An DC/AC rotary converter is a much more facinating piece of electrical machinery. It has a single rotating armature in a stationary magnetic field. One end has a commutator just like common motors used in most small appliances.
When fed with DC it runs just like a motor. However the opposite end of the armature is fitted with sliprings. AC appears on these connections. Once running, energy can be converted in either direction.
They have a very good sinewave output. Seventy percent of the power flows directly from the commutator to the sliprings without interacting with the magnetics so converters can be very powerful for their size.
They were commonplace when the AC system took over from earlier DC grid systems and allowed for a smooth transition in technology. However they were of little use once AC became the norm and very few survive.
Many years ago I found a small 24 volt converter on the rubbish tip. It was designed to work from batteries and the AC was passed through a transformer to normal mains voltage.
These devices should not be confused with the crude motor generator sets used to make AC power from batteries.