AD7874

REV. C

–12–

APPLICATIONS

Vector Motor Control

The current drawn by a motor can be split into two compo-

nents: one produces torque and the other produces magnetic

flux. For optimal performance of the motor, these two compo-

nents should be controlled independently. In conventional

methods of controlling a three-phase motor, the current (or

voltage) supplied to the motor and the frequency of the drive are

the basic control variables. However, both the torque and flux

are functions of current (or voltage) and frequency. This cou-

pling effect can reduce the performance of the motor because,

for example, if the torque is increased by increasing the fre-

quency, the flux tends to decrease.

Vector control of an ac motor involves controlling phase in addi-

tion to drive and current frequency. Controlling the phase of the

motor requires feedback information on the position of the rotor

relative to the rotating magnetic field in the motor. Using this

information, a vector controller mathematically transforms the

three phase drive currents into separate torque and flux compo-

nents. The AD7874, with its four-channel simultaneous sam-

pling capability, is ideally suited for use in vector motor control

applications.

A block diagram of a vector motor control application using the

AD7874 is shown in Figure 17. The position of the field is de-

rived by determining the current in each phase of the motor.

Only two phase currents need to be measured because the third

can be calculated if two phases are known. Channel 1 and

Channel 2 of the AD7874 are used to digitize this information.

Simultaneous sampling is critical to maintain the relative phase

information between the two channels. A current sensing isola-

tion amplifier, transformer or Hall effect sensor is used between

the motor and the AD7874. Rotor information is obtained by

measuring the voltage from two of the inputs to the motor.

Channel 3 and Channel 4 of the AD7874 are used to obtain this

information. Once again the relative phase of the two channels

is important. A DSP microprocessor is used to perform the

mathematical transformations and control loop calculations on

the information fed back by the AD7874.

VOLTAGE

ATTENUATORS

DAC

DAC

DAC

TORQUE

SETPOINT

IN1

V

V

IN3

V

IN2

*ADDITIONAL PINS OMITTED FOR CLARITY

TORQUE & FLUX

CONTROL LOOP

CALCULATIONS &

TWO TO THREE

PHASE

INFORMATION

TRANSFORMATION

TO TORQUE &

FLUX CURRENT

COMPONENTS

FLUX

SETPOINT

DRIVE

CIRCUITRY

ISOLATION

AMPLIFIERS

AD7874*

DSP

MICROPROCESSOR

I

C

I

B

I

A

V

B

V

A

3

PHASE

MOTOR

Figure 17. Vector Motor Control Using the AD7874