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ABSTRACT. Induction heating is the most progressive method for brazing of squirrel cage (SC) type rotors of electric motors. Frequencies from 3 to 10 kHz are typically being used for brazing of relatively large rotors (diameter more than 200 mm). If the ring thickness exceeds its height, flat single or two-turn inductors with concentrator are located under the ring instead of the round coil surrounding the ring. The rotor is standing on the top of the coil providing high pressure onto the joint components; a gap between the coil and ring is minimal and constant during the heating process.
This study describes a modified system with concentrator made of magnetic composite Fluxtrol 100. Frequency was much higher (around 50 kHz) than traditionally used (3-10 kHz). Electromagnetic and thermal coupled simulation with Flux 2D used to compare the process parameters and temperature distribution dynamics at 3 and 50 kHz. It was found that at higher frequency the brazing quality and time are approximately the same as at lower frequency. Electrical efficiency is slightly higher at 50 kHz while the coil current is significantly lower. Computer simulation at different powers showed that for a larger rotor the minimum required power is 70-75 kW. At lower power brazing time quickly increases and at 50 kW reaches 16 min instead of 5 min at 75 kW. Electrodynamic forces between the coil and rotor at 75 kW equal to 250 N at 50 kHz and almost 950 N at 3 kHz.
Thermal simulation of the coil proved that the maximum temperature of Fluxtrol 100 concentrator is below 200 C, which is acceptable for this material. Experimental and then industrial tests confirmed the results of simulation.

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  1. 1. Confidential and Proprietary Information of Fluxtrol, Inc. Auburn Hills, MISimulation of InductionSystem for Brazing ofSquirrel Cage RotorDr. Valentin Nemkov(1), Dr. Valentin Vologdin(2)Dr. Vl. Vologdin Jr.(2), Kevin Kreter(1)(1)Fluxtrol, Inc., USA; (2)Freal, Ltd., RussiaPadua, Italy, May 21-24, 2013
  2. 2. Specs and System Description• Copper ring must be heated up to temperature 800-850 C• Single-turn coil has П-shaped concentrator made of Fluxtrol LRM• Concentrator is attached to the coil with a thermally conductiveglue, k = 0.01 W/cmK• Ceramic fiber insulation pad between the coil and ring hasthermal conductivity k = 0.002 W/cmK; thickness of pad is 2 mm• Motor bars are substituted for a tube filling the whole groove;thermal conductivity and heat capacity of the tube reduced byspace factor g = 0.42• Simulation was made at a constant coil voltage corresponding tomean power equal to 50 or 75 kW during the heating time• Program Flux 2D has been used for simulation2
  3. 3. Brazing Setup and Joint Cross-section3№RotortypeD1,mmD25, mm D21, mm D22, mm D23, mm D24, mm h, mm H2, mm1 АDТ-3 170 151 155 211 165 206 14 3482 DТА-1 242 223 222 298 243 294 20 500Case 1
  4. 4. Temperature at Braze Joint401002003004005006007008009000 50 100 150 200Temperature(C)Time (s)3 kHz50 kHzCase 1, 50 kWStudy was made fortwo rotor sizes(case 1 and case2), two levels offrequency (3 kHzand 50 kHz) andtwo levels of power(50 and 75 kW)
  5. 5. Braze Thermal Profile5Case 1, 50 kHz, 50 kW• Ii = 6300 A• Ui = 21 V• Coil losses: 23.6 kW• Ring: 26.2 kW• Fluxtrol: 0.2 kW• t = 205 seconds
  6. 6. Rotor Cool Down601002003004005006007008009000 500 1000 1500 2000Time (s)Case 1 - 50kHzCase 1 - 3 kHz75 kW 50 kWTemperatureat braze jointTemperature curves show that heat losses in bars are very high andthermal efficiency at 75 kW at the end of heating is less than 50%
  7. 7. Coil Thermal Profile at the End of theProcess7Case 1, 50 kWFrequency 50 kHz, Tmax = 160 0CFrequency 3 kHz, Tmax = 150 0C
  8. 8. Braze Thermal Profiles8Case 2, 50 kHz, 75 kWCase 2, 3 kHz, 75 kWTemperature profiles are almost the same for 3 kHz and 50 kHz
  9. 9. 0200400600800100012000 50 100 150 200 250 300Temperature(C)Time (s)3 kHz50 kHzTemperature at Braze Joint9Case 2, 75 kW
  10. 10. Coil Thermal Profile at the End of theProcess10Case 2, 50 kHz, 75 kWTmax = 200 0CCase 2, 3 kHz, 75 kWTmax = 190 0CTmax of concentrator is close to upper limit for Fluxtrol material and must be keptunder control by material selection, concentrator design and proper manufacturing.Intensive coil cooling is required
  11. 11. Simulated and Experimental ProcessParameters11RotortypePower, kW FrequencykHzTime sec NotePtotal Pcoil Pring PмagnАТD-3Case 150 22.8 26.7 0.5 50 185 Model (Case 1)50 - - - 54 170 Experiment50 23.6 26.2 0.2 3 205 Model (Case 1)DТА-1Case 250 21.6 28.1 0.3 50 925 Model (Case 2)50 - - - 46 930 Experiment75 34 40.6 0.4 50 310 Model (Case 2)75 35 39.8 0.2 3 335 Model (Case 2)
  12. 12. 0200400600800100012000 50 100 150 200 250 300Axial Forceon Ring(N)Time (s)3 kHz50 kHzElectrodynamic Forces on Ring1275 kWForces are rather high especially at 3 kHz. They can cause vibration and noise
  13. 13. End of Test Brazing ProcessOperatorcontrols brazingquality and addssolder whenrequiredRed spot in thecircle shows apoint of thetemperaturecontrol usinglaser aiming
  14. 14. 14Conclusions• 2D simulation gives accurate enough results despite several assumptions• Frequency variation in the wide range from 3 to 50 kHz does not influencethe process efficiency and temperature distribution much• The use of frequency below 8 kHz may be limited by forces and noise• The use of the higher range of frequency may be limited by high coilvoltage and elevated temperature of SMC concentrator• Optimal frequency for the studied rotors is in a range of 10-30 kHz• Process is sensitive to a minimum power level. Brazing of rotor no. 2 at50 kW is marginal. Heating time is almost triple compared to 75 kW• Simulation and experiments showed that SMC concentrators may beeffectively used in rotor brazing• Computer simulation may be used as a powerful tool for design of rotorbrazing processes
  15. 15. Thankyou!15I Protest against InductionBrazing of My Cage!