Fairchild AN-7511 - Manual (Page 6)

©2002 Fairchild Semiconductor Corporation Application Note 7511 Rev. A1
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 Q
1
through Q
6
will
conduct for 180
o
. However, in a practical situation, it’s neces-
sary to provide some time delay (typically 10
o
to 15
o
×) 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
165
o
of every 360
o
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
L
1
limits the di/dt during fault conditions; freewheeling diode
D
7
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.
FIGURE 10A. COMPONENT SELECTION IS IMPORTANT. THE IGT SELECTED CIRCUIT HANDLES 10A, 500V AT 150
o
C. THE ANTI-
PARALLEL DIODES HAVE A SIMILAR CURRENT RATING.
FIGURE 10B. SELECT R TO YIELD THE DESIRED TURN-OFF TIME. FINALLY, L1’S VALUE DETERMINES THE FAULT-CONDITION
ACTION TIME.
D
7
0 TO 325V
10A
SWITCHES ON” (1, 4, 5),
(1, 3, 6), (2, 3, 6),
(2, 3, 5), (2, 4, 5)
L
1
D
1
Q
1
R
C
1
D
5
Q 5
R
R
Q 6 D
6
TO
LOAD
D
13
D 10
R
Q 4 D
4
D 9
R
Q 2 D
2
D 8
D
3
Q 3
R
D
12
D
11
+
10
1
0.1
100 1k 10k
R
GE
t D(OFF)
t
F1
t F2
t F2
t F1
t
D(OFF)
I
C
0.9I C
0.1I
C
0
Application Note 7511