Sunday, July 3, 2016

Electrical machines

Been a while since I blogged, so I thought I should post. Most of my updates on the software is on my facebook page:

https://www.facebook.com/pythonpowerelectronics/

The latest case I posted was of a single-phase three winding transformer. I spent almost two weeks on this and most of that time was trying to build a transformer model taking into account the magnetic circuit of the transformer. A magnetic circuit would allow a detailed model to be built if necessary - magnetizing current, core losses etc. The problem was that modeling the magnetic circuit involved calculating the flux and from this flux the induced emfs.

flux1 = L1*i1 + Lm*i2
emf1 = d/dt (flux1)

Where i1 is the primary current, i2 is the secondary current, L1 is the inductance of the primary and Lm is the mutual inductance between the windings. The problem is with the differentiation. Once you perform a differentiation, you magnify any noise and errors in the simulation. Which also means, when a non-linear component such a diode rectifier is connected, the simulation becomes unstable. This is the reason why the test case considers a diode rectifier load as it was needed to check whether the simulation was stable with this.

The second point was the method of simulating the transformer. It would have been convenient for the user to simply add a single transformer component with connections rather than to have to connect all the components together and furthermore to take into consideration the polarity of the ammeters and voltmeters. As of now I can't think of a way of doing this.

An advantage of this form of transformer modeling could be stated that a user has more control over the connections. But only a small fraction of users would really need this level of control on the modeling of a single component and that too one as basic as a transformer. For most people a transformer is basically for providing isolation or transformation between two circuits at different voltages. Without a transformer, all components would need to be transformed to one of the terminals and that means conversion of the impedances etc which could be cumbersome.

This also brings up another issue. As the circuit grows bigger, it would always be advisable to test parts out separately before integrating them together. For example, consider the case of two three-phase voltage source converters operating in current control mode connected to another three-phase voltage source converter operating in voltage control mode. It would be advisable to test each converter out separately. The problem comes when you begin to put them together. Let's say inverter 1 and inverter 2 have been tested. When putting them together, inverter 2 will have to be copied into the same spreadsheet as inverter 1. This might mean that inverter will need to be moved to another part of the spreadsheet. And this implies having to change all the parameters of Inverter 2 such as polarity of switches and diodes which could only make this integration error-prone.

A way to minimize this is that there is no need to have the entire circuit in one spreadsheet. Say all three inverters were in separate spreadsheets while testing. After testing, they remain in separate spreadsheets but are only connected together by jump labels. This would mean the parameters of these three inverters in their own spreadsheet do not change. This could make the entire process of building up a circuit so much simpler.

And this in turn might to some extent solve the problem of modeling transformers and machines. If the transformer circuit and its parameter spreadsheet always remain separate, a user can just use ready made spreadsheets over and over and not bother about changing any parameters. Only jump labels need to be added to connect them to other spreadsheets.

So this will be the next major release. The simulator should be able to deal with a circuit in multiple spreadsheets. This might take me a while.

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