Frequency-controlled induction motor drive systems

Voltage-sourced inverter


Output parameters:

Brake operation of the motor:

The inverter output voltage may be treated as a vector which occupies six constant positions:

For six-step operation the voltage vector changes its position stepwise by π/3. The vaweforms of driving signals of inverter transistors kA, kB, kC:

Motor phase to phase voltages:

Motor phase voltages:

Output voltage vector of the inverter for first step:

The general relation describing the inverter output voltage vector:

Voltage inwerter with Pulse Width Modulation


Output parameters:

Brake operation of the motor:

Types of pulse width modulation:

PWM with sinusoidal modulation function

This method requires a triangle waveform of amplitude Ai and frequency fi and a comparator. When the value of the reference signal (sinusoidal waveform of amplitude Am and frequency fm) is more than the triangle signal, the PWM signal is in the high state, otherwise it is in the low state.

Parameters:

Advantages:

Hysteresis modulation

In this method the motor current iA is compared with a reference waveform iAref and based on the comparation result the inverter transistors are switched.

Advantages:

Disadvantages:

Space vector modulation

This method allows to obtain a mean value of voltage vector (within the sampling interval) by means of set of voltage vectors available from voltage inverter.

Depending on the transistors switching states:

it is possible to obtain at the output of the voltage inverter one of six active voltage vectors or one of two zero vectors:

Voltage vector vst could be obtained as a combination of active vectors v1, v2 and zero vectors v0, v7.


The duration times of two active vectors could be calculated basing on the equations:

The switching sequence is:

Influence of PWM voltage inverter on AC line

The classic drive system, connected to the AC line through a diode rectifier, is not able to put energy back to the source during brake operation of the motor. The other disadvantage is that the AC current is strongly distorted.

Nowadays a PWM rectifier is often used instead of a diode rectifier.

In case of using the PWM rectifier, the AC line current is quasi-sinusoidal and there is a possibility to put energy back to the AC source.

Properties of drive system with induction motor supplied with a PWM voltage inverter

Advantages:

Disadvantages:

Solution: filters and low-capatitance shielded cable.

Multi-level inverters

Multi-level inverters provide better shape of the output voltage (lower THD coefficient) and allow to obtain higher value of output voltage.


Current-sourced inverter


Output parameters:

There is no problem with putting energy back to the AC source during brake operation of the motor.

The inverter output current may be treated as a vector which occupies six constant positions:

Motor phase currents:

Output current vector of the inverter for first step:

The general relation describing the inverter output current vector:

where:

The electromagnetic torque, which depends on stepwise changing y-axis component of motor current isy, could be expressed:

The amplitude and angular speed of rotor flux linkage ψr is constant.

Due to the rotation of the rotor, for a given current vector the angle between the rotor flux linkage vector and the current vector varies in the range of δmin to δmax, resulting in the ripple of isy and therefore the torque ripple.

The frequency of torque ripples is six times higher than the frequency of motor current.

Torque ripple lead to speed ripple, however, electromechanical time constant smoothes the waveform of speed.

Current inwerter with Pulse Width Modulation


By using a capacitive filter the quasi-sinusoidal motor current is obtained. The waveform of motor torque is significantly smoother in comparision with the control without PWM.



Advantages of drive system with current inverter:

Disadvantages of system without PWM modulation and capacitive filter: