generator rotor brakes 1

[TransD]

Hello,
I work at a hydroelectric plant that uses the field of the rotor for a brake once the unit is taken offline. As you can imagine if there is a little wicket gate leakage, it can take a long time to stop. We have had issues with the exciter tripping due to raising the voltage too high to apply extra brake. We realize with this type of system, the wicket gates need extra attention as well. These are 3 to 5 megawatt units. Does anyone know of a mechanical brake supplier that could pulse a brake to help us get the unit to slow down? The sync speed of the unit is 164 rpms. Thanks for any help.

 

[Skogsgurra]

I assume that the stator is short-circuited when you brake the machine?

Is there also a set of resistors in the circuit? Without them, you will usually not get any effective braking. That is because the circuit is almost purely inductive and your stator current will be at almost 90 degress, lagging, and there will not be much torque produced.

If there are no resistors, you should add sometning like 0.1 to 0.5 ohms between the phases. That will produce a current that is less reactive and more resistive so that the machine stops quicker. The 100 to 500 milliohms are a rough preliminary guess and the value depends, of course, on the rated voltage and power of the machine.

A mechanical brake shouldn't be needed unless you want to keep the machine at exact stand-still. But there are usually other means to do that.

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[TransD]

Using the field voltage as a brake gradually lowers the stator voltage as the rotor rpms comes down on our horizontal units. For the past 20 years we have used a GE exciter without V/HZ protection. Recently we switched to an updated exciter with V/hz protection turned on and that tends to trip when the rpms goes below 10 hz. Thanks

 

[crshears]

The slower the rotational speed of the unit becomes, will not the dynamic braking effect be diminished?

There were hand-operated mechanical brakes on the 2 MW hydaulic units I ran a few years back, and since those units did not have oil lift pumps there were instructions to take the units off line, close and pinch the wicket gates as far as possible [which wasn't always successful due to debris, most often chunks of wood, getting wedged in there] and, once the speed had dropped to about one-third or so of normal, apply the brakes firmly to bring the unit to rest. Once the oil film had squeezed out of the bearings, which we usually gave an hour for just to be on the safe side, the brakes could be released [in preparation for the next start]. As long as the wicket gate leakage wasn't too severe, the units wouldn't creep after that.

In another plant I worked in later, some of the 65 MW units did not have lift pumps either, and much the same procedure was followed - - except that these units had pneumatic brakes automatically applied on descending speed once the dynamic braking became relatively inneffective. Unfortunately the auto sequence was designed to release the brakes five minutes or so after shutdown, which wasn't always enough; if we received a creep alarm we'd have to proceed post-haste to the unit and re-apply the brakes manually [bearing saving requirement].

If I had my druthers, I would want mechanical brakes installed, and would make using them a matter of standard procedure, oil lift pumps or not.

After-market unit brakes? Now there's a poser; is there a convenient flat area on the underside of the rotor which could either serve as a braking surface for shoes to press against or to which a braking surface could be attached?

I've tried internet searches, but the correct keywords seem to be eluding me...

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[Skogsgurra]

Yes, the EMF is proportional to speed. So it goes towards zero as speed goes to zero.

But, so does the reactance in the rotor winding. So, it may very well be that you have more braking torque at lower speeds (not at all like a DC motor). By adding resistors, you will keep current in phase with the induced voltage so that the torque is high all the way from sync speed and down to near stand-still.

Then, at stand-still, you can use mechanical means - perhaps what crshears says.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.