DC & AC Drives – PWM, DTC (1)

Evolution of Drives :
1. DC Motor Drives
2. AC Drives, frequency control, PWM
3. AC Drives, flux vector control, PWM
4. AC Drives, direct torque control

1. DC Motor Drives

Features:

  • Field orientation via mechanical commutator
  • Controlling variables are Armature Current and Field Current, measured DIRECTLY from the motor
  • Torque control is direct

In a DC motor, the magnetic field is created by the current through the field winding in the stator. This field is always at right angles to the field created by the armature winding. This condition, known as field orientation, is needed to generate maximum torque. The commutator-brush assembly ensures this condition is maintained regardless of the rotor position. Once field orientation is achieved, the DC motor’s torque is easily controlled by varying the armature current and by keeping the magnetising current constant.
The advantage of DC drives is that speed and torque – the two main concerns of the end-user – are controlled directly through armature current: that is the torque is the inner control loop and the speed is the outer control loop (see Figure 1).

Advantages:

  • Accurate and fast torque control
  • High dynamic speed response
  • Simple to control

Initially, DC drives were used for variable speed control because they could easily achieve a good torque and speed response with high accuracy.

A DC machine is able to produce a torque that is:

  • Direct – the motor torque is proportional to the armature current: the torque can thus be controlled directly and accurately.
  • Rapid – torque control is fast; the drive system can have a very high dynamic speed response. Torque can be changed instantaneously if the motor is fed from an ideal current source. A voltage fed drive still has a fast response, since this is determined only by the rotor’s electrical time constant (i.e. the total inductance and resistance in the armature circuit)
  • Simple – field orientation is achieved using a simple mechanical device called a commutator/brush assembly. Hence, there is no need for complex electronic control circuitry, which would increase the cost of the motor controller.

Drawbacks:
• Reduced motor reliability
• Regular maintenance
• Motor costly to purchase
• Needs encoder for feedback

The main drawback of this technique is the reduced reliability of the DC motor; the fact that brushes and commutators wear down and need regular servicing; that DC motors can be costly to purchase; and that they require encoders for speed and position feedback.
While a DC drive produces an easily controlled torque from zero to base speed and beyond, the motor’s mechanics are more complex and require regular maintenance.

AC Drives Introduction

• Small size
• Robust
• Simple in design
• Light and compact
• Low maintenance
• Low cost

The evolution of AC variable speed drive technology has been partly driven by the desire to emulate the performance of the DC drive, such as fast torque response and speed accuracy, while utilising the advantages offered by the standard AC motor.
2. AC Drives – frequency control using PWM
Features:

• Controlling variables are Voltage and Frequency
• Simulation of variable AC sine wave using modulator
• Flux provided with constant V/f ratio
• Open-loop drive
• Load dictates torque level

Unlike a DC drive, the AC drive frequency control technique uses parameters generated outside of the motor as controlling variables, namely voltage and frequency. Both voltage and frequency reference are fed into a modulator which simulates an AC sine wave and feeds this to the motor’s
stator windings. This technique is called Pulse Width Modulation (PWM) and utilises the fact that there is a diode rectifier towards the mains and the intermediate DC voltage is kept constant. The inverter controls the motor in the form of a PWM pulse train dictating both the voltage and frequency. Significantly, this method does not use a feedback device which takes speed or position measurements from the motor’s shaft and feeds these back into the control loop. Such an arrangement, without a feedback device, is called an “open-loop drive”.

Advantages:
• Low cost
• No feedback device required – simple
Because there is no feedback device, the controlling principle offers a low cost and simple solution to controlling economical AC induction motors.
This type of drive is suitable for applications which do not require high levels of accuracy or precision, such as pumps and fans.
Drawbacks:
• Field orientation not used
• Motor status ignored
• Torque is not controlled
• Delaying modulator used
With this technique, sometimes known as Scalar Control, field orientation of the motor is not used. Instead, frequency and voltage are the main control variables and are applied to the stator windings. The status of the rotor is ignored, meaning that no speed or position signal is fed back.
Therefore, torque cannot be controlled with any degree of accuracy. Furthermore, the technique uses a modulator which basically slows down communication between the incoming voltage and frequency signals and the need for the motor to respond to this changing signal.

To be continue……….

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