PIC18FXX39 Datasheet, PDF (114/322 Pages) Microchip Technology – Enhanced FLASH Microcontrollers with Single Phase Induction Motor Control Kernel

PIC18FXX39
14.3 Software Interface
A sine table, stored in the ProMPT kernel, is used as
the basis for synthesizing the DC bus using the PWM
modules. The table values are accessed in sequence
and scaled based on the frequency or the speed at
which the motor is intended to run. The intended fre-
quency input can be from an A/D channel or a digital
value.
Parameters in the ProMPT modules can be accessed
using the pre-defined Application Program Interface
(API) methods. A list of the APIs is given in
Section 14.3.3.
For example, to run the motor at 40 Hz, the user would
invoke the PromMPT_SetFrequency API:
i = ProMPT_SetFrequency(40);
where i is an unsigned character variable. In this case,
if i = 0 on return, the command has been successfully
executed. If the frequency input is out of range, or if
there is an error in setting the frequency, i is returned
with a value of FFh.
Similarly, to check the frequency set by the ProMPT
kernel, use the ProMPT_GetFrequency API:
i = ProMPT_GetFrequency(void);
where i is an unsigned character variable. Upon return
from the ProMPT kernel, i will contain the frequency
value in the ProMPT kernel.
14.3.1 THE V/F CURVE
The ProMPT kernel contains a default V/F curve stored
in memory. The default curve is linear, as shown in
Figure 14-2. Table 14-1 shows the data points used to
construct the curve.
Users may require a different V/F curve for their appli-
cation, based on the load on the motor, or based on the
characteristics of the motor used. The curve can be
changed in the application program using the API
method SetVFCurve(X,Y), where X is the frequency
and Y is the level of modulation of the DC bus voltage.
As a rule, in customizing the curve, the input frequency
corresponding to the point on the V/F curve that gives
100% modulation should match the motor’s rated fre-
quency. Similarly, full modulation should occur at the
motor’s rated input voltage. (See Figure 14-2 for
details.)
Examples of the characteristics for V/F curves for typi-
cal motor applications are shown in Section 14-2
(page 115).
14.3.2 PARAMETERS DEFINED BY THE
ProMPT API METHODS
Frequency: The frequency (in Hz) of the supply
current for steady state motor operation.
Modulation: The level of modulation (in percentage)
applied to the DC supply voltage by the PWM through
the H-bridge to produce AC drive current.
Acceleration rate: The rate of increase of motor
speed, achieved by ramping up the supply frequency.
Expressed in Hz/s.
Deceleration rate: The rate of decrease of motor
speed, achieved by ramping down the supply
frequency. Expressed in Hz/s.
Boost: The mode for starting a stopped motor by vary-
ing the supply current frequency and modulation until
steady state speed is reached. Boost is defined in
terms of a frequency, a starting and ending modulation,
and a time interval for the transition between the two.
PWM Frequency: The sampling rate (in kHz) at which
the PWM module operates.
FIGURE 14-2:
DEFAULT V/F CURVE FOR
THE ProMPT KERNEL
150
V_rated of motor
should equal
125 _{at 100% modulation}
100
75
50
25
0
0
f_rated of motor
should equal f at
100% modulation
20 40 60 80 100 120 140
Input Frequency (Hz)
TABLE 14-1: DATA POINTS FOR THE
DEFAULT V/F CURVE
Frequency (Hz)
% Modulation
0
0
8
14
16
28
24
42
32
57
40
71
48
86
56
100
64
110
72
133
80
133
88
133
96
133
104
133
112
133
120
133
128
133
DS30485B-page 114
Preliminary
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