low and high acceleration. The low acceleration linear generators are of linear synchronous design. This means the stator has a winding movement on one side of an air gap and a range of alternate pole magnets on the other side. The energy is caused by a moving electromagnetic field applied on conductors. The eddy currents of any conductor appearing on the field will be induced, producing an opposing electromagnetic field. Since the opposing fields distracts each other, it forces the conductor to move away, bringing it along the moving magnetic field.
This category of linear generator is commonly used in magnetic levitation trains and other ground based transportation such as roller coasters. The stator of this kind of linear generator is quite long and can reach for miles.
The other design category is the high acceleration linear induction generator. The high acceleration linear induction generator is of linear induction design. This means that the stator has an active three phase winding on one side of the air gap and a passive conductor plate on the other side.
These linear generators are usually short but very powerful for its length. Frequently they are used for experiments on weapons such as rail guns or as mass drivers for alternative spacecraft propulsion.
High acceleration linear induction generators are hard to design. First, they have to be extremely powerful, requiring a lot of energy for just a short period of time. Second, they need large, complicated capacitors to supply that amount of energy. Finally, they need very strong magnetic fields which could be too much for superconductors. Therefore, they need to be carefully designed.Whatever the case, both types of linear induction generators are expensive and complex.