PIC18F4539-ESLASHMLSQTP datasheet(116/322 Pages) MICROCHIP

116 / 322 page

PIC18FXX39

DS30485A-page 114

Preliminary

 2002 Microchip Technology Inc.

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

TABLE 14-1:

DATA POINTS FOR THE

DEFAULT V/F CURVE

Frequency (Hz)

% Modulation

814

100

110

133

104

133

112

133

120

133

128

133

100

125

150

20406080

100

120

140

Input Frequency (Hz)

frated of motor

should equal f at

100% modulation

Vrated of motor

should equal

at 100% modulation

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