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 spreadsheet programs such as Excel and Lotus 123
After the first economical 8bit 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, easytouse tools for use throughout the industry.
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:
 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 threephase motor drive. The 3phase mains Uin supplies the controlled rectifier R through the 3phase commutation choke CC. The DC voltage Udc is regulated by the rectifier and smoothed with the capacitor C. The 3phase AC voltage Uout is produced at the inverter outputs. The amplitude, frequency and form of this 3phase AC voltage are regulated with the inverter and rectifier.
Fig. 1
The typical form of the inverter output voltage per phase is illustrated in Fig.2.
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 (N1)*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 (N1) * 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*(N1)*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
Fig.3
Modulation frequency and inductance
The selection of the modulation frequency N*f plays a major role in interpreting the motor filter choke. 45 years ago, it was at approximately 23kHz. 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 shortcircuit 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:
 It had an excellent technical documentation.
 It seems to have mastered problems with thermal aging of the powder core.
 It manufactures a range of singlephase EECores (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 threephase motor filter is realised with 3 singlephase chokes.
 The motor filter choke can be built for a threephase motor power up to maximum 35 kVA with the largest EE core in the family (E610). In order to cover the threephase powers over 35 kVA, several chokes must be connected in parallel.
 The value and linearity of the shortcircuit 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 System
The threephase motor filter is realised with 3 singlephase chokes. The singlephase choke is to be calculated with the following parameters:
 Motor voltage = 3x400V, 230V per Phase
 Fundamental frequency = 50Hz
 Modulation frequency =20kHz
 Threephase motor power = 34.5kVA
Phase current =34500/3/230=50A
 Shortcircuit voltage Ucc=5%
Inductance L= (Ucc/100)*U/(2* ¶*f)/I=0.05*230/314/50=0.732mH
 The shortcircuit 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 E61026 from Micrometals. The gaps are implemented between two Eparts.
 Winding with Cufoil with layer insulation of 0.05mm, vacuumimpregnated.
All the above mentioned parameters are taken into consideration in the following Rale Design System input mask for calculating the chokes.
Fig.4
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:
Fig.5
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.
Fig.6
This contains the mechanical data of the choke. The winding has 44 turns with the Cufoil 0.4 x 70mm. The gaps of approximately 4.75 mm are placed in the positions 2 and 5 (see Fig. 6) between two Eparts 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.7
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.
Fig.8
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.
Fig.9
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 EEcores from the Micrometals Powder –26
 Motor voltage = 3x400V, 230V per phase
 Fundamental frequency = 50Hz
 Modulation frequency =20kHz
 Shortcircuit voltage Ucc=5%
 Ambient temperature =40 ºC, natural cooling
 Temperature rise =85ºK, insulation class F
 The gaps are realised between two Eparts
 Winding is vacuumimpregnated
Core
Powder
Centre leg
Window width
Window height
Stack
Gaps
Weight 

mm
mm
mm
mm
mm
g

E305
26
23.7
15.0
53,8
23.7
2x0.90
728

E305A
26
23.7
15.0
53.8
31.5
2x1.23
973

(1/2)E450
26
34.9
22.1
56.7
17.4
2x1.41
980 
E450
26
34.9
22.1
56.7
34.8
2x2.45
1960 
2E450
26
34.9
22.1
56.7
69.6
2x3.30
3928 
E610
26
47.2
30.0
77.0
47.2
2x4.85
4821 
Turns
.Diameter or
Foil
Resistance cold
Weight 

mm
mm
ohm
g 
140
1.60
0.172
350 
116
1.80
0.123
402 
108
2.36
0.074
713 
73
0.18x50
0.028
1263 
48
.0.28x50
0.016
1673 
44
0.40x70
0.007
3042 
Inductance
at current
and induction 
.mH
A
T 
5.92
8.50
0.60 
4.45
11.2
0.54 
3.00
16.9
0.63 
1.80
31.0
0.54 
1.18
42.3
0.39 
0.721
70.5
0.419 
Thermal current 
A 
6.0 
8.0 
12.0 
20.0 
30.0 
50.0 
Culosses warm
with Rac/Rdc 
W 
8.71
1.13 
11.2
1.16 
15.1
1.16 
17.5
1.10 
23.3
1.14 
30.3
1.22 
Core losses tot.
At 50Hz
current
Induction
Losses
At 19050Hz
current
induction
losses
At 20050Hz
current
induction
losses 
W
A
T
W
A
T
W
A
T
W 
15.8
6
0.60
3.97
0.30
0.030
5.90
0.30
0.030
5.94 
17.34
8.0
0.54
4.44
0.40
0.027
6.43
0.40
0.027
6.47 
23.1
12.0
0.63
5.74
0.60
0.0315
8.67
0.60
0.0315
8.71

28.5
20.0
0.54
7.38
1.0
0.027
10.5
1.0
0.027
10.6 
36.3
30.0
0.39
13.3
1.5
0.0195
13.3
1.5
0.0195
13.4 
51.1
50.0
0.419
13.5
2.5
0.021
18.9
2.5
0.021
19.0 
Temp. rise 
ºK 
79 
83 
81 
84 
84 
84 
No thermal
aging up to years 

>10

>10

>10

>10

>10

>10

