Datasheet AD7874 (Analog Devices) - 13

AD7874 APPLICATIONS
A block diagram of a vector motor control application using the
Vector Motor Control
AD7874 is shown in Figure 17. The position of the field is de- The current drawn by a motor can be split into two compo- rived by determining the current in each phase of the motor. nents: one produces torque and the other produces magnetic Only two phase currents need to be measured because the third flux. For optimal performance of the motor, these two compo- can be calculated if two phases are known. Channel 1 and nents should be controlled independently. In conventional Channel 2 of the AD7874 are used to digitize this information. methods of controlling a three-phase motor, the current (or Simultaneous sampling is critical to maintain the relative phase voltage) supplied to the motor and the frequency of the drive are information between the two channels. A current sensing isola- the basic control variables. However, both the torque and flux tion amplifier, transformer or Hall effect sensor is used between are functions of current (or voltage) and frequency. This cou- the motor and the AD7874. Rotor information is obtained by pling effect can reduce the performance of the motor because, measuring the voltage from two of the inputs to the motor. for example, if the torque is increased by increasing the fre- Channel 3 and Channel 4 of the AD7874 are used to obtain this quency, the flux tends to decrease. information. Once again the relative phase of the two channels Vector control of an ac motor involves controlling phase in addi- is important. A DSP microprocessor is used to perform the tion to drive and current frequency. Controlling the phase of the mathematical transformations and control loop calculations on motor requires feedback information on the position of the rotor the information fed back by the AD7874. 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.
DSP MICROPROCESSOR IC DAC TORQUE & FLUX CONTROL LOOP I V DRIVE B B 3 CALCULATIONS & PHASE DAC CIRCUITRY TWO TO THREE V MOTOR PHASE A INFORMATION I DAC A TORQUE SETPOINT FLUX ISOLATION SETPOINT AMPLIFIERS VIN1 TRANSFORMATION TO TORQUE & FLUX CURRENT VIN2 COMPONENTS AD7874* VIN3 VIN4 VOLTAGE ATTENUATORS *ADDITIONAL PINS OMITTED FOR CLARITY
Figure 17. Vector Motor Control Using the AD7874 –12– REV. C