Application Note 7511
Polyphase motors, controlled by
Figure 8 defines the drive’s block diagram. A 3-phase rectifier converts the 220V ac to dc; the switching regulator varies the output voltage to the IGT inverter. At the regulator’s output, a large filter capacitor provides a stiff voltage supply to the inverter.
The motor used in this example has a low slip characteristic and is therefore very efficient. You can change the motor’s speed by varying the inverter’s frequency. As the frequency increases, however, the motor’s
Fiber-Optic Drive Eliminates Interference
In the example given, the switching regulator varies the IGT inverter’s output by controlling its dc input; the
Sensing the dc current in the negative rail and inhibiting the gate signal protect the IGT from overload and
(simultaneous conduction) conditions. If a fault continues to exist for an appreciable period, inhibiting the switching regu- lator causes the inverter to shut off. The inverter’s
In Figure 9’s circuit, it appears that IGTs Q1 through Q6 will conduct for 180o. However, in a practical situation, it’s neces- sary to provide some time delay (typically 10o to 15o×) dur- ing the
Because of the time delay, the maximum conduction time is 165o of every 360o period. Because the IGTs don’t have an integral diode, it’s necessary to connect an antiparallel diode externally to allow the freewheeling current to flow. Inductor L1 limits the di/dt during fault conditions; freewheeling diode D7 clamps the IGT’s collector supply to the dc bus.
The peak
D7 |
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L1 |
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D11 | Q1 | D12 | Q3 |
| D13 | Q5 |
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C1 |
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| D3 | R |
| D5 | TO |
R |
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0 TO 325V |
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10A |
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D8 | Q2 | D2 D9 | Q4 | D4 | D10 | Q6 | D6 |
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| SWITCHES ON” (1, 4, 5), |
R |
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| (1, 3, 6), (2, 3, 6), |
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| (2, 3, 5), (2, 4, 5) |
FIGURE 10A. COMPONENT SELECTION IS IMPORTANT. THE IGT SELECTED CIRCUIT HANDLES 10A, 500V AT 150oC. THE ANTI-
PARALLEL DIODES HAVE A SIMILAR CURRENT RATING.
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tD(OFF) |
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| tF1 | IC |
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1 |
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| C | 0 C | |
t | F2 | tD(OFF) | tF2 |
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0.1 | 1k |
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100 |
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RGE
FIGURE 10B. SELECT R TO YIELD THE DESIRED TURN-OFF TIME. FINALLY, L1’S VALUE DETERMINES THE FAULT-CONDITION ACTION TIME.
©2002 Fairchild Semiconductor Corporation | Application Note 7511 Rev. A1 |