DC & AC Drives – PWM, DTC (2)

3. AC Drives – flux vector control using PWM

Features:
• Field-oriented control – simulates DC drive
• Motor electrical characteristics are simulated – “Motor Model”
• Closed-loop drive
• Torque controlled INDIRECTLY

To emulate the magnetic operating conditions of a DC motor, i.e. to perform the field orientation process, the flux-vector drive needs to know the spatial angular position of the rotor flux inside the AC induction motor. With flux vector PWM drives, field orientation is achieved by electronic means rather than the mechanical commutator/ brush assembly of the DC motor.
Firstly, information about the rotor status is obtained by feeding back rotor speed and angular position relative to the stator field by means of a pulse encoder. A drive that uses speed encoders is referred to as a “closed-loop drive”. Also the motor’s electrical characteristics are mathematically modelled with microprocessors used to process the data. The electronic controller of a flux-vector drive creates electrical quantities such as voltage, current and frequency, which are the controlling variables, and feeds these through a modulator to the AC induction motor. Torque, therefore, is controlled INDIRECTLY.

Advantages:

• Good torque response
• Accurate speed control
• Full torque at zero speed
• Performance approaching DC drive

Flux vector control achieves full torque at zero speed, giving it a performance very close to that of a DC drive.

Drawbacks:

• Feedback is needed
• Costly
• Modulator needed

To achieve a high level of torque response and speed accuracy, a feedback device is required. This can be costly and also adds complexity to the traditional simple AC induction motor. Also, a modulator is used, which slows down communication between the incoming voltage and frequency signals and the need for the motor to respond to this changing signal. Although the motor is mechanically simple, the drive is electrically complex.

4 AC Drives – Direct Torque Control

Controlling variables:
With the revolutionary DTC technology, field orientation is achieved without feedback using advanced motor theory to calculate the motor torque directly and without using modulation. The controlling variables are motor magnetising flux and motor torque.
With DTC there is no modulator and no requirement for a tachometer or position encoder to feed back the speed or position of the motor shaft. DTC uses the fastest digital signal processing hardware available and a more advanced mathematical understanding of how a motor works.
The result is a drive with a torque response that is typically 10 times faster than any AC or DC drive. The dynamic speed accuracy of DTC drives will be 8 times better than any open loop AC drives and comparable to a DC drive that is using feedback.
DTC produces the first “universal” drive with the capability to perform like either an AC or DC drive.

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