Microchip Technology MCP6 series Manual de usuario descargar pdf (Pagina 2)

AN900

DS00900A-page 2  2004 Microchip Technology Inc.

SYSTEM OVERVIEW

Motor Drive Requirements

Practically speaking, control of a 3-phase AC induction

motor requires pulse-width modulated control of the six

switches of a 3-phase inverter bridge connected to the

motor’s stator windings (Figure 1). The six switches

form 3 pairs of “half-bridges”, which can be used to con-

nect the leg of a winding to the positive or the negative

high-voltage DC bus.

As shown in the figure, two switches on the same

“half-bridge” must never be on simultaneously, other-

wise the positive and negative buses will be shorted

together. This condition would result in a destructive

event known as “shoot-through”. If one switch is on,

then the other must be off; thus, they are driven as

complementary pairs. It should also be noted that the

switching devices used in the half-bridge (in this case,

IGBTs) often require more time to turn off than to turn

on. For this reason, a minimum dead time must be

inserted between the off and on time of complimentary

channels.

In AN843 and AN889, three PWM outputs were used to

drive a 3-phase inverter bridge. In these cases, however,

it was necessary to use external circuitry to generate the

complimentary control signal for the lower leg and insert

the proper dead time between them. For 3-phase

control, what is ultimately needed to drive a 3-phase

bridge is three pairs of complementary PWM outputs,

with dead time between the complimentary channels.

Hardware Overview

The motor control applications described in this

document were developed and tested on a production

model of Microchip’s PICDEM™ MC Development

Board. While the application software has been

designed with this platform’s control and communica-

tion requirements in mind, the control methods

discussed are applicable to any 3-phase ACIM control

application based on the PIC18F4431.

For a complete description of the board and its capabil-

ities as a development tool, please refer to the User’s

Guide for the PICDEM MC Development Board

(DS51453A). To give the reader a more clear idea of

the hardware platform, a brief overview and schematics

of the board are provided in Appendix A: “PICDEM™

MC Board Overview”.

FIGURE 1: A 3-PHASE INVERTER BRIDGE DRIVEN WITH 6 PWM INPUTS

PWM1

PWM4PWM2PWM0

PWM5PWM3

DC+

DC-

Stator Windings

1 2 3 4 5 6 7 ... 23 24

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