Microchip Technology MCP6 series Bedienungsanleitung PDF herunterladen (Seite 4)

AN900

DS00900A-page 4  2004 Microchip Technology Inc.

V/f Control with Current Feedback

A disadvantage of open-loop V/f control is that the

motor can stall if the speed is ramped up too quickly or

the load otherwise changes rapidly. Without some form

of feedback, it is impossible to detect whether the

motor is turning as expected, or if it is stalled.

A stall causes high currents and the motor loses torque.

By monitoring current, excessive slip can be detected,

and the motor frequency can be adjusted downward

accordingly. A high-current condition may also be

caused by a malfunction of the inverter bridge. If a

high-current condition persists, the drive should be shut

down to prevent motor overheating or other damage.

A conceptual diagram is illustrated in Figure 5. The

speed reference is provided by the user, in this case via

a potentiometer connected to an ADC channel. The V/f

function in firmware calculates the maximum PWM

duty cycle (amplitude) based upon the speed refer-

ence. The DC bus (bridge) current is measured using a

shunt resistor, which produces a voltage proportional to

the current through it. This voltage is amplified and

compared with an external comparator to a reference

level that corresponds to the maximum allowable bus

current. The comparator output drives the Fault A input

of the PIC18F4431. If the Fault signal is asserted, the

PWM output is inhibited for the following PWM period.

To detect a persistent overcurrent condition, the number

of times the Fault signal is asserted is monitored in

firmware. For example, if the Fault occurs more than 20

times within the last 256 PWM cycles, the motor is

stopped and an overcurrent condition is indicated by

blinking an LED. (The threshold number of events to

trigger the overcurrent Fault can be changed in the

firmware.)

The shunt voltage can also be monitored by using an

ADC channel to detect increasing current. This way,

corrective action can be taken by decreasing the drive

frequency before the hardware Fault is activated.

FIGURE 4: CLOSED-LOOP V/f CONTROL WITH CURRENT FEEDBACK

V/f Control with Velocity Feedback

In open-loop V/f control, the rotor is assumed to follow

the rotating flux generated in the stator, with a certain

degree of slip present depending upon the load. In

many applications, the load can vary widely and the

resulting motor speed will vary accordingly. To improve

speed control, a form of speed feedback can be added.

A simple implementation of closed-loop speed control is

illustrated in Figure 5. The reference speed is still set by

a potentiometer, as above. However, instead of directly

using the reference speed to determine the drive

frequency, it is compared to the actual motor speed to

generate a speed error signal. Actual motor speed is

established by a speed measurement with either the

Quadrature Encoder Interface (QEI) in Velocity mode, or

input capture of a tachometer signal. In this particular

application, the Quadrature Encoder Interface is used.

The speed error signal is then used as an input to a

Proportional-Integral (PI) controller, which determines

the desired drive frequency to the motor windings. The

standard V/f process determines the amplitude of the

drive waveform. The drive frequency and amplitude are

then used to update the PWM duty cycles of the six

PWM channels that drive the three-phase bridge.

Current feedback may also be used concurrently with

velocity feedback. For clarity, it is not shown in this

example.

FIGURE 5: CONCEPTUAL BLOCK DIAGRAM OF V/f CONTROL WITH VELOCITY FEEDBACK

PWM

3-Phase

Inverter

Bridge

ACIM

Current Feedback

V/f Function

Current

Limit

Current

Fault

Speed

Reference

PWM

3-Phase

Inverter

Bridge

ACIM

Speed

Reference

Speed

Error

Speed Feedback

PI Controller

Velocity

Calculation

∫

V/f Function

1 2 3 4 5 6 7 8 9 ... 23 24

Kommentare zu diesen Handbüchern

Keine Kommentare

Microchip Technology MCP6 series Bedienungsanleitung Seite 4

Kommentare zu diesen Handbüchern

Verwandte Produkte und Handbücher für Motherboards Microchip Technology MCP6 series