MA828
HARDWARE INPUT/OUTPUT FUNCTIONS
successive 8-bit sample linearly represents the instantaneous
amplitude of the waveform. It is assumed that the waveform is
symmetrical about the 9o°, 180° and 270° axes. The MA828
reconstructs the full 360° waveform by reading the 0°-90°
section held in ROM and assigning negative values for the
second half of the cycle.
These samples are used to calculate the instantaneous
amplitudes for all three phases, which will be 120° transposed in
the normal R-Y-B orientation for forward rotation or B-Y-R for
reverse rotation. The 384 8-bit samples are regularly spaced
over the 0° to 90° span, giving an angular resolution of
approximately 0·23°.
Set Output Trip (SET TRIP input)
The SET TRIP input is provided separately from the micro-
processor interface in order to allow an external source to
override the microprocessor and provide a rapid shutdown
facility. For example, logic signals from overcurrent sensing
circuitry or the microprocessor ‘watchdog’ might be used to
activate this input.
When the SET TRIP input is taken to a logic high, the output
trip latch is activated. This results in the TRIP output and the six
PWM outputs being latched low immediately. This condition can
only be cleared by applying a reset cycle to the RST input.
It is essential that when not in use SET TRIP is tied low and
isolated from potential sources of noise; on no account should it
be left floating.
Waveform segment
0°- 30°
Sample number
0 - 127
SETTRIPislatchedinternallyatthemasterclockrateinorder
to reduce noise sensitivity.
30·23°- 60°
128 - 255
60·23°- 89·77°
256 - 383
Output Trip Status (TRIP output)
The TRIP output indicates the status of the output trip latch
and is active low.
Table 8 90° of the 360° cycle is divided into 384 8-bit samples
Reset (RST input)
TheRST inputperformsthefollowingfunctionswhenactivbe
(low):
255
1. All PWM outputs are forced low (if not already low) thereby
turning off the drive switches.
2. All internal counters are reset to zero (this corresponds to 0°
for the red phase output).
3. TherisingedgeofRSTreactivatesthePWMoutputsresetting
the output trip and setting the TRIP output high –
assuming that the SET TRIP input is inactive (i.e. Iow).
POWER
WAVEFORM
VALUE
OF
8-BIT
SAMPLE
Zero Phase Pulses (ZPPR, ZPPY and ZPPB outputs)
The zero phase pulse outputs provide pulses at the same
frequency as the power frequency with a 1 : 2 mark-space ratio.
When in the forward mode of operation the falling edge of ZPPR
corresponds to 0° for the red phase, the falling edge of ZPPY to
0° for the yellow phase and the ZPPB falling edge to 0° for the
blue phase. In the reverse mode, the rising edge of a zero phase
pulse corresponds to 0° for the relevant phase PWM output.
0
0°
45°
90°
PHASE (384-BIT RESOLUTION)
Fig. 18 90° sample of typical power waveform
Waveform Sampling Synchronisation (WSS output)
This output provides a square wave signal of 50% duty cycle
at a frequency 1536 times higher than the fundamental of the
power waveform. Each successive pulse of WSS corresponds
to the MA828 reading the next location of the waveform ROM. It
may be used in conjunction with the ZPP signals to monitor the
position of the machine rotor and may form part of a closed loop
control system such as slip compensation.
PRODUCT DESIGNATION
Two standard option exist, defining waveform shape. These
are designated MA828-1 and MA828-2 as follows:
MA828-1
Sine1third harmonic at one-sixth the amplitude of the
fundamental:
1
Clock (CLK input)
x(t) = A [sin (vt)1 sin 3(vt)]
6
The CLK input provides a timing reference used by the
MA828 for all timings related to the PWM outputs. The
microprocessor interface, however, derives all its timings from
the microprocessor and therefore the microprocessor and the
MA828 may be run either from the same or from different clocks.
MA828-2
Pure sinewave:
x(t) = A [sin (vt)]
WAVEFORM DEFINITION
Additional wave shapes can be implemented to order, provided
theyaresymmetricalaboutthe90°,180°and270°axes.Contact
your local GEC Plessey Semiconductors Customer Service
Centre for further details.
The waveform amplitude data used to construct the PWM
output sequences is read from the internal 38438 ROM. This
containsthe90° spanofthewaveformasshowninFig. 18. Each
10
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