AC to AC Power Converters with Intermediate DC Link: Types, Features & Applications

This category of AC drives that is often referred to as “Variable Frequency Inverters” is one of the most extensively used drives in industrial motor control applications. Generally, the concept of these inverter drives can be summarized into three parts:

• Rectification of fixed frequency.
• Smoothing.
• Inverting to give variable frequency/variable voltage to feed an AC machine.

We have two main classifications of these AC drives that are based on the aforementioned concept:

• Voltage source in the sense that the converter impresses a voltage on the machine, and the machine impedances determine the current.
• Current source in which the converter impresses a current on the machine, and the machine impedances determine the voltage.

Voltage Source Inverters

The fixed frequency mains supply is a voltage source behind impedance. Voltage source inverters can be considered in the same way and as a result are very flexible in their application. The basic features of this category of inverters include:

• Multi-motor loads can be applied – this can be very economical in applications such as spinning machines, roller table drives, and so forth.
• Inverter operation is not dependent upon the machine – as a matter of fact, various machines such as induction, synchronous or even reluctance can be utilized provided the current drawn is within the current rating of the inverter. Precaution should be taken where a low power factor is used e.g. reluctance to ensure that the inverter can provide the needed VARs.
• In-built open circuit protection, very helpful in application where the cables between the inverter and motor are in some way insecure such as fed via slip-rings, subject to damage and so on.
• Motoring operation only in both directions is possible without the addition of resistive dumps for braking energy or costly regenerative converters to feed back to the supply.
• Facility to “ride-through’ mains dips can easily be provided by buffering the dc voltage link with capacitance or where necessary, a battery.

Pulse-Width Modulated (PWM) Voltage Source Inverter (VSI)

In the PWM inverter drive, the dc link voltage is uncontrolled and derived from a simple diode bridge rectifier (which only allows energy flow from the supply to the dc link). The output voltage can be controlled electronically within the inverter by employing PWM techniques. In this technique, the transistors are switched ON and OFF many times within a half cycle to generate a variable-voltage output which is typically low in harmonic content.

Most low-power inverters use MOSFET switching devices in the inverter bridge, and may switch at ultrasonic frequencies, which naturally results in quiet operation. Medium and larger power inverters use insulated gate bipolar transistors (IGBTs) which can be switched at high enough frequencies to be ultrasonic. However, a key point to keep in mind is that the higher the switching frequency, the higher the inverter losses and hence the lower the efficiency and therefore a compromise must be made.

Several PWM techniques exist each having different performance notably in respect to the stability and audible noise of the driven motor.

Utilizing the PWM technique, low-speed torque pulsations are virtually eradicated since negligible low-order harmonics are present. Thus, this is an ideal solution where a drive system is to be used across a wide speed range.

As voltage and frequency are both controlled with the PWM quick response to changes in demand voltage and frequency can be achieved. Additional, with a diode rectifier as the input circuit, a high power factor, approaching unity, is offered to the incoming AC supply over the entire speed and load range.

The efficiency of PWM inverter drive normally approaches 98% but this figure is heavily affected by the choice of switching frequency; the higher the switching frequency, the higher the losses in the drive. In practice, the maximum fundamental output frequency is typically restricted to 100 Hz in the case of gate turn-off thyristors (GTO) or about 1 kHz for a transistor based system. The upper frequency limit may be enhanced by making a transition to a less complex PWM waveform with a lower switching frequency and ultimately to a square wave if the application demands it. Nevertheless, with the introduction of faster-switching power semiconductors, these restrictions to switching frequency and minimum pulse-width have been reduced.

Six Step Square Wave Inverter

A typical dc link square wave voltage-fed inverter is illustrated below:

The three-phase AC supply is converted to DC in the phase-controlled rectifier stage. The rectified DC power is then filtered and fed to the inverter. The DC link reactor is usually small compared to that used in current source designs. As a matter of fact, in drives up to about 4 kW, it is not practically necessary. Some manufacturers omit the reactor in designs to 400 kW and above, however this has a significant effect upon supply harmonics and unduly stresses the rectifier and filter capacitor.

The inverter switching elements shown as transistors T1 to T6 are gated at 60° intervals in the sequence in which they are numbered in the diagram, and each transistor conducts for 180°. The feedback diodes D1 to D6 are connected in inverse parallel with the transistors and allow the return of energy from the reactive or regenerative loads through the inverter to the DC link.

The phase-controlled rectifier regulates the DC link voltage and this, in turn, determines the magnitude of the output voltage from the inverter. Hence, the output voltage/frequency relationship may be controlled to regulate the motor flux in the desired manner.

As a demonstration, the synthesis of inverter output voltage waveforms of a star-connected motor is shown below:

In reference to the above diagram, the phase-to-phase neutral voltage of the inverter has six-step wave-shape while the corresponding phase-to-phase voltage has 120° conduction angle. The output frequency is controlled by the rate at which the inverter transistors are triggered into conduction by the inverter control circuitry. Reversing the firing sequence of transistors in the inverter changes the direction of rotation of the motor, and no switching of power leads, either on the incoming supply or to the motor itself is needed.

In a square wave inverter each of harmonic voltage amplitude is inversely proportional to the harmonic order and hence there are no pronounced high-order harmonics. These are filtered by the motor leakage inductances.

Very high-speed motor operation is possible by increasing the output frequency. Faster switching devices like MOS transistors and insulated gate bipolar transistor (IGBT) can be used to achieve this performance.

It is well known fact that square wave inverter gives objectionable torque pulsations at low frequency operation, below approximately 5 Hz. This pulsating torque is due to the interactions of low order harmonics with the fundamental voltage, causing a stepping or cogging motion to the rotor running at low speed. Thus, the pulsating torque limits the low frequency operation of the square wave inverter. Suitable feedback control techniques or flux weakening can attenuate the low speed pulsating torque issue.

The existence of a phase-controlled rectifier to control the voltage of the inverter as shown in Figure 1.1 above is an inherent shortcoming of this circuit. The phase-controlled rectifier will present at low power factor to the AC supply, at low speeds, and since the DC link filter capacitor is large, it reduces the response time to voltage and hence speed changes. If the drive system is one for which regenerative braking operation is a requirement, the rectifier has to be inverse-parallel type. The input power factor and response time of the drive can be increased by replacing the phase-controlled rectifier with a diode rectifier feeding a DC chopper which regulates the input voltage to the inverter. For recovering regenerative energy of the load, a two-quadrant chopper will be needed. The alternative supply converter arrangement of a diode bridge plus a chopper also provides a fixed voltage link which is more economically buffered if mains dip ride-through is required.

Advantages of Square-Wave Inverter

The advantages of the square-wave inverter are: high efficiency (98%), suitability to standard motors, potential good reliability and high-speed capability. But, it suffers from low-speed torque pulsations as mentioned earlier and possible low-speed instability.
Applications of Square-Wave Inverters

The voltage-fed square-wave drive is typically employed in low-power industrial applications where the speed range is limited to 10 to 1 and dynamic performance is not of significance.

In modern industrial motor control applications, this type of drive has largely been superseded by PWM type voltage-fed inverters. Nonetheless, the voltage-fed square-wave inverter can be easily adapted to multi-motor drives where the speed of a number of induction motors can be easily tracked.

Square-wave drives are also used in some high frequency (> 1 kHz) and some high-power applications.

Current Source Inverters

Most of inverters utilized in motor drives are voltage source inverters (VSIs), in which the output voltage to the motor is controlled to suit the operating conditions of the motor. Notwithstanding, current source inverters (CSIs) are still sometimes used, especially for high-power applications.

Current source drives are usually, but not always, single-motor systems, and since current is controlled, have a simple short-circuit protection.

In contrast to voltage source inverters, full four quadrant operation is inherently possible with current source inverters.

Current source inverters are typically used with induction and synchronous motors.

We will look at three types of current source inverters in the following sections.

Forced-Commutated Current-Fed Induction Motor Drive

This was one of the most extensively used current source inverters. The figure below illustrates this type of motor drive:

This particular motor drive was strongly favored for single motor applications for a long period and was available at power levels in the range 50-3500 kW at voltages typically up 690 V. High-voltage versions at 3.3/6.6 kV were also developed but they have proved not to be cost effective, hence most manufacturers stopped producing them.

The DC link current I_d taken from a “stiff” current source (usually in the form of a thyristor bridge and a series inductor in the DC link), is sequentially switched at the required frequency into the stator windings of the induction motor. The capacitors and extra series diodes provide the mechanism for commutating the thyristors by exploiting the reversal of voltage resulting from resonance between the capacitor and the motor leakage reactance. The resultant motor voltage waveform is approximately sinusoidal apart from the superposition of voltage spikes caused by the rise and fall of machine current at each commutation.

The operating frequency range is typically 5-60 Hz, the upper limit being set by the relatively slow commutation process. Below 5 Hz, torque pulsations can be problematic but PMW control of the current can be used at low frequencies to alleviate the problem.

This drive system was typically used for single motor applications such as fans, pumps, extruders and compressors, where very good dynamic performance is not required and a supply power factor which decreases with speed is permitted.

Converter-Fed Synchronous Machine Drive

Once rotating, a synchronous machine generates AC voltages which can be used for the natural commutation of a converter connected to its terminals. As a matter of fact, the connected synchronous machine behaves as the mains in respect to AC to DC converters.

The figure below shows the basic components of this type of drive:

A low-impedance or ‘stiff’ DC current source is needed and is obtained from a controlled rectifier and a series reactor. With a ‘stiff’ current source, the output current wave is not greatly affected by the magnitude of the load.

The synchronous machine can be approximately represented by a counter-emf in series with an equivalent leakage inductance. The DC current is switched through the inverter thyristors (Th1…) so as to establish three-phase, six-stepped symmetrical line current waves. Each thyristor conducts for 120° and at any instant one upper thyristor and one lower thyristor remain in conduction.

It is an essential requirement to maintain an approximately constant angular relationship between the rotor and stator magnetomotive forces (MMFs) and hence automatically maintain the correct inverter frequency. The inverter doesn’t impose a frequency upon the machine; rather the machine itself determines the frequency. The motor cannot therefore pole-slip. The drive is accelerated by increasing the current fed to the drive which then accelerates and thereby increases the frequency.

Just like with DC drives, the AC supply power factor is poor at low speeds. Full four quadrant operation is possible without additional components.

To start these drives, special procedures are necessary, due to the fact that at standstill the machine voltage is not available to commutate the current. Basically this is usually accomplished by momentarily switching OFF the DC link current every sixth of the cycle. This permits the thyristors in the inverter to turn OFF so that the next pair can be fired. Above approximately 5% of rated speed the machine generates enough voltage for natural commutation and control is done in the same way to that of a DC drive.

Applications of a Converter-fed synchronous machine drive

The applications of this drive can be classified into two groups:

1. Starting converters for large synchronous machines, the converter being rated only for a fraction of the machine rating.
2. As large high power (and sometimes high speed) variable speed drives for a variety of applications. Power ratings, typically from 1.5 to 30 MW at speeds up to 8000 rpm are available. It is important to note that high voltage drives are offered with supply voltages up to 5 kV typical; however systems up to 25 kV are in service where the high voltage converter technology is the same as that used for HVDC power converters.

Converter-fed Induction Motor Drive

In contrast to synchronous machine, the induction motor is unable to provide the VARs or terminal voltage to commutate a converter connected to its terminals. But commercial schemes are available which are closely based upon the converter-fed synchronous machine drive having additional components to provide the VAR compensation. A typical illustration of a converter-fed induction motor drive is shown below:

In its simplest form, the compensator (refer to figure above) could comprise capacitors coupled with appropriate switches. The control of such a system is rather incorporated. Most often than not, it is better to employ a cycloconverter or even an auxiliary synchronous machine to provide the commutation and motor VARs.

This system is only suitable for high-power drives typically above 4 MW where an induction motor is preferred.

Related articles:

• DC to DC Power Converters: Function, Types, Operation & Applications
• Basic Features of Power Semiconductor Controlled Drives
• Key Facts about Thyristor DC Motor Drives Operation
• Single-Phase & Three-Phase Inverters: Function and Operation
• Three-Phase Diode Bridge Rectifier: Function & Operation
• Speed Control of a DC Compound Motor
• Power MOSFET Motor Control