Microchip Technology MCP6 series Manual de usuario descargar pdf (Pagina 3)

 2004 Microchip Technology Inc. DS00900A-page 3

AN900

OVERVIEW OF CONTROL

STRATEGIES

Open Loop V/f Control

AC induction motors are often operated in open loop

with no velocity or position feedback. The V/f ratio is

maintained constant to provide a constant (maximum)

torque over the operating range. This form of control is

relatively inexpensive and easy to implement. Feed-

back from the rotor is not utilized and the rotor is

assumed to follow the rotating flux generated in the

stator, with a certain amount of slip present depending

upon the load.

To drive an AC induction motor, the 3-phase inverter

bridge is driven by a microcontroller’s PWM outputs, as

shown in Figure 2. By changing the PWM duty cycles

in a regular manner, the PWM outputs are modulated

to synthesize sinusoidal waveforms (three-phase AC)

across the three motor windings (Figure 3).

AC is applied to the three stator windings as three

sinusoidal currents, equal in amplitude and frequency,

but offset from each other by 120 degrees. As a result,

the current in the stator windings generates a rotating

magnetic field. This rotating field induces electromotive

force in the rotor, which in turn produces a magnetic

field in the rotor that attempts to align with the rotating

magnetic field in the stator. This causes the rotor to

rotate. See AN887, “AC Induction Motor Fundamentals”

(DS00887), for more details on induction motor

construction and operating characteristics.

The operation of an ACIM is governed by two

principles:

1. Base speed is directly proportional to the

frequency of the alternating current applied to

the stator and the number of poles of the motor.

2. Torque is directly proportional to the ratio of

applied voltage and the frequency of the applied

AC current.

Therefore, speed can be controlled by varying the input

frequency of the applied alternating current and torque

can be maintained constant by varying the amplitude in

direct proportion to the frequency. These are the two

basic aims of open-loop V/f control.

FIGURE 2: OPEN-LOOP V/f CONTROL

BLOCK DIAGRAM

FIGURE 3: SYNTHESIS OF 3-PHASE SINE WAVE

PWM

3-Phase

Inverter

Bridge

ACIM

V/f Function

Speed

Reference

DC+

DC-

Time

Voltage

PWM1 Output

PWM2 Output

PWM3 Output

1 2 3 4 5 6 7 8 ... 23 24

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