Why Alternator Mfgs Fit PMGs to Non-Black Start Alternators

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In summary, alternator manufacturers fit PMGs to alternators that are not black start machines in order to provide fault ride through and support the grid when a fault appears.
  • #1
Jobrag
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Why do alternator manufacturers fit PMGs to alternators that are not black start machines.
On site today we are doing some maintenance on a 27MW alternator, for this machine to run there must be electrical power available (to run oil pumps fans etc), so why do we need a PMG when we could supply power for excitation from another source?
 
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  • #2
Not quite sure what type of excitation you are talking about. But by using permanent magnets directly or indirectly to supply the rotor field winding, the excitation current which again controls reactive and to some degree active power are independent of the terminal voltage.

This is an important property when it comes to fault ride trough and supporting the grid when a fault appears. In case the excitation where supplied from the terminals and a short circuit appears near the terminals the voltage would drop substantially and the field winding would have trouble providing enough current to increase induced voltage in the armature winding's (EMF).

On the other hand, pumps, fans etc. have auxiliary power supply like UPS, battery banks or even diesel generators, so they can run without being connected to the grid.

If you are talking purely permanent magnet synchronous generators, permanent magnets give the possibility of building larger generators without increasing the weight, increasing the efficiency (no I^2 R losses in the field winding) etc.
 
  • #3
OOPS disregard this post I mis-read the question and answered a different question than was asked..


[STRIKE]I have wondered the same thing for decades. No reason not to put a small magnet there so as to make residual magnetism enough to bootstrap the solid state regulator.[/STRIKE]

Automobiles use a resistor behind the "ALT" light to provide a trickle of field current from the battery and that's why you can't push start today's cars with the battery removed .

old jim
 
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  • #4
Your question was:
so why do we need a PMG when we could supply power for excitation from another source?

If black start is NOT a requirement then you can get by without a PMG.
It is not uncommon to see excitation taken from machine terminals, with a small source to provide excitation for startup.
Two reasons to provide PMG excitation are:
1. What happens when somebody opens a breaker that's feeding your non-pmg exciter?
The machine immediately trips offline from loss-of-field.
So reliability of the generating machine is one answer.

2. What happens when a fault on the transmission lines close to generator lowers terminal voltage?
The source for your excitation is now reduced so you are limited on how much excitation you can provide, just at the instant you need max excitation to "get under" that fault and deliver lots of current to burn it clear .
So reliability of the system to which the generating machine is connected is another answer.

The main generator at my power station used a shaft driven 420 hz PMG for source, and the voltage regulator itself was magnetic amplifiers. The 420hz power source gave the magamps quicker response than possible with 60 hz.

So put yourself in generator manufacturer's shoes. You'd offer customer the most robust and reliable machine you can build. If customer decides to sacrifice reliability of shaft driven excitation for simplicity of separate excitation that's up to him.

I apologize for my mistaken answer above. Haste makes waste.

Utilities go to what seems extreme lengths to get reliability. Our emergency diesels had huge cooling radiators with shaft driven fans and pumps so they'd be independent of any other cooling water pumps. You recall at Fukushima the only diesel they had left was air cooled, like ours.
Your system is only as reliable as its weakest link.

I should have repeated your question at top of that other post. Then i'd have realized Sir Askalot already answered it. (chagrin icon)old jim
 
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  • #5


There are several reasons why alternator manufacturers choose to fit permanent magnet generators (PMGs) to non-black start alternators.

Firstly, PMGs provide a reliable and efficient source of power for excitation, which is crucial for the proper functioning of the alternator. PMGs are able to generate a constant output of power, regardless of the speed of the alternator, making them a more stable and consistent source of power compared to relying on an external power source.

Secondly, PMGs are also more compact and lightweight compared to traditional excitation systems, making them easier to install and maintain. This is especially beneficial for larger alternators, such as the 27MW alternator mentioned, where space and weight considerations are important factors.

Additionally, PMGs are also more cost-effective in the long run. They require minimal maintenance and have a longer lifespan compared to traditional excitation systems, reducing overall operating costs for the alternator.

Lastly, PMGs also offer better control and protection for the alternator. They have built-in voltage and frequency regulators, which help maintain stable and safe operating conditions for the alternator. This is particularly important for non-black start alternators, as they may be connected to a larger power grid and need to adhere to strict regulations and standards.

In conclusion, the use of PMGs in non-black start alternators offers several advantages in terms of reliability, efficiency, cost-effectiveness, and control. Alternator manufacturers choose to fit PMGs to their machines to ensure optimal performance and minimize potential risks and issues during operation.
 

1. Why do alternator manufacturers fit PMGs to non-black start alternators?

Alternator manufacturers fit PMGs (permanent magnet generators) to non-black start alternators in order to provide a reliable and efficient source of power during start-up. PMGs are able to provide a source of excitation power to the alternator, which is necessary for it to start generating electricity. This is especially important in situations where the main power source is unavailable or unreliable.

2. What are the benefits of using a PMG in a non-black start alternator?

Using a PMG in a non-black start alternator offers several benefits. Firstly, PMGs are able to provide a higher level of excitation power compared to traditional methods such as residual magnetism. This ensures a more reliable start-up process. Additionally, PMGs are more compact and lightweight, making them easier to install and maintain.

3. Are there any drawbacks to using a PMG in a non-black start alternator?

While there are many benefits to using a PMG in a non-black start alternator, there are also some drawbacks. One potential issue is the higher cost of PMGs compared to traditional excitation methods. PMGs also require a small amount of DC power to operate, which may be a concern in certain applications.

4. How does a PMG work in a non-black start alternator?

A PMG works by using the permanent magnets to create a magnetic field, which then induces a current in the stator windings of the alternator. This current provides the necessary excitation power for the alternator to start generating electricity. Once the alternator is running, it can then provide power to the PMG, creating a self-sustaining system.

5. Can a non-black start alternator function without a PMG?

Yes, a non-black start alternator can function without a PMG. However, using a PMG offers several advantages, such as a more reliable start-up process and increased efficiency. In situations where a reliable start-up is critical, using a PMG is highly recommended.

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