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SIMULATION OF INDUCTION SYSTEM FOR BRAZING OF SQUIRREL CAGE ROTOR

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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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SIMULATION OF INDUCTION SYSTEM FOR BRAZING OF SQUIRREL CAGE ROTOR

  1. 1. Confidential and Proprietary Information of Fluxtrol, Inc. Auburn Hills, MI Simulation of Induction System for Brazing of Squirrel Cage Rotor Dr. Valentin Nemkov(1), Dr. Valentin Vologdin(2) Dr. Vl. Vologdin Jr.(2), Kevin Kreter(1) (1)Fluxtrol, Inc., USA; (2)Freal, Ltd., Russia Padua, 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 conductive glue, k = 0.01 W/cmK • Ceramic fiber insulation pad between the coil and ring has thermal 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 by space factor g = 0.42 • Simulation was made at a constant coil voltage corresponding to mean power equal to 50 or 75 kW during the heating time • Program Flux 2D has been used for simulation 2
  3. 3. Brazing Setup and Joint Cross-section 3 № Rotor type D1, mm D25, mm D21, mm D22, mm D23, mm D24, mm h, mm H2, mm 1 АDТ-3 170 151 155 211 165 206 14 348 2 DТА-1 242 223 222 298 243 294 20 500 Case 1
  4. 4. Temperature at Braze Joint 4 0 100 200 300 400 500 600 700 800 900 0 50 100 150 200 Temperature (C) Time (s) 3 kHz 50 kHz Case 1, 50 kW Study was made for two rotor sizes (case 1 and case 2), two levels of frequency (3 kHz and 50 kHz) and two levels of power (50 and 75 kW)
  5. 5. Braze Thermal Profile 5 Case 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 Down 6 0 100 200 300 400 500 600 700 800 900 0 500 1000 1500 2000 Time (s) Case 1 - 50 kHz Case 1 - 3 kHz 75 kW 50 kW Temperature at braze joint Temperature curves show that heat losses in bars are very high and thermal efficiency at 75 kW at the end of heating is less than 50%
  7. 7. Coil Thermal Profile at the End of the Process 7 Case 1, 50 kW Frequency 50 kHz, Tmax = 160 0CFrequency 3 kHz, Tmax = 150 0C
  8. 8. Braze Thermal Profiles 8 Case 2, 50 kHz, 75 kWCase 2, 3 kHz, 75 kW Temperature profiles are almost the same for 3 kHz and 50 kHz
  9. 9. 0 200 400 600 800 1000 1200 0 50 100 150 200 250 300 Temperature (C) Time (s) 3 kHz 50 kHz Temperature at Braze Joint 9 Case 2, 75 kW
  10. 10. Coil Thermal Profile at the End of the Process 10 Case 2, 50 kHz, 75 kW Tmax = 200 0C Case 2, 3 kHz, 75 kW Tmax = 190 0C Tmax of concentrator is close to upper limit for Fluxtrol material and must be kept under control by material selection, concentrator design and proper manufacturing. Intensive coil cooling is required
  11. 11. Simulated and Experimental Process Parameters 11 Rotor type Power, kW Frequency kHz Time sec Note Ptotal Pcoil Pring Pмagn АТD-3 Case 1 50 22.8 26.7 0.5 50 185 Model (Case 1) 50 - - - 54 170 Experiment 50 23.6 26.2 0.2 3 205 Model (Case 1) DТА-1 Case 2 50 21.6 28.1 0.3 50 925 Model (Case 2) 50 - - - 46 930 Experiment 75 34 40.6 0.4 50 310 Model (Case 2) 75 35 39.8 0.2 3 335 Model (Case 2)
  12. 12. 0 200 400 600 800 1000 1200 0 50 100 150 200 250 300 Axial Force on Ring (N) Time (s) 3 kHz 50 kHz Electrodynamic Forces on Ring 12 75 kW Forces are rather high especially at 3 kHz. They can cause vibration and noise
  13. 13. End of Test Brazing Process Operator controls brazing quality and adds solder when required Red spot in the circle shows a point of the temperature control using laser aiming
  14. 14. 14 Conclusions • 2D simulation gives accurate enough results despite several assumptions • Frequency variation in the wide range from 3 to 50 kHz does not influence the 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 coil voltage 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 at 50 kW is marginal. Heating time is almost triple compared to 75 kW • Simulation and experiments showed that SMC concentrators may be effectively used in rotor brazing • Computer simulation may be used as a powerful tool for design of rotor brazing processes
  15. 15. Thank you! 15 I Protest against Induction Brazing of My Cage!

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