A&D October 2006 Inverter commutation failure in line ... - Siemens
A&D October 2006 Inverter commutation failure in line ... - Siemens
A&D October 2006 Inverter commutation failure in line ... - Siemens
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thyristor must be fired simultaneously, a process which has given rise to the term "double<br />
pulses". Each thyristor conducts the load current for 2 successive fir<strong>in</strong>g cycles (twice with<strong>in</strong> a<br />
maximum of 60 degrees <strong>in</strong> steady state).<br />
The average value of the rectified output voltage u dα can be adjusted by the so-called delay<br />
or gat<strong>in</strong>g angle α. The delay angle α is measured from the so-called "natural <strong>commutation</strong><br />
<strong>in</strong>stant“ N. This is the po<strong>in</strong>t at which current <strong>commutation</strong> would commence <strong>in</strong> a new l<strong>in</strong>e<br />
phase <strong>in</strong> a diode bridge (<strong>in</strong>tersection between u 12 and u 13 , u 13 and u 23 , u 23 and u 21 , u 21 and<br />
u 31 , u 31 and u 32 , u 32 and u 12 ).<br />
One segment at a time (with <strong>in</strong>terval that can be varied by delay angle α) from one of the<br />
l<strong>in</strong>e-to-l<strong>in</strong>e supply voltages (u 13 , u 23 , u 21 , u 31 , u 32 , u 12 ) is "switched through" cont<strong>in</strong>uously to<br />
the DC mach<strong>in</strong>e via a pair of thyristors (V1+V2, V2+V3, V3+V4, V4+V5, V5+V6, V6+V1).<br />
Due to the <strong>in</strong>ductance L k <strong>in</strong> the circuit, however, the load current I d cannot transfer<br />
<strong>in</strong>stantaneously from the outgo<strong>in</strong>g ("turn<strong>in</strong>g off") thyristor arm to the <strong>in</strong>com<strong>in</strong>g ("turn<strong>in</strong>g on")<br />
thyristor arm. With a cont<strong>in</strong>uous load current I d , the "<strong>commutation</strong> period" or "overlap"<br />
commences <strong>in</strong> each case when the "new" (<strong>in</strong>com<strong>in</strong>g) thyristor is fired, while the load current<br />
I d from one term<strong>in</strong>al of the DC mach<strong>in</strong>e is split between two l<strong>in</strong>e phases via the outgo<strong>in</strong>g and<br />
<strong>in</strong>com<strong>in</strong>g thyristors. The other term<strong>in</strong>al of the DC mach<strong>in</strong>e is connected to the rema<strong>in</strong><strong>in</strong>g<br />
supply connection via the thyristor fired <strong>in</strong> the previous cycle, which is now <strong>in</strong> the second<br />
conduction phase and conduct<strong>in</strong>g the full load current I d .<br />
While the 6-pulse three-phase bridge connection can supply only a positive load current I d , it<br />
is capable of generat<strong>in</strong>g positive or negative average values of the rectified output voltage<br />
u dα through variation of delay angle α. Depend<strong>in</strong>g on whether the DC mach<strong>in</strong>e is operat<strong>in</strong>g<br />
as a motor or generator, the converter bridge is said to be operat<strong>in</strong>g <strong>in</strong> "rectifier mode" or<br />
"<strong>in</strong>verter mode". A s<strong>in</strong>gle three-phase bridge connection is therefore suitable for a drive with<br />
only positive torque <strong>in</strong> different directions of rotation. A typical application for this type of<br />
operation <strong>in</strong> the 1st and 4th speed/torque quadrant would be the hoist<strong>in</strong>g or lower<strong>in</strong>g of a<br />
hoist load. If the DC mach<strong>in</strong>e is also required to operate with negative torques, then a double<br />
converter (also referred to as "reversible converter") must be used. The most common<br />
design of reversible converter is based on a circulat<strong>in</strong>g-current-free anti-parallel connection<br />
of two three-phase bridge connections.<br />
Version vom 5.10.<strong>2006</strong> Seite 4 von 17 Wöhrer, Hofmüller, Himmelstoss