Transformers and Turbo Generators

I had that exact same question on one of my final exams............

Wow! A smack down on the first post!!!!!!!!!!!!!

Did you get the answer right?

Welcome.

Don't hold back!

Any magnetic core is designed for a certain maximum flux density. Flux density is Flux/Core Area. During overfluxing, this maximum flux density is crossed and the core is driven to saturation. The analogy is Population Density. Imagine a confined space where there is enough space for only, say, ten persons. What would happen if ypu pack 20 persons in the same space or 30 or 50...? There is bound to be choking and physical harm to the persons present.Isn't it? It is the same what is happening to a magnetic core during over fluxing.

The concept was answered by #3. What harm? Lots of things but if you cause the secondary to go to high in voltage the circuit you are driving may be damaged. Also the insulation can break down and burn out your transformer or generator.

Hello friends,

I am completely happy with the suggestion of above said friends.

Further, imagine if some iron piece is permanently magnetised, then what will happen? We have constant magnetic flux. There will no change in flux with respect to change in current. Hence, efficiency of the machine will reduce drastically. And we loose the loss of generation. Above answer is typically for large size generators & transformers.

But as per the latest technology, sometimes OEM's uses permanent magnet in rotor of motor up to 355frame. This is know as optimum flux technology. This will increase efficiency and also very compact. Such motors uses inverters for their operation.

Suvek Kumar

As users of magnetic devices such as transformers, we specify certain parameters such as primary and secondary voltage, KVA, etc. and trust the transformer designer and builder have done their jobs. Every magnetic core material has a property called saturation flux density. When the transformer is used beyond its capacity the core will saturate and no longer function as a transformer. Slight overloading will cause distortion of the waveform. As loading becomes greater, the tranformer will make audible sounds, cease to function entirely, overheat, and can eventually destroy itself.

Transformers are designed with a certain number of turns, both for the primary and the secondary part. Applying the respective voltage (at the respective frequency) will not lead to saturation. The only case of saturation may appear if a voltage higher than the voltage for which the transformer has beend designed is applied or if coil turns are damaged (shortcircuited) leading to a total lower number of turns and thus to a higher required current.

Under normal circumstances, in order to operate in the same point on the hysteresis curve (B-H) of the material, the ratio of applied voltage /frequency has to remain constant. This is valid for a frequency range close to the nominal frequency ( +/- 20-30%) . At higher frequencies ( 2 or 3 x) the characteristic changes and losses go up, etc.

Many many good answers are appearing from participants to this question.

I will try to explain my concept of "over fluxing" to further understand it. Tranformers and Generators and mostly all AC circuit equipments are designed with fixed ratio of V/f. Typically almost all generator excitaion circuits ( Generator field switches are interlocked to get switched on after the rated or about 90 % of speed is reached) . Now overfluxing takes place ( if this interlock fails or bypassed for some reasons) when we excite the genertor below rated speed ( low frequency) means we are trying to build up voltage which has tendency going towards DC( Zreo frequency) and now imagin if we apply such voltage having tendency as DC voltage ( but not pure DC) what would happen to the transformation process or transformer which has very low resistance winding.It would act as low resistance load for full applied voltage and less back e.m.f to oppose applied voltage resulting high current and damaging/burning of winding . So as the excited voltage reaches full value and speed or frequency is less it would try to damage the winding.

I do not understand what you described earlier. Synchronous generator excitation is in DC. What is the "generator field switch" supposed to do ?

In order for a generator designed to rotate at speed "n" to deliver voltage "U" at the frequency "f", to deliver the same voltage "U" at a lower speed (0.5 x n, and implicitely at 0.5xf)), the necessary excitation current IE needs to be at least 2 x IE if the generator has been designed somewhere on its material linear characteristic (hysteresis). If it is close to the knee-point, an increase in current brings it on its saturation curve part, so the the necessary excitation current is > 2 x IE.

Transformers are designed to work within a range of frequencies ( nominal frequency +/- 10%), a normal transformer can definitely not work in DC. (Applying DC to its windings, is basically equivalent to shortcircuiting it).

Yes Tomad, exactly generator field excitation is DC, but here we are talking about over fluxing which happens in generator winding and generator main step-up transformer. Field switch is in generator exciter field which is interlocked with speed of turbine/generator so that only can be switched on if the speed is more than 90 % of rated speed.

Many ways to express the same concept. For me, the easiest way to understand the princple of saturation is that there is a volt x time product that will cause saturation in any core. So, for a given core, you can have a high voltage on a winding for a short time or a low voltage for a long time. But you can't have a high voltage for a long time. This means that if there is even a small DC component in your transformer exitation, you will eventually saturate the core.

The AVRs on all but the smallest generators have an "Under Frequency Rolloff" characteristic to avoid driving loads with too much voltage as the gen slows down (or speeds up).

You have answered your own question. X_L ∝ f thus the windings of transformer or motor loads will draw more current until they trip a protective device or burn out.

Very small generators, without an AVR, usually have a declining voltage as part of their excitation system anyway (capacitor or compound transformer excitation)

I suppose the field switches mentioned are for the big boys in the MW range, my experience only extends to a bit over 100kW,

regards Chas