PWM Motor Drives and EMC in installations and systems Mart Coenen
PWM Motor Drives and EMC in installations and systems Intro Problem definition Why What How Problem solutions Application results Conclusions
Intro
Problem definition With large systems and installations, the EMC- requirements are primary determined by: the needed functionality (#1) and the needed reliability (#2) of the (end-) system.
Problem definition
Problem definition; why? Need for measures to: Handle fast switching (power efficiency) Minimize external RF emission/ noise Cost effective integral solutions; drive, filter(s), cable, motor sensor/encoder Overvoltage reduction lacking reference to PE in -systems Bearing erosion reduction
Problem definition, what? PWM drive: hard/soft switching w/o di/dt limiter Output filter(s), Cable: shielded/non-shielded Motor(s): DC, 3-phase, multi-phase/stepping
Problem definition, what? PWM drive output impedance determined by drive s output topology: Break-before-make (typical) Clamp diodes Capacitances at output di/dt limiter; typ. 3-phase reactor or single inductors often driven, so floating from PE
Problem definition, what? Output filters: 2-phase 3-phase 4-phase Sine-wave Power losses and efficiency shall be considered in drive, filter, cable and motor
Problem definition, what? Cable(s) Shielded/ non-shielded Cross-sectional symmetry Characteristic impedance Length, propagation delay Number of wires
Problem definition, what? Motor(s): Phase winding inductance Phase winding capacitance Phase winding to enclosure capacitances
Problem solution, how? Determine equivalent drive characteristics; impedance, dv/dt, di/dt Determine cable characteristics Determine equivalent motor characteristics
Problem solution, how?
Problem solution, how? Current confinement to ensure that i n (t) = 0 within the shielded cable for all frequencies Reduce RF on motor lines by filtering over various decades of frequency Terminate electrically long cable(s) to avoid standing waves/ resonances Add all measures together
Current confinement I I R S ( Z ( Z G G R R j ( M w j ( L Rw M T L )) RT )) Plot1 IDB(IR24), IDB(IR23) in db(amperes) 1 IDB(IR23) 2 IDB(IR24) -30.0-50.0-70.0-90.0-110 1 2 I S = Source current I R = Return path current Z G = Shield and/or frame impedance M W = Mutual coupling between wire and return wire M T = Mutual coupling due to the CM-choke R R = Resistance of the return wire L RW = Inductance of the return wire L RT = Inductance of the CM-choke 10 100 1k 10k 100k 1Meg 10Meg 100Meg FREQUENCY in hertz
RF (di/dt) filtering V5 V6 15 26 14 26 X7 SSWITCH X8 SSWITCH D7 40EPS16 D8 40EPS16 R4 1 IR4 16 R7 30 L4 6u V17 17 C4 10n 160 1 IR1 2 IR4 80.0 26 X9 SSWITCH D9 40EPS16 R5 1 R8 30 L5 6u V21 Plot1 IR1, IR4 in amperes 0 21 V7 19 18 X10 SSWITCH D10 40EPS16 20 21 C5 10n -80.0-160 26 R9 30 465.6u 466.0u 466.4u 466.8u 467.2u TIME in seconds V8 23 22 X11 SSWITCH X12 SSWITCH D11 40EPS16 D12 40EPS16 R6 1 24 L6 6u V25 25 C6 10n Series reactor in combination with cable and motor capacitance may cause resonances
Avoid cable resonances 90.0 70.0 Plot1 VDB(V4), VDB(V8), VDB(V16) in db(volts) 50.0 30.0 100 90 80 70 60 Ov e rshoot 50 [%] 40 2 13 30 20 10 1u 0 10.0 Se rie s Inductance [uh] 10u 100u 100 1000 10 Paralle l Resistance [Ohm] 1000u 1 1k 10k 100k 1Meg 10Meg FREQUENCY in hertz 1 VDB(V4) 2 VDB(V8) 3 VDB(V16)
Application results Ref 0 dbm Att 10 db 0-10 SWT 5 s 100 khz Ma rker 1 [T1 ] 1 MHz -69.54 dbm 1.000000000 MHz 0.000000000 Hz Ref 0 dbm Att 10 db A 0-10 SWT 5 s 0.000 000000 Hz 100 khz Marker 1 [ T1 ] 1 MHz -93.11 dbm 1.000 000000 MHz A 1 SA MAXH -20 1 SA MAXH -20 2 SA CLRWR -30 2 SA CLRWR -30-40 -40-50 -50-60 3DB -60 3DB -70 12-70 -80-80 -90-90 1-100 Center 100 khz Sp an 990 khz -100 Cente r 100 khz Span 990 khz 2 Measured common-mode current through motor cable before (left) and after (right) modifications
Conclusions Concept is in use for more than 5 years in low and high power applications Common-mode currents on cables can be reduced by 40 db (100 x) in less volume and less power losses!! Overvoltages/ bearing currents can be effectively reduced by factor 10 Spice analyis possible on MoR circuit topology Most components needed are of-the-shelf available Easy to apply in existing installations
Conclusions Applying the combined measures eliminate cable screen currents: NO current = NO crosstalk ALL cables can then be routed adjacent to one another with minimum separation (when thermally allowed)