AD7874
AP P LICATIO NS
Vector Motor Contr ol
A block diagram of a vector motor control application using the
AD7874 is shown in Figure 17. T he 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.
T he 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. T his 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.
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.
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. T he AD7874, with its four-channel simultaneous sam-
pling capability, is ideally suited for use in vector motor control
applications.
DSP
MICROPROCESSOR
IC
DAC
TORQUE & FLUX
CONTROL LOOP
CALCULATIONS &
TWO TO THREE
PHASE
IB
VB
VA
3
DRIVE
CIRCUITRY
PHASE
MOTOR
DAC
DAC
IA
INFORMATION
TORQUE
SETPOINT
ISOLATION
AMPLIFIERS
FLUX
SETPOINT
V
IN1
TRANSFORMATION
TO TORQUE &
FLUX CURRENT
COMPONENTS
V
IN2
AD7874*
V
IN3
V
IN4
VOLTAGE
ATTENUATORS
*ADDITIONAL PINS OMITTED FOR CLARITY
Figure 17. Vector Motor Control Using the AD7874
–12–
REV. C
ADI [ ADI ]
