Frequency-controlled induction motor drive systems

Output parameters:

- output voltage depends on the thyristor rectifier voltage V
_{d}, - output frequency depends on the voltage inverter operation.

Brake operation of the motor:

- in case of brake operation of the motor the additional thyristor rectifier is needed with inverter operation.

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 k_{A}, k_{B}, k_{C}:

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:

Output parameters:

- output voltage and frequency depends on the the PWM voltage inverter operation.

Brake operation of the motor:

- diode rectifier doesn't allow to put back energy to the AC source during braking operation of the motor. The power is then dissipated on the resitor R
_{h}.

Types of pulse width modulation:

- with sinusoidal or trapezoidal modulation function,
- hysteresis modulation,
- space vector modulation,
- DTC-type modulation.

This method requires a triangle waveform of amplitude A_{i} and frequency f_{i} and a comparator. When the value of the reference signal (sinusoidal waveform of amplitude A_{m} and frequency f_{m}) is more than the triangle signal, the PWM signal is in the high state, otherwise it is in the low state.

Parameters:

- Frequency modulation coefficient:

- Amplitude modulation coefficient:

Advantages:

- simplicity,
- constant switching frequency.

In this method the motor current i_{A} is compared with a reference waveform i_{Aref} and based on the comparation result the inverter transistors are switched.

Advantages:

- simplicity,
- voltage source becomes a current source.

Disadvantages:

- variable switching frequency.

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 v_{st} could be obtained as a combination of active vectors v_{1}, v_{2} and zero vectors v_{0}, v_{7}.

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

The switching sequence is:

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.

Advantages:

- low total harmonic distortion coefficient of motor current,
- wide range of frequency control,
- low price.

Disadvantages:

- AC current distorted in case of using a diode rectifier,
- exposure of windings insulation to large and repeatable du/dt,
- capatitive currents in case of long cable,
- generation of electromagnetic interference.

Solution: filters and low-capatitance shielded cable.

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

Output parameters:

- output current depends on the thyristor rectifier current I
_{d}, - output frequency depends on the current inverter operation.

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 i_{sy}, could be expressed:

The amplitude and angular speed of rotor flux linkage ψ_{r} 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 δ_{min} to δ_{max}, resulting in the ripple of i_{sy} 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.

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:

- possibility of putting energy back to the AC source during brake operation of the motor,
- wide range of frequency control,
- possibility of using ordinary cables (normal insulation).

Disadvantages of system without PWM modulation and capacitive filter:

- high harmonic distortion of motor current,
- torque ripples,
- overvoltages.