Designing Motor Filter Chokes with Rale Design Software

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:

• First, companies developed their own computer programs to meet their own specific requirements. These usually used already available algorithms and experience. After reaching acceptable levels to meet the company’s needs both in technical capability and ease of use, further development ceased.
• Secondly, small companies began to develop professional computer programs which are sold or leased to the manufacturers of transformers and inductors. With continuous input from the various manufacturers, they were able to develop universal, powerful, easy-to-use tools for use throughout the industry.

Designing with the Rale Design Software

The Rale Design Software 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:

• The user only fills in the input mask with the global parameters (voltage, current, temperature rise, regulation, etc.) and runs the program.
• After the design is finished by the program the user can switch to the Test Mode and change by hand the parameters of the designed transformer (turns, wire sizes, steel, ...) and run the program in order to redesign it. In this phase the user can also test his design, changing the input voltage, frequency, load, duty cycle,...

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:

• Fundamental frequency U with the frequency f (50Hz or 60Hz)
• First harmonic U1=(0.45 - 0.90)*U with the frequency (N-1)*f.
• Second harmonic U2=(0.45 - 0.90)*U with the frequency (N+1)*f

Accordingly, the current through the motor filter choke Iout essentially comprises of 3 components:

• Fundamental frequency I with the frequency f. This current is "impressed" and its amplitude depends on the motor power.
• First harmonic I1 with the frequency (N-1) * f. The amplitude of this current depends on the voltage U1, the modulation frequency N*f and the inductance L of the motor filter choke: I1=U1/(2* ¶*f*(N-1)*L).
• Second harmonic I2 with the frequency (N+1) * f. The amplitude of this current depends on the voltage U2, the modulation frequency N*f and the inductance L of the motor filter choke: I2=U2/(2* ¶*f*(N+1)*L).

Fig.3 illustrates 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:

• Maximum permitted voltage drop at the fundamental frequency
• Modulation frequency and fundamental frequency
• Steel
• Induction
• Insulation class
• Core power
• Price

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 Software 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:

• It had an excellent technical documentation.
• It seems to have mastered problems with thermal aging of the powder core.
• It manufactures a range of single-phase EE-Cores (E305, E450 and E610) with the appropriate bobbins (the typical model of the powder core is toroidal core).

The special features and results of this decision are summarised as follows:

The three-phase motor filter is realised with 3 single-phase chokes.

The motor filter choke can be built for a three-phase motor power up to maximum 35 kVA with the largest EE core in the family (E610). In order to cover the three-phase powers over 35 kVA, several chokes must be connected in parallel.

The value and linearity of the short-circuit voltage of all chokes must be equal in the entire current range. Thus, the current distribution and the thermal load of the chokes connected in parallel proportional to the core power of the individual chokes is ensured.

At an operating temperature over 75 ºC, special attention must be paid to the thermal aging. Under circumstances, an "old" powder core can have higher losses than before its first operation. Because of this, a powder core choke must be equipped with a thermal fuse. This is recommended in particular when using the powder cores from companies that provide no information regarding the thermal aging and/or where the modulation frequency is over 10kHz.

Design Example with Rale Design Software

The three-phase motor filter is realised with 3 single-phase chokes. The single-phase choke is to be calculated with the following parameters:

• Motor voltage = 3x400V, 230V per Phase
• Fundamental frequency = 50Hz
• Modulation frequency =20kHz
• Three-phase motor power = 34.5kVA

Phase current =34500/3/230=50A

• Short-circuit voltage Ucc=5%

Inductance L= (Ucc/100)*U/(2* ¶*f)/I=0.05*230/314/50=0.732mH

• The short-circuit voltage Ucc was selected in such a way, that the harmonics I1 and I2 remain less than 5% of the motor current I. Thus, the motor filter could be implemented without the capacitors FC (see Fig.1).
• Harmonic I1=U1/(2* ¶*N*f)/L=0.9*230/414/N/0.000732=2.25A, 19050Hz

Harmonic I2=I1, 20050Hz

The harmonics I1 and I2 are increased by 10%. This compensates the influence of the harmonics that were not considered: I1=I2=2.5A.

We took into consideration the bipolar form of the inverter output voltage when applying the amplitude U1=U2=0.9*U (Fig.2).

• Ambient temperature =40 ºC, natural cooling
• Temperature rise =85ºK

The choke is manufactured in the insulation class F. For protection, the choke must be provided with a thermal fuse for the response temperature at 150ºC.

• Powder core E610-26 from Micrometals. The gaps are implemented between two E-parts.
• Winding with Cu-foil with layer insulation of 0.05mm, vacuum-impregnated.

All the above mentioned parameters are taken into consideration in the following Rale Design Software 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:

• All chokes are designed with EE-cores from the Micrometals Powder –26
• Motor voltage = 3x400V, 230V per phase
• Fundamental frequency = 50Hz
• Modulation frequency =20kHz
• Short-circuit voltage Ucc=5%
• Ambient temperature =40 ºC, natural cooling
• Temperature rise =85ºK, insulation class F
• The gaps are realised between two E-parts
• Winding is vacuum-impregnated