Advanced Applications

International English

Vector control is best understood by considering the operation of a DC machine. A DC machine consists of a field winding and armature winding. Therefore the armature Current (Torque) and field current (Flux) can be controlled independently. Independent control of the Flux and Torque producing currents permits optimum performance - Torque at zero speed, rapid response to load changes etc.

Field

Armature

Stator

Winding

Winding

Windings

DC Machine

AC Machine

Figure 8-6 Comparison. DC Motor/AC Motor

In an AC Machine, the stator winding currents set the Flux and the Torque; therefore it is difficult to control the torque and flux separately. Hence it is necessary to control the magnitude and phase - the Vector - of the current. To control the phase with reference to the rotor, the rotor position must be known. For full vector control, an encoder must be used to tell the inverter the rotor position.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Supply

AC

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Inverter

 

Encoder

 

 

 

 

 

 

 

 

AC Motor

 

 

Load

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Position Feedback

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 8-7 Position Fedback via a Motor Shaft Encoder

With the above arrangement, it is possible to achieve performance equal to that of a DC Machine provided the full parameters of the motor are known and the inverter is able to model its performance.

However, most AC motors are not fitted with encoders and the additional cost and complexity is an unnecessary expense.

Recent developments in motor control and modeling have allowed sensorless (that is without encoder feedback) vector operation to be possible. Sensorless Vector Control predicts the rotor position by mathematically modeling the motor. To do this the inverter must:

·Monitor the output voltage and current very accurately.

·Know the motor parameters (Rotor, Stator resistance, leakage reactance etc.).

·Know the motor history; that is, the previous load etc. in order to predict the motor temperature.

·Be able to calculate very rapidly.

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MICROMASTER Applications Handbook