72 Frequency Converters The PWM switching patterns are calculated for the inverter using the selected control algorithm. U/f control is suitable for applications involving • Special motors (for example, sliding rotor motor) • Motors connected in parallel In the case of the applications referred to above, no compensation of the motor is required. With the VVCplus control principle, the amplitude and angle of the voltage vector are controlled directly, as is the frequency. At the heart of this method lies a straightforward, yet robust, motor model. The type of control method involved is called Voltage Vector Control (VVC). Some of the features off ered include:
• Improved dynamic properties in the low speed range (0 - 10 Hz) • Improved motor magnetisation • Speed control range: 1:100 opened loop • Speed accuracy: ±0.5% of the rated speed without feedback • Active resonance dampening • Torque control • Operation at the motor current limit 2.8.1 Danfoss VVCplus Control Principle The Danfoss VVCplus control principle uses a vector modulation method for constant voltage-source PWM inverters. Depending on the application control demands, the motor equivalent diagram can be simplifi ed (that is, the iron, copper and air fl ow losses are ignored) or used in its full complexity. Example:
A simple fan or pump application control uses a simplifi ed motor diagram. However a dynamic hoist application requiring fl ux vector control requires the complex motor equivalent diagram, accounting for all losses in the control algorithm. The inverter switching pattern is calculated using either the SFAVM or 60° AVM principle, to keep the pulsating torque in the air gap very small. The user can select the preferred operating principle, or allow the control to select one automatically on the basis of the heatsink temperature. When the temperature is below 75° C, the SFAVM principle is used for control. At temperatures above 75°, the 60° AVM principle is applied.