Synchronous Motor/Generator

T

Thread Starter

Terence Sundgren

So I have this theoretical gen-set consisting of a petrol engine directly coupled to a synchronous generator. There are no electrical protections on the gen-set.

I connect the generator to the "grid" (my home DB) through a switch. The engine is not running and ignition is off. I flip the switch with the engine switched off. By my understanding, the generator will go into motoring mode, act like a starter motor, and start turning the engine until it is running at grid frequency, and in-phase - thus synchronised. I now switch on the ignition of my engine and introduce fuel, and as the engine torque output increases, the generator goes into generating mode and I am now putting power into the "grid", without any complex synchronizing controls.

As long as I do not put more power into the generator than it's rated power, and as long as I stay connected to the grid, the set should not over-speed and produce slip.

Does this make sense?
 
Terence Sundgren,

First of all, let me say that most utilities would have a conniption fit if anyone did this at their home without any electrical protections on the generator-set.

Second, I presume the theoretical generator is directly coupled to the petrol engine--meaning that when the generator is started as a motor it will have to be spinning the petrol engine (presumed to be a theoretical reciprocating engine) and that takes a fairly significant amount of power to compress the air in the cylinders while accelerating to rated speed--and maintaining rated speed--without any fuel burning in the engine. That power is going to be coming in through your watt-hour meter, and it's going to cost money for that power.

If there was a clutch between the theoretical generator and theoretical engine this would be a lot easier. You could then start the engine and engage it with the generator which was already synchronized. (Any electrical device, generator or motor, takes some current (power) to get to and maintain rated speed, also. So, even if the generator being used as a motor isn't coupled to the engine it's still going to require power just to get to and maintain rated speed.

Third, the title of your post was "Synchronous Motor/Generator." But in your third paragraph you talk about slip--which is NOT a characteristic of a synchronous electrical machine, but is a necessary characteristic of an induction machine.

But you are right--the only difference between a motor and a generator is the direction of torque and current flows. When amperes flow into a device, and the device produces torque (whether it's just to maintain rated speed or to pump water or spin a petrol engine) it's an electrical machine operating as a motor. When torque is applied to the shaft of that electrical machine by some device like a petrol engine then the electrical machine will produce amperes. Generators are electrical machines that convert amperes into torque, and motors are electrical machines the convert amperes into torque. Generators "power" motors--it's actually the prime mover driving the generator that's doing the work of the motor. Electricity is just how torque is transmitted from a prime mover (a hydro turbine, or a steam turbine, or a combustion turbine, or a petrol engine) located in one location to a motor or motors in locations very far away from the prime mover, all using wires.

A synchronous electrical machine would require lots more controls (an "excitation system" of some sort) that an induction electrical machine would require. Synchronous machines are generally more expensive than induction machines--sometimes a LOT more expensive. Wind turbines are many times started as motors to get them up to speed and then the wind flowing "through" the rotor blades develops torque and they become generators, until the wind flow/speed drops lower than the amount required to keep the rotor blades spinning at rated speed, and then the wind turbine(-generator) again becomes a motor. Some wind turbines are synchronous machines; others are induction machines.

Finally, overspeed is always an issue for any generator prime mover--it's one of the most critical aspects of the prime mover governor (control system). If something happened to cause your connection to the "grid" to open (you'll be connected through a circuit breaker at a minimum, right? If the breaker opened and the fuel to the petrol engine didn't immediately shut off it would overspeed. Even if it took a half-a-second or so to reduce the fuel flow the engine would speed up. And, without any generator protection to sense there was no current flowing (because the circuit breaker had opened) to tell the engine fuel control to reduce the fuel it would definitely overspeed--and that could damage the generator, as well as the engine, and could even hurt (or worse!) anyone near the engine-generator.

So, you've got some of the fundamentals right, and some a little off--but, I would venture a very educated guess that if the utility every found out about your gen-set they would be highly upset, up to and including a law-suit. It may even be illegal to connect a generating device if any kind to the grid where you live.... I know of places in North America where it is expressly against criminal law to do so without an agreement with the utility.

I know--this is all theoretical, and I hope this helps! Great idea to bypass synchronization requirements, but that's only one of the issues to be dealt with. If, for some reason--and by some freak of nature and situations, the utility power to your house was momentarily disconnected while your petrol engine was supplying torque to the generator, and the utility power was re-established again very quickly it's highly likely that the generator would be out of synchronism with the utility--and that could cause a LOT of damage to your gen-set, your DB, and even nearby utility equipment (transformers; switches; etc.). The list of reasons not to implement this theoretical set-up just keeps growing--but hopefully this has been a learning process for you!
 
I agree, the thought of having no protections on the generator is not just scary, it is down right DEADLY for any utility worker who is working on a downed power line when you connect that generator to the grid. DO NO ATTEMPT THIS! There are REASONS why people are prohibited from doing this very thing.

So what are you attempting to accomplish with this by the way? Your fuel costs will be far higher than any net revenue return you would get from the utility (unless you are stealing the fuel). And to the point above, in order to 'export' power to the grid, your utility will likely require what's called a "Net Metering Contract" in order to supply you a meter that reads both directions, otherwise you are just giving them power for free. But to attain that Net Metering Contract, they will INSIST on you having the required "protections", and they will not just take your word for it, you will need to have
 
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