Electric Generators: Mechanical Engineer Questions

[bhaazee]

I am a Mechanical Engineer and have less idea on Generators.

In wind turbines as the generators would be driven at variable speed by the wind, some type of controllers are used to provide a steady output, say, maintaining a constant voltage.

My question is what is the function of this controller? means what it actually does?

Also, in generators (small scale and large scale) which is the most preferred one? AC or DC? and why?

Regards.

 

[Bob S]

The "generators" used on wind turbines are generally alternators (that produce ac power), not generators (that produce dc power). In small alternators, the rotor is made of permanent magnets, and the power output is the stator. The alternator is multipole (to make it more efficient at low RPM), and multiphase (to make rectification to dc more efficient). The small DIY (up to ~1000 watt) often use axial magnetic field rotors like shown in

http://www.instructables.com/id/DIY-1000-watt-wind-turbine/#

The 3 phase ac output is rectified and converted to a voltage suitable for charging batteries.

The large (1 to 3 MW) public utility units are much more complex multipole asynchronous alternators with dc current excited rotors. The ac stator output (variable frequency) is rectified and then inverted to enable connecting to the grid (fixed frequency).

 

[cabraham]

Actually, the term "generator" can be used for AC as well as DC units. The power plants refer to their AC generating units simply as "generators".

It was the automotive industry, I'm pretty sure, that makes the distinction. Early auto units generated DC (commutation), hence they were referred to as generators. Then when the AC units were introduced, the term "generator" would still have been correct, but to distinguish they used "alternator".

For AC units, either "generator" or "alternator" is correct.

Also, induction generators maintain fixed frequency when connected to the grid even when speed varies.

Claude

 

[bhaazee]

Thanx for the reply guys.

Now assume I need to generate dc power (say 1 KW capacity) from some source.
Case 01: I am using a generator (dc). Therefore, I don't need a rectifier.
Case 02: The other case is I am using an Alternetor (AC). Here I need an rectifier.

In cost means case 01 is advantageous. However, in efficiency means which will be advantageous in long term. Assume that the specification for both Alternator and Generator are the same.

Regards.

 

[cmb]

What is the load for this DC power?
How far you want to send the power?
Do you want high volts/low current, or low V, etc..?
What is the duty cycle/life requirement (how long do you need it)/reliability requirement?

 

[bhaazee]

the voltage is 48 V.
The current is 1000 W/48 V
The power generated is stored in a battery say at 3 m distance.
The power generation is carried out at a frequency of 20 times/min

Simple example can be a energy recuperation in a Car. The generator life span has to be 10 to 20 years.

 

[cmb]

The current is 1000 W/48 V
...
Simple example can be a energy recuperation in a Car. The generator life span has to be 10 to 20 years.

You will need a capacity of at least 10kW to get any useful recovery from a car under electric braking, more likely 50kW would be better.

This is why electric power in vehicles is a few 100 volts, so, for example, a typical electric car might run, say, 300V-400V and 200A. Managing the power at these capacities requires heavy duty IGBT swithing modules, so whether you regard chopping up and re-constituting currents from a DC battery as 'DC' or 'AC', it becomes rather moot.

So, if you are talking electric vehicles then, no, DC makes no sense. You need commutation-free synchronous AC systems to handle that much power.

1000W is just about what your power steering or air-con will take at peak loads. Car traction is an order of magnitude (or two) power difference.

 

[bhaazee]

Thnx. One more query. consider a normal dc generator at home. If I reverse the terminals, the direction of rotation is reversed.

Now, in a motor with both Stator and Rotor as coil of wire, a rotation is created in clockwise direction. Consider, that stator carries the temporary magnets. Now, if I reverse the power supply to temporary magnets, then the attraction between stator and rotor becomes repulsion. This means, the motor will behave like brakes. Am I right? If yes, is this possible for both AC and DC?

 

[cmb]

Thnx. One more query. consider a normal dc generator at home. If I reverse the terminals, the direction of rotation is reversed.

Now, in a motor with both Stator and Rotor as coil of wire, a rotation is created in clockwise direction. Consider, that stator carries the temporary magnets. Now, if I reverse the power supply to temporary magnets, then the attraction between stator and rotor becomes repulsion. This means, the motor will behave like brakes. Am I right? If yes, is this possible for both AC and DC?

Sorry, I am unclear on what you mean. If you reverse the terminals on a generator then you get the 'negative output' of what you had, but that rather depends on where the wires go!

If you are generating a field for a DC rotor by means of an electromagnetic coil, and then you reverse the coil, then, yes, it would brake the rotor until it comes to a stop, then will act as a motor in the opposite direction. I am unclear on this question, it seems clear what would happen. Am I missing something?