E-learning Resources in Microelectronics Control of Induction Motors

Universität Paderborn

Document Information
 

1. Introduction 2. Pulse amplitude modulation (block-operation)

3. Pulse width modulation (PWM 4. Space-vector modulation

Author:

Staff of Universität Paderborn

Basics

Where p, the number of poles, cannot change constantly. Hence, for speed control it is necessary to vary the frequency with suitable electronic components. Several technical possibilities are available to drive a synchronous machines.

Stucture of a frequency converter
The most common inverter topology is a V-inverter or pulse inverter with a DC voltage link circuit.

The input-rectifier is normally not controlled, so the DC link capacitor is loaded with the rectified mains voltage. For the rectifier allows only energy flow from the mains to the DC link. In case of braking (reverse-energy flow from motor to DC link) a chopper with braking resistor is necessary to protect the capacitor from over-voltage. Depending on the DC link voltage level, the chopper loads the DC link with the braking resistor to convert the excess energy into heat. Recent developments with an inverter instead of a simple rectifier allow energy flow in both directions.
The output voltage can be adjusted in voltage and in frequency.

Operating characteristics
 

By altering the frequency and voltage with the inverter, the commonly known speed-torque characteristic of the AC induction machine with squirrel cage for mains operation can be displaced beyond the speed axis (see Fig.). In order to reach constant flux, frequency and voltage are proportional. In this region (A) the motor can be loaded with constant (rated) torque. When the voltage reaches the maximum value and the frequency is increased further, the flux and thus also the rms torque decrease in inverse proportion. This region is called field weakening (F).

For high-end applications such as servo drives, the operating characteristics of a DC machine can be achieved even with induction machines using the principle of field-orientated control.

Modulation methods
In order to generate three-phase sinusoidal output voltages, different techniques are used:

• pulse amplitude modulation (block operation)
• pulse width-modulation
• Space-vector-modulation
• Hysteresis current modulation

All the named methods are used, for example, in converters with intermediate voltage or current circuit. The following diagram show how to build up a frequency converter with a DC voltage-circuit.
 

The drivers shown are intelligent electronic components which allow control of the power switching IGBTs. An internal logic contains complete protection against overcurrent, undervoltage and overtemperature ... The main function is to fire on and fire off the IGBT´s with appropriate pulse- patterns using modulation methods.
 

This arrangement can be compared with an IPM in the first instance. The pulse patterns must be generated for example by microcontrollers or REAL TIME systems . Until the default speed is reached (given on the motor typeplate) it is important to hold the quotient UMotor/fMotor = constant in order to ensure that flux is also constant for equivalent torque.
 

2. Pulse amplitude modulation (block operation)

Pulse amplitude modulation refers to the principle that electrical tension, Ud, can vary in intermediate circuits. This is due to a controlled input rectifier. One example would be to use Thyristors or GTOs.

To control the motor voltage U, the value Ud must vary depending on the chosen speed of machine, . This is due to an increase or decrease in Ud by using of controlled rectifier modules like thyristors or GTOs. The form of pulse is constant against pulse width modulation. Only the time period varies in attaining motor speed. 

The equivalent block diagram shows the technical structure of PAM. The output stage controls only the frequency of the rectangular pulses.

It is necessary to produce a sinusoidal current for the motor. However PAM is not the best method of generating good harmonic currents, as shown in the example below.
 

The current is not sinusoidal. A better idea is to choose another method of modulation.

The next chapter describes the function and principle pulse width modulation (PWM).
 

3.Pulse width modulation (PWM)

Pulse width modulation is used for generating sinusoidal currents inside the motor. To provide sinusoidal voltages, pulses are generated in the field. The width of pulses varies according to the required voltage for the corresponding frequency. This is described by the following diagrams.

Pulse Width Modulation is based on the principle that a sinusoidal waveform overlaps higher frequency triangular osscilation. The times, where the value of the triangle oscillation is lower than the value of the sinusoidal waveform, pulses are produced and defined as 1. At other times no pulses are generated. See the diagram below.

The output stage controls U and f, So the input rectifier can easily be built using diodes. U is only variable due to varying the pulse width as shown.

Advantages of sinusoidal pulsing:

• Approximately sinusoidal machine currents (Low current overoscillation)
• Low additional heating inside the machine
• Lower pendular torque
• True running even at lower speed

 

4. Space-vector modulation

A better use of PWM is space vector modulation. The six valves of a frequency converter can accept 8 switching states. This behaviour also generates eight voltage vectors. In combination with the 120 degree phaze shift caused by inductances of the machine they form a regular hexagon. The seventh and eighth vectors are generated by connecting the motor input connectors to the + or - terminal of intermediate circuit voltage.
 
 

S3 S2 S1 Vector 0 0 0 1 0 0 1 1 0 1 1 1 1 1 0 1 0 0 0 0 0

The vectors describe the switching states of one sector of the hexagon. The other sectors are similarly to the one shown but use other switching series. It is now possible to define a rotating voltage-vector on a circle generated by switching the valves at different times, as shown in the following picture.
 
 

For the switching time: The time for zero vector is: Polar coordinates are used to represent vector P:. This can be written as: Substituting in the first expression:

Exercise: Calculate from the last term the times for the left and right vector. Show the intermediate steps to generate the times in the following order:

 

These terms can be used for programming the space vector modulation method to generate pulses by microprocessor systems.

Hysteresis Current Modulation

Hysteresis Current Modulation is a direct method (a group of on-line optimized methods) of controlling the speed of electrical drives. The main principle is direct pulsing of motor currents. The machine currents are measured by the control unit and they approximate to a predefined value.
 

A hysteresis block compares the currents Imust and Ireal. With a defined difference between both values, the valves Vup and Vdown switch on or off. The next diagram shows the correlation.

 
 

The diagram shows the switching states of both valves. The pulses are generated when the real current reaches the upper or lower value of hysteresis.