Fairchild AN-7511 manual Fiber-Optic Drive Eliminates Interference

Application Note 7511

Polyphase motors, controlled by solid-state, adjustable-fre- quency ac drives, are used extensively in pumps, conveyors, mills, machine tools and robotics applications. The specific con- trol method could be either 6-step or pulse-width modulation. This section describes a 6-step drive that uses some of the pre- viously discussed drive techniques (see page 11, “Latch-Up: Hints, Kinks and Caveats”).

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 air-gap flux diminishes, reduc- ing developed-torque capability. You can maintain the flux at a constant level (as in a dc shunt motor) if you also vary the volt- age so the V/F ratio remains constant.

Fiber-Optic Drive Eliminates Interference

In the example given, the switching regulator varies the IGT inverter’s output by controlling its dc input; the voltage-con- trolled oscillator (VCO) adjusts the inverter’s switching fre- quency, thereby varying the output frequency. The VCO also drives the 3-phase logic that provides properly timed pulsed outputs to the piezo couplers that directly drive the IGT.

Sensing the dc current in the negative rail and inhibiting the gate signal protect the IGT from overload and shoot-through

(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 power-out- put circuit is shown in Figure 9A; the corresponding timing diagrams show resistive-load current waveforms that indi- cate the 3-phase power Figure 9B and waveforms of the out- put line voltage and current Figure 9C.

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 positive-to-negative transition periods in the phase current. This delay allows the complementary IGTs to turn off before their opposite members turn on, thus preventing cross conduction and eventual destruction of the IGTs.

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 full-load line current specified by the motor manu- facturer determines the maximum steady-state current that each transistor must switch. You must convert this RMS- specified current to peak values to specify the proper IGT. If the input voltage regulator had a fixed output voltage and a constant frequency, each IGT would be required to supply the starting locked-rotor current to the motor. This current could be as much as 15 times the full-load running current.

D7
L1
D11	Q1	D12	Q3		D13	Q5
C1		D1		D3	R		D5	TO
R		R						LOAD
0 TO 325V
10A
D8	Q2	D2 D9	Q4	D4	D10	Q6	D6
								SWITCHES ON” (1, 4, 5),
R		R			R			(1, 3, 6), (2, 3, 6),
								(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.

10
tD(OFF)				tF1
		tF1	IC
1			0.9I		0.1I
				C	0 C
t	F2	tD(OFF)		tF2
0.1	1k			10k
100

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