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AD7874 A block diagram of a vector motor control application using the AD7874 is shown in Figure 17. The position of the field is derived 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.

APPLICATIONS Vector Motor Control

The current drawn by a motor can be split into two components: one produces torque and the other produces magnetic flux. For optimal performance of the motor, these two components 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 coupling effect can reduce the performance of the motor because, for example, if the torque is increased by increasing the frequency, the flux tends to decrease.

Simultaneous sampling is critical to maintain the relative phase information between the two channels. A current sensing isolation 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 addition 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 components. The AD7874, with its four-channel simultaneous sampling capability, is ideally suited for use in vector motor control applications. DSP MICROPROCESSOR TORQUE & FLUX CONTROL LOOP CALCULATIONS & TWO TO THREE PHASE INFORMATION

IC DAC DRIVE CIRCUITRY

DAC

IB

VB VA

3 PHASE MOTOR

IA

DAC

TORQUE SETPOINT ISOLATION AMPLIFIERS

FLUX SETPOINT VIN1 TRANSFORMATION TO TORQUE & FLUX CURRENT COMPONENTS

VIN2

AD7874* VIN3 VIN4 VOLTAGE ATTENUATORS *ADDITIONAL PINS OMITTED FOR CLARITY

Figure 17. Vector Motor Control Using the AD7874

–12–

REV. C


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