Electromechanical Frequency Converter

Your question is not clear . I think you want to know about a Medium frequency Motor generator set with 3 phase 50/60 cycles Induction Motor as prime mover. and generator out put single phase 2 KHz /8 KHz .These out puts are normally used for Industrial purposes such as Induction heating ,very high speed motors etc.These generators are available from very low capacity range to high capacity depends on utility ( 2 KW to even 500 Kw) These motors and generators are mechanically coupled .The principle of of design of these MF generators I have already given in reply to one of the earlier threads on same subject which you can read.

You may be right about being unclear. Let me (try and) elaborate. The freq converter ties a 3phase 60Hz system with a 3 phase 50Hz system, both at 6.6kV. Power can flow from the 60 to the 50 and vice versa (there are generating sources at the 60 and at the 50 sides). I see this as different than an MG set since power flow is bidirectional as opposed to the M simply driving the G. Is this any clearer?

If you have a prime mover at either side, it is just a generator for whatever side you connect. The only issue with one side being a synchronous motor is that it can operate in "island mode" meaning not connected to another grid for excitation current. otherwise, you could do it with 2 induction motors.

If power is applied to an induction motor and it is turned by another motor at faster than synchronous speed, it will act as a generator. To deliver torque, the induction motor runs at less than synchronous speed. So if the motor is on the 60Hz side, power will go from the motor to the generator. If the the generator is on the 60Hz side, power will go from the generator to the motor. Moving the generator from the 50 Hz side to the 60 Hz side makes it go faster turning the induction motor into a generator. However, in either case, you are moving power from the 60Hz power to the 50Hz power.

Freq converter that were built in 1927 are still used on Amtrack's northeast corridor line. The converter 60 cycle power to 25 cycle power.

There are 2 old Transit Authoity frequency converters where I am presently working. One is a 25 to 60Hz converter and the other is a 60 to 25Hz converter. They are both in the process of being demolished and removed.

A synchronous motor (or generator) is essentially the same machine, but will motor or generate depending on the power absorbed or delivered to the shaft.

An induction motor (or generator) does the same thing, however to act as a motor its speed will be less than synchronous, at synchronous speed its power is 0, and if you supply power and drive its shaft above synchronous speed it will generate.

Both machines can behave both as a motor and generator, it only depends on how you apply them.

The system is very similar to the traditional DC MG set, where the prime mover was a synchronous motor, the generator was a DC machine with adjustable field (hence armature voltage), providing adjustable DC voltage to attached DC "motors" on the bus. If you overhauled the DC motors the power was smoothly transferred through the DC generator through the shaft and back into the prime mover that dumped the power back into the grid. This was a typical control system for Paper Machine controls from the 1920's until being replaced with SCR controls 20 years ago. The system does have its benefits in being able to smoothly transfer power without harmonics and has "bump-less" transition from motoring to regenerating.

This same principal is used with modern VFDs that typically use induction machines. They can smoothly deliver power or absorb power. However, if the VFD is to absorb significant power you need to ensure it is a design that can "regenerate".

The Equipment is called "Cycloverter' or "Cyclo-converter"

There had been a post about it, but not much of info then.

It is some 30-40 years back I had a chance to work on such device but it was Hybrid;

Elctronic /Mechanical.