Lecture Power Electronics Interactions Between Electrical Machine and Power Electronics Technische Universität München Lehrstuhl für Elektrische Antriebssysteme und Leistungselektronik Prof. Dr.-Ing. Ralph Kennel

Additional Losses

Current Harmonics Quelle : Prof. A. Binder, Technische Universität Darmstadt with increasing switching frequency the current harmonics caused by the inverter decrease

Iron Losses under Inverter Supply Quelle : PTB f / Hz (fundamental oscillation)

Additional Losses Influence of Switching Frequency Quelle : Prof. A. Binder, Technische Universität Darmstadt mains supplyinverter supply 2 pole squirrel cage induction machine 3 kW, 380 V, Y connection rated frequency 50 Hz, slip 4.5 %, torque 10 Nm voltage source inverter 8.3 kVA, 400 V at frequency 9.6 kHz motor efficiency is high (  less temperature rise) the overall efficiency, however, is the same as at Frequency 4.8 kHz At frequency 19.2 kHz motor current harmonics are low, but switching losses increase

EMC

Electro Magnetic Compatibility EMCEMC even more confusion

Cabinet Design with Modern Servo Drives signal electronics power electronics

Shielding and Grounding

following these requirements and advice there is „conductive EMI“ only when using power electronics inverters (… usually no „radiation EMI“) Shielding and Grounding

Reactive Power

Motor (e. g. induction machine) U0U0 induction machines need reactive power for magnetization … in this cable reactive power can be measured ! this is a DC link  there is no reactive power by definition … in this cable reactive power cannot be measured !

Reactive Power Motor (e. g. induction machine) U0U0 induction machines need reactive power for magnetization this is a DC link  there is no reactive power by definition … in this cable reactive power cannot be measured ! … where does reactive power come from ??? … as the sum of reactive power in all 3 phases is zero („0“) !

Reactive Power Motor (e. g. induction machine) U0U0 induction machines need reactive power for magnetization … where does reactive power come from ??? … it is no problem for the inverter to provide it … as the sum of reactive power in all 3 phases is zero („0“) !

Reactive Power Motor (e. g. induction machine) U0U0 … it is no problem for the inverter to provide it … as the sum of reactive power in all 3 phases is zero („0“) ! … with regard to reactive power the inverter is like a marshalling yard (switching station) for trains !

Reactive Power Motor (e. g. induction machine) U0U0 … with regard to reactive power the inverter is like a marshalling yard (switching station) for trains ! … therefore inverters can be used easily for compensating reactive power in grids ! … especially in regenerative energy applications like wind power farms or solar power arrays !

Reactive Power voltage current reactive power t t t

Reactive Power voltage current reactive power t t t Active Power active power

voltage current t t t Active Power active power …, of course, this can be split mathematically into a constant term (which is active power) and a fluctuating term (which can be considered reactive power)

t Active Power active power …, of course, this can be split mathematically into a constant term (which is active power) and a fluctuating term (which can be considered reactive power) … this is, however, a mathematical operation only … because there is no moment with power flowing in backward direction … in fact this is pulsating active power only !!! … from physical perspective there is not really reactive power

t Active Power active power … in fact this is pulsating active power only !!! Pulsating Active Power ≠ Reactive Power

P F C = Power Factor Control Power Factor = P W (active power) P S (apparent power)

PFC … when using line commutated (Thyristor-)converters this was an issue indeed voltage t current t cos  = 10,60,2- 0,3

PFC … when using line commutated (Thyristor-)converters this was an issue indeed voltage t current t cos  = firing angle control takes care for a phase shift between current wave and voltage wave ! 10,6 … for that reason the „inductive“ reactive power had to be compensated by „kapacitive“reactive power

PFC … in case of diode rectifiers the power factor is usually cos  = 1 voltage t current t cos  = 1 and in case of fully controlled rectifier bridges … PFC is meant to include harmonics as well ! – in spite of harmonics having nothing to do with „PFC“ „PFC“ is often meant to compensate harmonics

Rectifiers

not allowed in the public grid (with respect to impact to the grid) Rectifiers B4 bridge with capacitive load current shape … today‘s discussion is dealing with harmoncs ! – in spite of harmonics having nothing to do with „PFC“ „PFC“ is often meant to compensate harmonics

… filtering effort would be significant! harmonics spectrum Rectifiers B4 bridge with capacitive load not allowed in the public grid (with respect to impact to the grid)

… different (better) solution : fully controlled front end rectifier current shapeharmonics spectrum Rectifiers B4 bridge with capacitive load … filtering effort would be significant!

Quelle : Prof. A. Binder, Technische Universität Darmstadt Additional Losses Current Harmonics with increasing switching frequency the current harmonics caused by the inverter decrease

… different (better) solution : fully controlled front end rectifier Netz U0U0 ≈ current shape Rectifiers

Netz U0U0 ≈ harmonics spectrum Rectifiers

current shapeharmonics spectrum fully controlled front end rectifier … nevertheless !!! … even with a good 1phase „PFC“ … either the load has to be charged by pulses (law of energy conservation !) or an energy storage device must be implemented … no tricky control scheme can change that !!! … or – after all – can it? Rectifiers

PFC … how must the current shape look like to provide a constant power flow ? voltage t current t power t … of course, at u = 0 and/or i = 0 no power can be transmitted (law of energy conservation !) The time to be bypassed by the energy storage device (e. g. capacitance), (= energy), however, is significantly smaller !

PFC … is this current shape allowed ??? voltage t current t power t … please calculate the harmonics spectrum … it is surprising, how close one can get to this current shape without exceeding the standard limits of grid harmonics

PFC … is this current shape allowed ??? voltage t current t power t … please calculate the harmonics spectrum … some companies make use of this effect, to reduce the size of the DC link capacitance, … but, of course, they do not tell that in public.

Travelling Waves

typical „voltage pattern“ at the output of a PWM voltage source inverter Travelling Waves

typical „voltage step“ at the output of a PWM voltage source inverter

M time  terminal voltage  Travelling Waves

M … what now ? … which case is it ?  „fix“ end  „loose“ end  adaptation … „it depends“… on what ? … whether we consider currents or voltages !!! … in our case :  voltages … for voltages the motor is a „loose“ end Travelling Waves time  terminal voltage 

M … what now ? … which case is it ?  „fix“ end  „loose“ end  adaptation … for voltages the inverter is a „fix“ end … if the inverter output voltage did not change meanwhile, the wave is inverted and travels back again Travelling Waves time  terminal voltage 

M  „loose“ end … on the inverter side the voltage remains constant (fix end !) … on the motor side voltage oscillations occurr up to the double value of DC link voltage (loose end !) Travelling Waves time  terminal voltage 

… so far … so good … the matter, however, is getting much worse, as soon as the inverter switches simultaneously „into“ the back travelling voltage wave Travelling Waves … on the inverter side the voltage remains constant (fix end !) … on the motor side voltage oscillations occurr up to the double value of DC link voltage (loose end !)

M Travelling Waves time  terminal voltage 

M  „loose“ end … until here everything is like before … Travelling Waves time  terminal voltage 

M … what now ?  „fix“ end … in case the inverter has switched the voltage at its output meanwhile, the wave travels back with amplification Travelling Waves time  terminal voltage 

M  „loose“ end Travelling Waves time  terminal voltage 

… on the inverter side the voltage is „impressed“ (fix end !) … on the motor side voltage oscillations occurr up to 2,7 times the DC link voltage (loose end !) … what is so critical ? Travelling Waves

Voltage Flashover within Winding … what is so critical ?

„Horror“ Picture … the danger is real … - behind such pictures, however, there is a commercial interest !! … what is so critical ?

Extract from IEC paper IEC 2 (CD) (in Germany) : appendix to IEC 34 as long as you supply standard induction motors by inverters with voltage peaks below 1000 V voltage rise times below 500 V/µs you should not expect any danger for the motor Compatibility between Inverter and Motor … these are realistic values for modern inverters !!!

Insulation of Wire Reasoning the critical voltage resulting in a flashover does not depend at all on the diameter of the wire doubling the thickness of wire insulation increases the critical voltage by 15 % (the must significant effect results from covering faults of the first layer by the second layer)  that is „state of the art“ today !!! increasing the operation temperature to 155 °C lowers the critical voltage by 15 %

Voltage Stress on Partial Coils

Voltage Stress on (Partial) Coils

… to explain this effect, the representation as a simple equivalent circuit containing a serial connection of concentrated inductances is not sufficient!!! … in this case the motor winding has to be represented – like an electric cable – by a serial network of two-ports Voltage Stress on (Partial) Coils

… voltage “waves“ spread out within ther motor windings – as in an electrical cable – according to the laws of cable equation Voltage Stress on (Partial) Coils

… the motor windings only has inductive behaviour, if the rising time of the voltage edge is significantly larger than the group delay of the complete motor winding if the rising time of the voltage edge is smaller than the group delay of the complete motor winding, the capacitive behaviour is predominant !!! Voltage Stress on (Partial) Coils

… that is alarming !!! the first voltage pulse appears nearly completely at the entrance coil Voltage Stress on (Partial) Coils

… using cost effective winding processes the single wires are distributed randomly in the slot !! … therefore the insulation of the single wire must be designed with respect to the full voltage stress !!! Voltage Stress on (Partial) Coils

… remember: … on the motor side entstehen voltage oscillations occurr up to 2,7 times the DC link voltage Voltage Stress on (Partial) Coils … using cost effective winding processes the single wires are distributed randomly in the slot !! … therefore the insulation of the single wire must be designed with respect to the full voltage stress !!!