The quad timer is an extremely flexible module, providing all required services relating to time events. The application uses four channels of the quad timer for:
-- One channel for PWM-to-ADC synchronization
-- Two channels for reading quadrature encoder signals (one channel in case the tachogenerator is used instead of a quadrature encoder)
-- One channel for system base of slow control loop (1ms period)
An adaptive closed loop rotor flux estimator enhances control performance and increases the overall robustness of the system. Sensitivity of the parameter drift can be considerably minimized in this way. Minimizing system cost the algorithm implements a single shunt current sensing reducing three current sensors to one. The high range of motor operating speeds up to 20,000RPM is another advantage of the presented design.
Such high speed is required e.g. by washer producers. The ratio between motor and drum is about ten in horizontal washers. Thus, for achieving drum speed 2,000RPM, motor has to run in 20,000RPM. Three-phase induction motors for washers are designed for nominal speed much lower than 20,000RPM (mostly about 6,000RPM). Higher speed is achieved utilizing so-called field-weakening algorithm which allows exceeding the nominal motor speed while the motor magnetizing flux maintaining at a level to prevent it from exceeding the nominal motor voltage. This feature introduces next cost and energy saving by using motor designed for lower nominal speed but that is possible to run up to 20,000RPM with a field-weakening algorithm.
Three-Phase Current Reconstruction
The vector control algorithm requires the sensing of the three motor phase currents. A standard approach is to sense the phase currents directly through current transformers, or Hall Effect sensors, directly coupled to the motor phase lines that carry the current between the switches and the motor. To reduce the number of current sensors and overall cost of the design, the three-phase stator currents are measured by means of a single DC-Link current shunt sensor; see Fig. 5. 
Fig. 5 DC-Link Current Sensor
The DC-Link current pulses are sampled at exactly timed intervals. A voltage drop on the shunt resistor is amplified by an operational amplifier inside the 3-phase driver and shifted up by 1.65V. The resultant voltage is converted by the ADC.
Based on the actual combination of switches, the three-phase currents of the stator are reconstructed. The AD converter measures the DC-link current during the active vectors of the PWM cycle. When the voltage vector V1 is applied, current flows from the positive rail into the phase A winding and returns to the negative rail through the B and C phase windings. When the voltage vector V2 is applied, the DC link current returning to the negative rail equals the T phase current. Therefore, in each sector, two phase current measurements are available. The calculation of the third phase current value is possible because the three winding currents sum is zero. The voltage vector combination and corresponding reconstructed motor phase currents are shown in Table 1.
Table 1. Measured Current
