Scaling Issues for Common Mode Chokes to Mitigate Ground Currents in Inverter-Based Drive Systems

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1 Scaling Issues for Common Mode Chokes to Mitigate Ground Currents in Inverter-Based Drive Systems Annette Muetze Electrical and Computer Engineering University of Wisconsin-Madison, USA Inverter Motor 3 Common mode chokes in series Source: Magnetec GmbH A. Muetze - 1

2 Outline Motivation to use common mode chokes in the inverter-output requirements? Influence of the Inductance Value on the Current Reduction Equivalent circuit, general case and un-damped case Physics of Common Mode Chokes Fundamentals, wound and feed-through chokes Design Criteria Window area, current-to-diameter ratio, minimum diameter, Measurement Results Summary A. Muetze - 2

3 Motivation Common mode current 20 A/Div Stator ground current 20 A/Div 1 µs/div Circulating bearing currents motor speed n = 3000 /min Bearing current (NDE) (DE) 5 A/Div Bearing current (NDE) (DE) 5 A/Div Rotor ground current 10 A/Div 1 µs/div Bearing currents due to rotor ground currents, motor speed n = 15 /min, rotor grounded on DE-side Inverter Motor Squirrel-cage induction motor, frame size 400 mm, 500 kw rated power, bearing temperature T b 70 C (Measurements were obtained in the frame of a research project at Darmstadt University of Technology, Institute of Prof. Binder) Common mode choke A. Muetze - 3

4 Modeling: Lumped Parameter Circuit > Approach: RLC-Circuit (L: function of cable, R and C: function of motor) [Ogasawara, Akagi, 1996] L R > The peak HF common mode current amplitude is searched for: v i com C L com R v dv/dt = m slope m t I com = i com,max C wf A. Muetze - 4

5 Lumped Parameter Circuit: Parameters Common mode inductance required value??? Related to the motor only Damping due to motor frame resistance including skin effect; up to some tens of Ω L com R v dv/dt = m slope m t I com = i com,max C wf Stator winding to frame capacitance > Assumptions: short motor leads (L cable 0) All values are per phase values. I com = i com,max occurs during rise time t r A. Muetze - 5

6 Influence of L com on the Current Reduction: Problem Formulation > > where 1. Under-damped case 2. Critically-damped case 3. Over-damped case A. Muetze - 6

7 Influence of L com on the Current Reduction: General Case Simulated peak HF ground current [A] 80 under-damped (Lower bound) C wf = 25 nf C wf = 20 nf C wf = 15 nf C wf = 10 nf C wf = 5 nf practically irrelevant R = 1 Ω dv/dt = 2 kv/µs region to obtain current reduction 1.E-12 1.E-10 1.E-08 1.E-06 1.E-04 1.E-02 1.E+00 Common mode inductance [H] A. Muetze - 7

8 Influence of L com on the Current Reduction: General Case cont. Simulated peak HF ground current [A] 30 C wf = 25 nf C 25 wf = 20 nf C wf = 15 nf 20 C wf = 10 nf R = 10 Ω dv/dt = 2 kv/µs C wf = 5 nf 5 practically irrelevant region to obtain current reduction 0 1.E-12 1.E-10 1.E-08 1.E-06 1.E-04 1.E-02 1.E+00 Common mode inductance [H] A. Muetze - 8

9 Influence of L com on the Current Reduction: Un-damped Case > > Minimum value of common mode inductance [uh] Series2 C wf = 5 nf Series4 C wf = 10 nf Series6 C wf = 15 nf Series8 C wf = 20 nf Series10 C wf = 25 nf t r = 5 µs Rise time [µs] A. Muetze - 9

10 Outline Motivation to use common mode chokes in the inverter-output requirements? Influence of the Inductance Value on the Current Reduction Equivalent circuit, general case and un-damped case Physics of Common Mode Chokes Fundamentals, wound and feed-through chokes Design Criteria Window area, current-to-diameter ratio, minimum diameter, Measurement Results Summary A. Muetze - 10

11 Physics of Common Mode Chokes Wound choke: N c turns V a,inv V V b,inv c,inv V a,motor V V b,motor c,motor Diameter d c Core cross-sectional area A c Core window-area A w with window diameter dw Inductance value: (Un-gapped cores) A. Muetze - 11

12 Wound choke: Physics of Common Mode Chokes cont. Feed-through choke: N =1 c turn V a,inv V V b,inv c,inv Diameterd c V a,motor V V b,motor c,motor Core cross-sectional area A c Core window-area A w with window diameter dw (Un-gapped cores) Inductance value: A. Muetze - 12

13 Outline Motivation to use common mode chokes in the inverter-output requirements? Influence of the Inductance Value on the Current Reduction Equivalent circuit, general case and un-damped case Physics of Common Mode Chokes Fundamentals, wound and feed-through chokes Design Criteria Window area, current-to-diameter ratio, minimum diameter, Measurement Results Summary A. Muetze - 13

14 Accommodation of the Motor Leads Minimum core window area and bore of feed-through toroidal cores for different cable cross-sectional areas: N c = 1, k w = 0.4 Cable crosssectional area Current carrying capability per individual cable *) Core minimum window area Core minimum bore diameter 70 mm A 525 mm 2 26 mm 150 mm A 1125 mm 2 38 mm 300 mm A 2250 mm 2 54 mm *) At 30 C ambient temperature, according to DIN VDE 0276 Part 1000 More than a factor of 2 smaller than the minimum bore diameter to avoid saturation! Already at 1 turn! A. Muetze - 14

15 Maximum d c /I com Ratio for Avoiding Core Saturation (Un-gapped cores) A. Muetze - 15

16 Minimum Core Diameter for Avoiding Core Saturation High value by intention Order of magnitude is significant! (Un-gapped cores) A. Muetze - 16

17 Cross Sectional Area as Function of L com Core cross sectional area [mm 2 ] Series2 I com = 50 A, B s = 2 T Series4 I com = 10 A, B s = 2 T Series6 I com = 50 A, B s = 1.2 T Series8 I com = 10 A, B s = 1.2 T region to obtain current reduction Common mode inductance [H] (Un-gapped chores, constant µ) A. Muetze - 17

18 Use of Several Feed-Through Cores in Series > Large cross sectional area required to achieve L com > Increase of the N c not an option because of saturation > Use several cores in series Inverter Motor 3 Common mode chokes in series A. Muetze - 18

19 Outline Motivation to use common mode chokes in the inverter-output requirements? Influence of the Inductance Value on the Current Reduction Equivalent circuit, general case and un-damped case Physics of Common Mode Chokes Fundamentals, wound and feed-through chokes Design Criteria Window area, current-to-diameter ratio, minimum diameter, Measurement Results Summary A. Muetze - 19

20 Measurement Results Commercially available un-gapped toroidal core ( Cool Blue of Magnetec GmbH) Nano-crystalline material with µ r = 30,000 (10 khz) Tabulated inductance (20 30)µH Drive with C wf = 8 nf, dv Lg /dt = 2 kv/µs Reduction of I com = 28 A 1 core: by 15% 2 cores: by 36% Minimum d c = 280 mm twice actual value Reduction of the choke performance by 50% Compares well with above Figures Simulated peak HF ground current [A] E-12 1.E-10 1.E-08 1.E-06 1.E-04 1.E-02 1.E+00 Common mode inductance [H] A. Muetze - 20

21 Summary 1. Minimum inductance value of the chokes: in the order of 10 µh 2. Rather large values of I com require a minimum bore diameter for avoiding saturation that results in a core window area large enough to accommodate the motor leads 3. Any number of turns > 1 will likely result in increased saturation ( not improve the choke performance) (Un-gapped cores) 4. Aiming for a higher value of relative permeability than 10,000 is arguable (Un-gapped cores) 5. Use of several cores in series A. Muetze - 21

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