Improved high speed efficiency of induction motors/rotors for xev traction

Transcription

1 Improved high speed efficiency of induction motors/rotors for xev traction Malcolm Burwell International Copper Association May Tokyo

2 Summary This talk presents Why it is important to improve induction motors (IM s) for xev traction Practical work that increases the high-speed efficiency of die cast copper rotors in IM s Plausible mechanisms to explain the normally poor high-speed efficiency of die cast rotors AC Propulsion s AC-75 xev motor and rotor This work moves IM s towards becoming effective replacements for today s high cost permanent magnet motors (PMM s) Motor performance measured on dynamometer 2

3 Why optimize induction motors for xev s? xev s must lower costs to reach mass-marketsmarkets >90% of xev s today use magnet motors Rare earth metals used in magnet motors Magnet prices are high, rising, volatile xev motor suppliers considering move to IM s Rare earth prices per kg peak 2014 Dysprosium $80 $3500 $630 (Dy) Neodymium (Nd) Source: metal-pages.com $30 $460 $90 3

4 Conclusions Die cast copper rotors show Lower efficiencies than fabricated rotors Increasingly lower efficiencies as rotor rpm increases There are at least two plausible mechanisms that explain this phenomenon Increased bar-to-lamination shorting in die cast rotor cages Lower AC resistance of die cast rotor cages Speed-dependent inefficiencies in die cast copper rotors can be removed by Using a ceramic coating on the interior of the bar slot Quenching the rotor immediately after ejection from the die 4

5 The problem In many IM designs with die cast copper rotors: Efficiencies lower than identical fabricated rotors Worse at the higher speeds of xev motors Important to solve this problem because: Die casting costs $12/motor less than fabricating* Copper rotor enables highest IM efficiency 5 4% higher average efficiency than aluminum rotor 2% less efficient than a PMM = 900kWh higher lifetime losses = only $260 higher lifetime running cost* Fabricated rotor IM cost is less than PMM cost by: $60* (2014 magnet prices) $390* (2011 magnet prices) * For a 50kW HEV traction motor Source: MBurwell-ICA-EV-Traction-Motor-Comparison-v1.8-Eng1.pdf Die cast rotors less efficient than fabricated At high xev speeds, die cast copper rotor performance can even be as low as aluminum

6 Experimental method 14 rotors were cast and these parameters varied: Conductor material: copper, aluminum Coating in bar slot: ceramic coating, no coating Quench after ejection: water quench, no quench Post-cast heat treat: reheat-and-quench, no treat Then each rotor was: Non-destructively tested for casting quality Assembled into a stator DC-to-shaft efficiency measured on dynamometer 10kW and 50kW 8,000rpm and 10,000 rpm 50 C and 100 C Ceramic coating of bar slot in lamination stack Die cast rotor being released into quench tank 6

7 Results Efficiency of die cast rotor raised above fabricated rotor efficiency by: Applying ceramic coating to bar slots before die casting, together with Quenching in water immediately after ejection from die Parameter Efficiency difference from fabricated rotor* Coating + Quench after +0.1% ejection (both together) Coating only -0.5% Post-cast heat treat -1.2% No coating + no quenching -1.3% Aluminum -1.4% * at 10,000rpm/50kW/ rpm/50kW/100 o C rotor temperature and measurement error of ±0.1% 7

8 Causal mechanisms Five possible causes of the reduced high-speed efficiency of die cast rotors were explored Two causes remain plausible Increased bar-to-lamination shorting Low rotor-cage AC resistance Three causes were eliminated after experimental investigation Contamination of molten copper used for die casting High die casting temperatures (1100 o C) compared to fabrication temperatures (700 o C), causing either: Degradation of lamination magnetic properties, or Breakdown of C5 lamination coating Copper pellets added to die casting crucible Induction brazing of fabricated rotor 8

9 Plausible cause (1): Increased bar-to-lamination shorting Bar-to-lamination resistance of die cast rotor lower than in fabricated rotor Intimate bar contact from bar/slot conformance Ceramic coating is abraded away in places This bar-to-lamination shorting could cause Circumferential parasitic currents from bar to bar Inter-lamination currents Those losses could increase with speed Centrifugal force pushes bars closer into contact Closer contact causes even lower resistance Quenching reduces this effect due to breakaway Bar-to-lamination shorting remains a possible cause of speed-related die cast rotor inefficiency Short length of rotor bar taken from die cast rotor -- white ceramic coating material visible ibl -- lamination marks confirm intimate bar contact (cast) Measured bar-to-lamination resistance 9

10 Plausible cause (2): Low rotor-cage AC resistance Computer modeling suggests Frequency-dependent eddy current losses are created by space MMF and time current harmonics Above a threshold frequency/motor rpm, increasing cage AC resistance decreases those losses Fabricated rotors have higher cage AC resistance than die cast rotors Due to high resistance brazing alloy in end rings This higher-than-copper resistance may lower eddy current losses at high speeds This project did not experimentally examine this phenomenon Low rotor cage AC resistance remains a possible cause of speed-related die cast rotor inefficiency Rotor end rings before brazing DC resistance of fabricated rotor cage is higher than that of a die cast rotor cage 10

11 Conclusions Die cast copper rotors show Lower efficiencies than fabricated rotors Increasingly lower efficiencies as rotor rpm increases There are at least two plausible mechanisms that explain this phenomenon Increased bar-to-lamination shorting in die cast rotor cages Lower AC resistance of die cast rotor cages Speed-dependent inefficiencies in die cast copper rotors can be removed by Using a ceramic coating on the interior of the bar slot Quenching the rotor immediately after ejection for the die 11

12 Thank you For more information please contact Phone:

13 Eliminated cause (1): Contamination of molten copper Copper bar and end-ring material Sampled after die casting Analyzed for contaminants that can affect copper s conductivity Levels of all metal contaminants too low to affect conductivity Oxygen level was the exception Increased 0.12% w/w in end rings Bar sample too small to measure However, oxygen is not soluble in copper and does not affect its conductivity Contamination of copper is not the cause of die cast rotor inefficiency i 13

14 Eliminated cause (2): Degradation of lamination magnetic properties Two samples were compared As received new laminations Laminations removed from rotor after die casting Coils wound around two lamination areas Slot base to shaft hole (a) Slot top to lamination outer diameter (b) Changes in magnetic properties due to die casting Area (b) is of most interest because of high magnetic flux during motor operation Not significant enough to account for inefficiencies Degradation of magnetic properties are not the cause of die cast rotor inefficiency Windings to measure magnetic properties Resultant measured magnetic B/H curves 14

15 Eliminated cause (3): Breakdown of C5 lamination coating Laminations use standard C5 inorganic coating to prevent lamination-to-lamination shorting Experimental method: Two C5 coatings tested Remisol 5308 and 5620 Coated coupons dipped in molten copper or aluminum Coating resistance measured before/after dipping Coating resistance results Infinite resistance for aluminum-dipped coupons 22,000Ω resistance for copper dipped samples Although resistance is lower for copper, it is not low enough to account for the inefficiencies seen Breakdown of C5 lamination coating is not the cause of die cast rotor inefficiency Test coupon of C5-coated lamination after rightmost half dipped in molten metal Resistance measurements on dipped coupons 15