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8.5Vector and FCC Control
8.5.1What is a Vector Drive?
Vector control is a complex mathematical control function that improves the performance of an AC drive. It is necessary to understand conventional Voltage to frequency control and Flux current control in order to understand Vector Control.
Voltage to Frequency Control
A simple inverter drive outputs a variable frequency and voltage to the motor, with an additional voltage boost (see section 5.8.9) to overcome losses and non linearity at low frequency. In this case, the voltage rises linearly as the frequency increases, which theoretically maintains the correct flux level in the motor up to the base frequency. For pump and fan applications the load is reduced at frequencies below base frequency, so a reduced voltage is acceptable. In these cases a quadratic (pump and fan curve) can be specified.
These voltage levels are suitable for many applications. However, for higher performance a better control strategy is needed. In particular, to offer the same capability as a DC drive and motor (fast response, torque control, low speed operation) complex mathematical models of the motor are needed. The processing power to achieve this has only become available recently at low cost. Siemens standard drives have offered different solutions such as Flux Current Control and now offer Sensorless Vector control in some variants.
8.5.2What is Flux Current Control?
The Siemens Standard Drives have developed an improved current monitoring system which allows accurate measurement of the output current with reference to the motor voltage. This enables the monitoring system to separate the total output current into the real (load) and imaginary (flux) part.
I total 
I load
I flux
Figure
The flux part can then be controlled, and therefore the flux on the motor optimized for all conditions. This is Flux Current Control (FCC). It has proved very successful in a wide variety of applications. It offers improved motor efficiency and better torque and transient response compared to standard Voltage to frequency operation. It is not as effective as full vector control or sensorless vector control.
Vector Control
MICROMASTER Applications Handbook | 53 |