The single spindle drive was realized as a brushless DC motor with sensorless detection of the rotor position by monitoring the third harmonic of the EMF. This fits perfectly to the simple frequency converter of the brushless motor, which can only produce voltage blocks. The minimum and maximum of the third harmonic voltage correspond with the commutation points.
Each drive has its own frequency converter and DC/DC power supply. The converter bridge is controlled by a programmable logic device (PLD) - or later on by an ASIC. The logic devices can be connected to almost any micro controller.
Fig. 7 shows the schematic diagram of the single spindle drive.
The micro controller realizes the communication with the textile machine controller. It also manages the start up of the drive. The sensorless rotor position detection does not work at very low speed (or standstill), so there is a special strategy needed for starting the motor. In a first step, the rotor is forced into a defined position by applying a voltage to the three motor phases. If voltage is applied to each of the three motor phases simultaneously, two of the three phases are therefore shortened in conjunction with the simple PWM strategy. This EMF in the shortened phases effects a current which will suppress the oscillation of the rotor.
Out of this position the motor can be started by performing two commutations shot after each other. Now, the axis of the magneto-motive forces of rotor and stator perform an angle of (el.), the torque is positive, the rotor will start rotating. After a delay time, depending on the moment of inertia of rotor and spindle, the rotor position detection is activated and the motor commutates itself. After a second delay time the speed controller is activated and the spindle runs up to operation speed of the spindle.
The micro controller supervises the textile process by monitoring the spindle torque which is proportional to the motor current. The detection of the current in the DC link can be achieved with low expense. There is no need for a high precision measuring because the breaking of a filament is recognized by the changing of the torque and not by the absolute value.