Motor Load Damping Factor

[cdee333]

Can someone please explain load damping factor as it relates to electric motors?

What information is required to calculate the load damping factor?

 

[electricpete]

I remember studying power system stability, there was a damping parameter D which related to the aggregate loading in a system. Is that what you're talking about?

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(2B)+(2B)' ?

 

[cdee333]

Yes, that is exactly what I'm talking about.

 

[electricpete]

I looked in a few Power System Stability books including the huge EPRI textbook on Power System Stability by Kundur and surprisingly found absolutely nothing on a damping factor associated with the power system loads. But lots of discussion for damping associated with sync generators.

So I dug out from the crates in the corner of my garage my old course notes from Power System Stability with Professor Begovic at Georgia Tech. For induction motor with frequency fe applied he draws motor torque speed curve and load curve similar to a centrifugal pump. He computes the mechanical power at the operating point and calls it Pm.

Then he shifts the electrical frequency fe up a little bit to fe' which creates new torque speed curve and new operating point with corresponding power Pm'.

He defines D = dP/df = (Pm' - Pm) / (fe' - fe)

There are also ways to compute D for other loads besides motors. Somehow they are rolled into a composite D.

The use of this D in the power system swing equation which is shown on upper right corner of page 5 attached (article by Begovic that I found thru google)


=====================================
(2B)+(2B)' ?

 

[electricpete]

Apparently a block of loads would be grouped with each generator for purposes discussed above.

Hopefully you can see that in the swing equation matches a simple SDOF system:
position x <-> angle delta
force F < - > power P
damping c = F/(dx/dt) <-> D = P / [d/dt(delta)] <-> P / w
or linearized as dP / dw
There may be a factor of 2*pi missing somewhere

=====================================
(2B)+(2B)' ?

 

[cdee333]

Well thank you for digging out your old notes!

It doesn't seem like the damping factor is something that I can calculate with the information I have. I am doing a stability study with some large induction motors, and I only have their nameplate data. Perhaps the manufacturer can provide the damping factor of each motor?

 

[cdee333]

Also, I found some information in the Handbook of Electric Motors By Hamid A. Toliyat. He says that the theoretical determination of the damping factor is extremely complex and must be determined experimentaly. He says the damping factor is usually in the range of 0.01 to 0.03 for induction motors. Does this seem correct? I wish there was a way that I could make a better judgement as to what the value would be for my motors.

 

[cdee333]

So, I just discovered that this damping factor is not a parameter of the motor, but a parameter of the type of load connected to the motor.

The loads of the motors I am working with are screw compressors. Any ideas of what the damping factor of a screw compressor is?

In the model I'm using, the damping factor exists in the Load Torque equation:

Tload = Tnom*(1 + delta_w)^D

where:
delta_w is the motor speed deviation from nominal (per unit slip)
Tnom is the motor load torque at synchronous speed
D is the damping factor