Losses in Toroidal CTs 2

[nuzelac]

I am performing testing on toroidal CTs and have hard time undestanading the results. WOuld like to hear your thoughts.

Test set up is extremaly simple. Constant current generator is applying the primary current through the CT, and I am measuring voltage across resistor that is connected to the CT. I am finding that at low level primary currents (20A for 600A CT), the losses of the CT are increasing when the external pressure is applied to the CT. For example, 1000:1 CT will have 19mA secondary current witout pressure, and then it will drop to 16-17mA with the external pressure.

The working hypethesis is that losses inside the CT are increasing, thus decreasing secondary current for constant primary current. The problem is that I can't relate the pressure influence to the hysteresis or/and eddy current losses.

Any thoughts on your side?

 

[dpc]

How are you applying pressure to the CT?

 

[scottf]

External pressure is known to negatively affect accuracy of CTs. The pressure works to change the magnetic properties of the core material, i.e. reducing the magnetic permeability. So, in a sense, yes, the core losses are being increased.

Some core material, such as "nickel"-type CT cores are softer material and even more susceptible to effects of external pressure. When we make high-accuracy CTs with this material, we often place them inside higher strength shells to take the winding and outside pressure off of the cores.

 

[nuzelac]

Thanks for answers!

The pressure is applied by molding the CT in the epoxy. Once the CT is molded in the epoxy, the losses are increasing. For example,on 500:1 CT, we are loosing 5-10mA on the secondary (with 20A on the primary) after molding.

I've simulated the same phenomena pressure using a good old method - wrapping and tighting a hose clamp around the CT.

Interestingly, the process is reversable: Decrease in pressure results in decrease in losses.

I don't know how the pressure will affect the hysteresis losses. If someone can explain, or send me some useful link, would be great. Theoretically, I undestand that edddy current losses can increase if the laminations are squezzed together, and instead of laminated toreoid we get the solid core.

 

[Skogsgurra]

It is probably not eddy current losses that increase, but permeability that is reduced when you apply a force to the core. The phenomenon is sometimes used to measure forces. See "Pressductor".

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[scottf]

nuzelac-

I'm very familiar with what you're seeing. We make cast-type CTs as well and one has to take measures to reduce the pressure on the core during the molding process. Part of the "art of manufacturing"

 

[nuzelac]

Thanks Guys! I guess we need to improve the art of manufacturing

For example we found out that wrapping the CT with the soft silicone tape helps to reduce pressure and thus improve CT performance. However, it doen't work 100%.

BTW, I've looked at the Pressductor and found out that "The permeability of the steel is reduced in the direction of the force and increases in the right angle direction to the force.". Anyone have a white paper on this?

I've started to search IEEE Xplore papers, no luck so far