# Damping in generators

1. Mar 5, 2014

### Fluidman117

Hey,

I understand that damper windings are used to stabilize the speed of the rotor under varying loads. The rotor speed is decreased or increased by the induction principle.

Physically the damped windings are some sort of copper bars inserted into the pole faces of a rotor and all connected together by a copper ring.

Here comes my problem:

I am working on a project, where I have a generator that needs to have a certain damping coefficient (units in my case are kg/s). Is there a formula to determine the damping coefficient of my generator?

2. Mar 5, 2014

### jim hardy

They're simply a squirrel cage winding. So their resistance will determine torque/slip relationship.

I've never run across the specific formula you seek,

here was my first hit
http://itee.uq.edu.au/~aupec/aupec99/chetty399.pdf

i'm told google personalizes search results - is that same link you got?

3. Mar 5, 2014

### Fluidman117

Thanks for the reply. Then I could say that the damping coefficient is just the rotor resistance. Thus it would also be possible to modify this resistance by adding an external resistor circuit to the main circuit and modifying the external resistance I could modify the damping coefficient.

I got the same first link. But it could also be that we have very similar interests when googling :)

4. Mar 5, 2014

### jim hardy

Well, the damping coefficient is a function of rotor resistance and probably some other physical things about the generator .
I'm not a generator design guy but i've known a few and they are amazing. Hopefully there's one on the forum , if i just 'prime the pump' it's about all i can do.

Since the damper/amortisseur windings are squirrel cage they don't come outside the machine which means you can't add resistance to them except in a computer simulation. Their resistance is controlled by their shape, and any good book on induction motors will tell you about that.

There exists inside the voltage regulator an adjustment called "damping" which basically slows the regulator down.
One adjusts that with machine not connected to grid;
apply a step change to voltage adjust, observe response, adjust for the slight overshoot typical of any 'critical damped' closed loop .

When the excitation is changed on a machine that's connected to the grid it will come to a new power angle, as you know. So there is a mechanical interaction between strength of excitation and the inertia of the rotating apparatus. Field strength (excitation) is the "spring constant", if you will, between torque and angle of rotor to stator fields(power angle) .
The natural frequency of that rotating spring-mass system was around 1hz for our big central station steam turbine-generators.
That's where your damper windings help out - they prevent rotational harmonic oscillation . Any 'speed reduction' is brief, only while slip exists ie coming to a new power angle.

I don't know what you're doing - just hope this clears up some basics.

If you get really interested in big generator rotors, here's an outfit that tests them for shorted field turns.
http://www.generatortech.com/B-Page2-Theory-Overview.html

old jim

5. Mar 6, 2014

### Fluidman117

You are right about the squirrel cage. However, I found this:
http://www.uwig.org:8080/index.php?...on_Generator_with_External_Resistance_Control

And seems that when opting for the wound rotor it is possible to modify the resistance by external circuit.

And here comes another questions. When I have an external power electronics network, that is able to modify the resistance, does the act of modifying the resistance value in mid operation consume any amount of power or it can be considered negligible compared to a the power output of a 500kW generator?

6. Mar 6, 2014

### jim hardy

You're headed into very interesting territory here.

A wound rotor machine can do a LOT of things...

If you allow rotor alternating current to flow you have an induction machine which can be generator or a motor. For your project we'll stick to generator...
Induction machine will generate if driven faster than synchronous , ie 'negative slip'. Indeed there's some power dissipated in the rotor resistance but not a lot at reasonable slip and resistance.
Look at speed - torque curves for induction machines.

In my college machinery lab course we had a 7.5kva wound rotor machine and dynamometer on which we ran speed-torque curves . We were studying its characteristics as a motor. Our field resistor bank was small compared to the machine. So yes there's power dissipated but small compared to the machine , and in proportion to slip.

Now comes the interesting part...
If you apply DC to that wound rotor it becomes an electromagnet and you have an everyday synchronous machine. It operates at zero slip,,,,
except while changing power angle - and that's where your damper windings come into play.

So - IF you have control of the field current
Could you modulate it to produce any damping you want?
Of course you could.
That's what is done in big machines, ours was 984,000 kva.
Your field current algorithm might include terms for slip and power angle, as well as terminal voltage and reactive power.
This is actually done on big utility machines to damp both "subsynchronous resonance" and "power system instability".

Google those terms and it takes you into another world.

Briefly, subsynchronous resonance is a phenomenon where something in the electrical distribution system resonates with the mechanical torsional natural frequency of the turbine-generator shaft. It can cause the shaft to fracture and blades to fly off it. Our shaft's torsional frequency was around seven hz. One avoids loading the generator with anything that has that natural frequency and the voltage regulator is low passed well below it..
"Power System Instability" is where the rotating inertia of all the machines in one region set up an oscillation against those in another region, say South Florida against North Florida and Georgia. It tends to happen where transmission lines are skimpy. A "Power System Stabilizer " detects variations in slip at the natural frequency of the power system and sends a correction term to field current that damps them. They'll be less than 1hz, i saw some at 2/3 hz.

and compensation for reactive current too
Plus the amortisseur windings
the machine was quite stable, because it had been designed so by geniuses at Westinghouse ( the late Chester Raczkowski comes to mind)
A power system stabilizer got added when we noticed one was needed. Prior to that we'd way overdamped our voltage regulators to suppress those 2/3 hz system swings..

I hope you get interested in this field - the country needs capable power system engineers.
I was a lowly plant maintenance guy who got to look over the shoulders of some experts.
Like all great men they were kind and would explain to an interested workingman what they were doing.
We stand on the shoulders of giants like Chester Raczkowski.

Last edited: Mar 6, 2014