Designing Motor Filter Chokes with Rale Design System
In the beginning…
Thirty years ago, designers calculated transformers on their pocket calculators. The designer had to pencil in all the input and output fields onto a form and then feed them into the calculator. Today, he can forget the pencil, but he still has to enter the figures into spread-sheet programs such as Excel and Lotus 123
After the first economical 8-bit computer became available in 1978, professionals could begin to develop programs to design transformers and inductors. This development went in two directions:
Designing with the Rale Design System
The Rale Design system automatically calculates designs for transformers and inductors. Consequently, its data base incorporates all the necessary materials including cores, bobbins, wires, steels, etc. in both metric and USA units. This data base is totally user expandable. To use the programs, the designer needs only a basic knowledge of transformers or inductors and their operation mode. The designer does not need to use any complicated formulas, he only needs to follow two simple phases:
About Motor Filter Choke
Fig.1 illustrates the main circuit diagram of a three-phase motor drive. The 3-phase mains Uin supplies the controlled rectifier R through the 3-phase commutation choke CC. The DC voltage Udc is regulated by the rectifier and smoothed with the capacitor C. The 3-phase AC voltage Uout is produced at the inverter outputs. The amplitude, frequency and form of this 3-phase AC voltage are regulated with the inverter and rectifier.
The typical form of the inverter output voltage per phase is illustrated in Fig.2.
At an inverter modulation frequency N*f, the output voltage Uout essentially consists of three components:
Accordingly, the current through the motor filter choke Iout essentially comprises of 3 components:
Fig.3illustrates the current through a motor filter choke under the following conditions: I=100A, I1=I2=5A and N=36
Modulation frequency and inductance
The selection of the modulation frequency N*f plays a major role in interpreting the motor filter choke. 4-5 years ago, it was at approximately 2-3kHz. Today, modulation frequencies of 16kHz to 20kHz are used. For this reason, motor filter chokes are almost exclusively equipped with powder cores.
Selecting the optimum inductance L of the motor filter choke is a difficult task and depends on several parameters:
An optimum solution can only be found with a close cooperation between the power electronics engineer and the manufacturer of the motor filter choke. Normally, the short-circuit voltage Ucc=100*(2* ¶*f)*L*I/U of the motor filter choke for the modulation frequencies between 2kHz and 20kHz lies between 20% and 5%.
Powder cores and their modular operation
Motor filter chokes are calculated below with the Rale Design System for the modulation frequency of 20kHz. After careful examination, we ascertained that the optimum solution today could be realised only with high flux density powder cores from Micrometals, USA.
The reasons for selecting this company are as follows:
The special features and results of this decision are summarised as follows:
Design Example with Rale Design System
The three-phase motor filter is realised with 3 single-phase chokes. The single-phase choke is to be calculated with the following parameters:
All the above mentioned parameters are taken into consideration in the following Rale Design System input mask for calculating the chokes.
The two graphs above depict the inductance vs. current and the form of the thermal current.
The calculation results are shown in the following Fig.5 and Fig.6:
In the first choke equivalent circuit, the leakage inductance of 0.226mH, the core inductance of 2.938mH and the gap inductance of 0.607mH at the current of 70.5A can be seen. The resulting inductance is 0.730mH. The program did not accept the specified induction of 0.97T due to thermal reasons and was optimised to 0.373T. The second equivalent circuit shows the same inductances at the thermal current peak of 77.55A.
The minimum, average and maximum induction in the core are shown to the right of the equivalent circuits. The induction Bx and By are responsible for generating the eddy current losses in the winding. It must be mentioned here that the magnetic fields was calculated with the help of numerical methods.
The core losses and the induction of each current harmonic as well as the core temperature are important for assessing the thermal aging of a powder core.
This contains the mechanical data of the choke. The winding has 44 turns with the Cu-foil 0.4 x 70mm. The gaps of approximately 4.75 mm are placed in the positions 2 and 5 (see Fig. 6) between two E-parts of the core. The desired inductance is calibrated with the help of the calibration voltage 7.87V, 50Hz and the current 34.31A.
Fig. 7 (thick, blue line) illustrates the inductance vs. current up to 300A. The inductance between 35A and 70A is linear, which is required for a modular operation in parallel with other chokes with the same linearity.
Fig.8 illustrates the induction in the core and through the winding. You can see that the core induction of approximately 0.4T optimised by the program is well below the saturation induction 1.4T. If the Powder –26 had three to four times lesser loss, then the optimum induction would be approximately 1T and the core powder E610 would be double.
Normally, not all the choke parameters calculated by the program (number of turns, foil dimensions, gap…) can be accepted. The designer can therefore make use of the test mode in the program (Fig. 9) to modify almost any choke parameter also manually and to check the choke under different operating conditions.
In this instance, the choke was calculated with the unipolar form of the inverter output voltage (Fig. 2) and with an output current increased by 20%: I=60A, I1=I2=1.25A. The calculated temperature rise of 82.5º K shows that a power increase of 20% is possible.
Construction specifications and properties of the calculated chokes
The table below shows the most important parameters of the 6 calculated chokes. It is possible to protect any motor up to 140kVA by connecting the chokes in parallel.
The following parameters are for all 6 chokes equal: