74 Frequency Converters The additional voltage U Comp is determined using the currents measured under the two conditions mentioned above (loaded and no-load) as well as the speed range: low or high speed. The voltage value and the speed range are then determined on the basis of the rated motor data. The control principle is illustrated in the block diagram below: Fig. 2.26 Basic principles of Danfoss VVCplus control As shown in Fig. 2.26 Basic principles of Danfoss VVCplus control, the motor model calculates the no-load references (currents and angles) for the load compensator (I SX , I SY ) and the voltage vector generator (I 0 , θ 0 ).
The voltage vector generator calculates the no-load voltage (U L ) and the angle (θ L ) of the voltage vector on the basis of the no-load current, stator resistance and stator inductance. The measured motor currents (I u , I v and I w ) are used to calculate the reactive current (I SX ) and active current (I SY ) components. Based on the calculated currents (I SX0 , I SY0 , I SX , I SY ) and the voltage vector actual values, the load compensator estimates the air-gap torque and calculates how much extra voltage (U Comp ) is required to maintain the magnetic fi eld strength at the reference value.
It then corrects the angle deviation (Δθ) that is to be expected due to the load on the motor shaft. The output voltage vector is represented in polar form (p). This enables direct overmodulation and facilitates connection to the PWM ASIC.