67 Frequency Converters Typical features: • Improved control properties compared with simple scalar control • Able to withstand sudden changes in load • No external feedback signal required • Unable to solve resonance problems • No torque control properties • Problems occur when attempting to control high-power motors • Problems in the event of load changes in the low speed range 2.7.3 Space Vector with and without Feedback The space vector control method is available with (“closed loop” ) or without (“open loop”) an external motor speed feedback.
As illustrated below, a feature allowing motor current polar transformation is added to the control (in the components responsible for magnetisation and torque-generating current). The voltage angle (θ) is regulated in addition to the voltage (U) and frequency (f ). Basic features: • Voltage vector control in relation to steady-state characteristic values (static) Typical features:
• Improved dynamic performance compared with scalar control • Very good at withstanding sudden changes in load (compared with scalar plus compensation) • Operation at the current limit • Possibility of active resonance dampening • Possibility of open-loop/closed-loop torque control • High starting and holding torque • Problems during rapid reversing compared with fl ux vector • No “rapid” current control 2.7.3.1 Space Vector (Open Loop) If the space vector without external speed feedback speed and position will be calculated by the control software and is based on information about motor current and motor frequency which is measured (see example on page 74, Fig.
2.26 Basic principles of Danfoss VVCplus control).