Sorry in advance if this is not necessarily EV related, but I know that there are some very knowledgeable members on here. And no, this is not a 'free energy' thread. 
I am curious to learn what happens when one electric motor is used as a generator to spin a second motor, which may be assumed to be identical for these scenarios. Imagine that the motors are decently large (EV size) and that the generator motor is attached to a water wheel, windmill, diesel engine, or some other means of mechanically turning it.
To better explain what I mean, please see this video at 18 seconds in:
https://www.youtube.com/watch?v=6kgzrXFSDwA
1. The motors in the video appear to be brushed DC. Could the same thing be achieved with two identical 3-phase BLDC motors? Since the phases/frequency are the same, could one motor spin the other, without going through a rectifier/inverter/controller? Or are 3-phase motors more complicated than that?
2. How efficient is this process and what are the primary factors in determining the efficiency? For example, if I spin the generator motor at specific rpm, how may I determine approximately what percentage of those rpm the driven motor will spin at, or how many volts it will be fed, or what its kw/torque output will be?
3. What happens if the driven motor is mechanically stalled? Imagine that there is a brake on the driven motor and it is stopped completely. Will the generator motor slow down significantly? Will it also stop completely, thus also bringing to halt whatever mechanism is powering it? Would this be damaging to either motor, or both, and how?
I am sure that I have many more questions but those are the three main points that I have been wondering about.
Thank you for any input!
I am curious to learn what happens when one electric motor is used as a generator to spin a second motor, which may be assumed to be identical for these scenarios. Imagine that the motors are decently large (EV size) and that the generator motor is attached to a water wheel, windmill, diesel engine, or some other means of mechanically turning it.
To better explain what I mean, please see this video at 18 seconds in:
https://www.youtube.com/watch?v=6kgzrXFSDwA
1. The motors in the video appear to be brushed DC. Could the same thing be achieved with two identical 3-phase BLDC motors? Since the phases/frequency are the same, could one motor spin the other, without going through a rectifier/inverter/controller? Or are 3-phase motors more complicated than that?
2. How efficient is this process and what are the primary factors in determining the efficiency? For example, if I spin the generator motor at specific rpm, how may I determine approximately what percentage of those rpm the driven motor will spin at, or how many volts it will be fed, or what its kw/torque output will be?
3. What happens if the driven motor is mechanically stalled? Imagine that there is a brake on the driven motor and it is stopped completely. Will the generator motor slow down significantly? Will it also stop completely, thus also bringing to halt whatever mechanism is powering it? Would this be damaging to either motor, or both, and how?
I am sure that I have many more questions but those are the three main points that I have been wondering about.
Thank you for any input!