# Electric motor used as generator to directly power another identical motor?



## Matej (Dec 4, 2015)

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!


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## dcb (Dec 5, 2009)

1. yes, but in either case the question is going to be "why would you?".

2. Not very efficient, in the video they aren't doing any real work, just spinning a shaft, so it is essentially 0% efficient in the demo. But in a real application with a load you would see large differences in input torque*rpm vs output torque*rpm. in the case of bldc, it has to stay in sync with the generator rpm, so the usable torque will be a fraction of the generator input torque, or it will stall, and the startup torque will be tiny as the stator winding resistances will dominate the low voltage output of the low rpm generator/motor. You would essentially need to convert the bldc generator output to dc and run an inverter (more losses) to do anything useful aside from some archaic type of instrumentation.

3. usually "driven" is a mechanical term AFIK, so assuming you mean the one acting as a motor instead of a generator. Well for the most part it will cause increased torque on the generator shaft, and increased current in the loop, and not do any useful work and may fail eventually. If there is insufficient torque applied to the generator it will slow down.

For a large part this is why pure series hybrids aren't a good idea. If you had an engine driving the generator (driving a motor driving a wheel), it wouldn't go nearly as far or carry as much load as if you had an engine driving a wheel. Some say you can optimize the engine load with series, but you can also do so with a parallel arrangement (gear selection, use drive motor as generator).


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## Sunking (Aug 10, 2009)

Bottom line is you just wasted energy or better put, threw energy away. 

No motor or generator is 100% efficient, and what you are suggesting is perpetual motion which gets you thrown out of science and engineering schools. Look up Above Unity Gain. 

On the same lines of the HHO , or Power Factor Correction scams. We even have a special section on this forum for such ideas. You do not want to go there.


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## Harmon20 (Oct 5, 2017)

I don't think what is being suggested here is perpetual motion but rather relocation of kinetic energy from the source to point of use. This answers the "why would you" question: For the same reason we use long wires to connect the "motors" on water wheels in Hoover Dam with motors on fans in Vegas HVAC units.

Question #2 demonstrates that the poster understands that this is not a perpetual motion machine and losses are inherent in the arrangement. The question wasn't if losses exist but rather how big are the losses.

To me the post read as, "How simple can I get a generation plant/point of use arrangement? Can it be as simple as wires between a driven and a driving motor with no equipment in between and if so a) can I use AC motors, b) how much loss should I expect, and c) how would the two motors affect each other when linked in this way?"

Harmon


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## dcb (Dec 5, 2009)

the context here is "EV Conversions and Builds" though, so it is safe to assume this is all on-board and not part of a stationary electrical grid.


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## Matej (Dec 4, 2015)

dcb said:


> the context here is "EV Conversions and Builds" though, so it is safe to assume this is all on-board and not part of a stationary electrical grid.


To be honest, what originally made me think about this was whether two motors could be used as a primitive 'electric transmission' and how efficient it would be. 

Although then I mainly just became curious how it works and how the motors would affect each other.

Thank you all for the replies.


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## Sunking (Aug 10, 2009)

Matej said:


> To be honest, what originally made me think about this was whether two motors could be used as a primitive 'electric transmission' and how efficient it would be.


OK tap the breaks for a moment. What you are suggesting has already been done, and done every day for at least the last 100 years. Figure it out yet?

Train Engines aka Diesel-Electric Transmission or Diesel Locomotive. The diesel engine turns a Generator to supply power to the Traction Motors. The whole point of doing it that way is because there is no good mechanical way to make a Transmission and Clutch assembly for direct drive from the diesel engine. 

Up to a few years the traction motors were Series Wound DC Motors and still in use today. Today's modern Locomotives use 3-phase AC Induction motors. 

However what you are inquiring about, using it as a Transmission is kind of pointless. Electric Motors do not need a transmission for what you suggest. All you would be doing is just adding more losses. 

With just one motor at best 80% efficiency. Now drive a Generator and another motor with that you have a huge compounded loss. 3 stages say each is 80% efficient and you start with 100 HP input out with 80 HP to the Generator and 64 HP out of the Generator o the drive motor and now you end up with 51 Hp on the wheels. Overall efficiency 51%. Not good. Go direct with one motor. 

Now what can be done and has been done is use a diesel engine to drive a genny to power the Traction Motor. Those are called Hybrid Electric Vehicles and do exactly what a train does.


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## dcb (Dec 5, 2009)

Sunking said:


> Those are called Hybrid Electric Vehicles and do exactly what a train does.


Well not exactly... There is a world of difference between Matej's setup, and a diesel electric, and a hybrid (i.e. prius). His is just a straight connection, a diesel electric controls the field strength of the generator (and trains use it for reliability even though it is less efficient, and they brake via heat instead of battery), and the prius splits the power and sends much of it in parallel to the wheels (the motor attached to the wheels is 50kw, the motor that can be driven by the engine (or assist/start the engine, or simply retard the sun gear so more torque goes from the engine to the wheels, or ??) is 10kw or so) and has a very sophisticated management system. 

Aside from electrons and rotary motion being involved, they are all very different. If matej had an alternator powering a motor, where he could control field strength, it would be more like a diesel electric, yes, but nothing like a modern hybrid, and even modern hybrids trade a bit of peak efficiency for better overall efficiency, for your typical driver.


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## Matej (Dec 4, 2015)

Sunking, series hybrids and diesel-electrics are of course known to me, but they generally rely on inverters, controllers, batteries, and other systems to optimize their input and output and maximize their efficiency. 

I was just curious how efficient a completely simplistic system consisting of only two motors would be at simply transferring mechanical energy from point A to point B.


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## electro wrks (Mar 5, 2012)

Matej said:


> Sunking, series hybrids and diesel-electrics are of course known to me, but they generally rely on inverters, controllers, batteries, and other systems to optimize their input and output and maximize their efficiency.
> 
> I was just curious how efficient a completely simplistic system consisting of only two motors would be at simply transferring mechanical energy from point A to point B.


You need to understand that a pure mechanical transference of energy (through gears, chains, etc.) maintains a fairly consistent high level of efficiency over a very wide speed range. Generators to motors(as you're asking about), hydraulic pumps to hydraulic motors, air pumps to air motors have a lower efficiency over a much narrower speed range. Think of the inefficiency as slippage or leakage that contributes to less work done and more energy wasted, mostly as heat energy.

Sometimes the convenience, flexibility, or simplicity of the non-mechanical energy transference, in machines such as the diesel-electric trains, some types of heavy equipment, remote sailboat drives, portable tools, etc., out-ways the need for maximum efficiency. This is usually not the case in say an EV, where you're trying to get the most energy out of a limited battery capacity.


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## dcb (Dec 5, 2009)

Matej said:


> I was just curious how efficient a completely simplistic system consisting of only two motors would be at simply transferring mechanical energy from point A to point B.


So the faster a generator spins the more volts it makes (like a magnet moving quickly past a wire vs slowly). And the faster a motor spins (motors are generators obvi.) the more back-emf it makes. The difference is when used as a generator there is an internal voltage drop (stator/armature resistance), and when used as a motor it requires additional voltage above the back-emf to overcome and create sufficient current (torque). That voltage drop IS a function of current. So when they are turning at the same speed no torque is created (much like the slip in an induction motor).

So for an identical motor-generator pair (brushed for now), the generator HAS to spin faster than the motor (assuming no controller) to create the same amount of torque, and below a certain amount of rpm difference, current(torque) will be small. And it also HAS to be less torque, not really the same torque, as there are frictional losses and eddy currents and whatnot. And the motor/generator has their own efficiency curves which get multiplied together in this case depending on operating their rpm independent operating conditions (current is identical in this case and is a function of the difference between motor and generator rpm).

A controller allows you to change the voltage and current ratios (the rpm and torque ratios, a bit like a gearbox, but lossier, but "infinitely" variable like a cvt instead of 4,5 or 6 ratios to choose from), so that the generator can spin fast (or like having a high voltage pack) and the motor can spin slow at higher torque, since the controller will "buck" the higher voltage and lower current into a lower voltage and higher current. But with a straight connection you don't have any control of the current (torque) or voltage(rpm) ratio. 

If you floored it from a stop, and generate a bunch of voltage, the motor and/or generator would possibly overload and die, without current control. Probably the motor first as it isn't spinning or is turning slowly and not circulating any air.


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