Generator Selection for Motor Design: Efficiency and Torque Considerations

[jjb23]

Hey guys

I'm working on the design of a motor and I'm hoping to convert the output shaft kinetic energy into electricity, nothing new there!

The motor will generate 100Nm/rev and I aim on achieving 500rpm...

P = τω

P = (100)(2pi)(500/60) = 5.236 kW

Can anyone advise as to the most efficient induction generator type/spec to use?

Also, does the torque required to run an induction generator increase dynamically? Or is the start-up torque constant?


Cheers.

 

[SirAskalot]

Just to clarify. Are you designing a internal combustion engine, the one you refers to as the motor, or are you designing a electrical motor?

Anyway, what "type of electricity" do you want? AC or DC? 50 or 60 Hz? What "grid/network" are you going to supply?

According to conservation of energy, the energy you put into the generator must be the same that comes out. Thus the torque required to run a generator is proportional to the power you are extracting from the generator.

If noting electrical is connected to the generator no torque is required to make it spin. Hence there will usually be no start up torque, because you disconnect the loads from the generator.

The right generator depends on what grid/network you are going to operate on. A induction motor/generator will not work by its self. It normally needs a grid to be connected to. (or some other elegant methods).

A permanent magnet synchrounus generator may be the simplest in many applications. A synchronous generator with wounded field winding may be better for other applications.

As for induction machines efficiency: http://en.wikipedia.org/wiki/Premium_efficiency

 

[jjb23]

According to conservation of energy, the energy you put into the generator must be the same that comes out. Thus the torque required to run a generator is proportional to the power you are extracting from the generator.

If noting electrical is connected to the generator no torque is required to make it spin. Hence there will usually be no start up torque, because you disconnect the loads from the generator.]

^Thanks, this confirmed my suspicions. So to confirm, the greater the load from the generator output, the greater the torque drawn from the generator input shaft?

The motor isn't an internal combustion engine but will serve the same function. As for AC/DC operation, haven't given it too much thought as I've assumed I can fit a rectifier for any DC application.

I basically just want to make the transition from kinetic shaft power into electric power as efficient as possible and then go from there.

 

[SirAskalot]

^Thanks, this confirmed my suspicions. So to confirm, the greater the load from the generator output, the greater the torque drawn from the generator input shaft?

Well, torque is not drawn from anything. But the generator creates a "counter torque" acting in the opposite direction than the torque created by your motor. A equal torque and opposing torque will prevent a acceleration (Newtons laws, right?).

As efficient as possible is one of the biggest challenges our generation are faced with. So that is a interesting and promising field of study.

I can´t say i´m up to date on the newest inventions on energy conversion systems. But a permanent magnet generator is one of the most efficient ways to convert energy. Can you think of why? What is the alternative to permanent magnets? The source of the rotating magnetic field?

 

[jjb23]

yeh sorry that's what I meant; a negative torque to that of the motor.

I'm guessing because a permanent magnet has a constant magnetic field without the need of a current such as that of an electromagnet?

 

[Windadct]

Does your motor turn at 3600 or 1800 RPM? ( typical induction generators will be 2 or 4 poles - And will you be able to throttle control the speed, to maintain this?

With a direct AC connection to the generator, the motor must maintain the proper speed, and your % speed regulation is important to making a "good" generator - physical control of the system can be a challenge. For example what happens when 50% load is applied. Also - some motors run much more efficient at a different speed, so a high efficiency generator, my still be rectified and then an inverter used to create "clean" power.

 

[jjb23]

Thanks for your input so far Windadct and Siraskalot.

I aim for it to turn at 500 rpm with no load. However I'm unsure how the angular velocity of the motor (generator input shaft) will vary with generator output load, can anyone help with that? I'm guessing it would be a conservation of power problem but I'm not sure.

 

[Windadct]

How ω will vary with load is up to the motor and the control system, the Motor should be able to maintain 500rpm up to the rated output at that speed. My point being that a basic generator - will be synchronous with the line Voltage ( 60Hz ?) , and depending on its construction needs a shaft speed of ω = 2 · 60 · f / n - where N is the number of poles in the generator.

You will need a transmission to match the shaft speed - which will affect the performance of the control system regulating the speed ( throttle in your case?) - this is a classic controls systems case / example.

Is this a "product" you are developing or a one off build? -- these issues come into play here as what is the best way to solve the problem.