Drives breakthrough on three levels

Yaskawa has produced a new AC inverter, the Varispeed G7, heralding the next generation of variable speed drive technology. The AC inverter has become a mature device for motor speed control that is widely used and continues to grow. With the introduction of energy conservation legislation in many parts of the world, including the UK with its Climate Change Levy, inverter application has been accelerated.

But first a little history…
The AC variable speed drive has come a long way in a relatively short time. The development of Pulse Width Modulation (PWM) technology enabled designers to create a digital simulation of an analogue sine wave and hence treat the motor control in a similar manner to that for DC machines. The invention of the Isolated Gate Bipolar Transistor (IGBT), now in its fourth generation, provided the high speed solid state switching required to pulse current as required by PWM techniques. Control of the voltage vector provided better speed control. Then a technique to control the flux vector (sensorless flux vector control) was pioneered by Yaskawa. This enabled vastly improved torque control by comparing current characteristics experienced at the drive with those expected from a "model" motor. It can be reasonably argued that improvements made in AC inverter technology since the introduction of sensorless flux vector have been minor refinements. Yaskawa's new G7 is an even more significant development than that of sensorless flux vector technology.

The compromise
However widely accepted inverters have become, they have remained beset by compromises. High levels of electrical and acoustic noise can be generated by the drives. It is still the case that poorly insulated motors can be damaged by line transients (electrical spikes) that are allowed to pass unimpeded through the drive's fast solid state switches. Motor bearings can become pitted through electrolytic corrosion caused by leakage currents from the motor shaft through the bearings. Where long motor cables are required, the high dv/dt generated by the PWM waveform is amplified and can create the need for chokes to be installed on the load side of the installation. All inverters are acoustically and electrically noisy and create potentially damaging harmonics on the electricity supply.

All of the above conditions have been well known since the birth of the AC inverter, but compromises have been made to exploit the inexpensive drive technology. European legislation governing the emission of Electro Magnetic Interference (EMC) and Radio Frequency Interference (RFI) was introduced a few years ago to make it an offence to create electrical interference. The EMC Directive created the need to filter interference and all electrical panels now have various filters fitted as countermeasures. One result of this is that drives manufacturers have been largely able to ignore the EMI/RFI emissions of their own equipment, since it is likely to be filtered within the application.

In the past year new legislation, in the form of the EC's G5/4 Recommendation, has placed control over the levels of harmonic interference that are allowed to be created on the supply network. This legislation has teeth, for the electricity companies have the right to refuse connection for new installations that create excessive harmonics. All devices with rectification circuits create harmonics, including TVs and PCs, but in the industrial field, AC inverters are a major culprit. G5/4 means that drives applications now require harmonic filters to be installed, to control harmonics down to the 50th harmonic (which is difficult to even measure).

Overcoming compromise
Yaskawa's Varispeed G7 has been designed to go a long way towards overcoming the compromises that plague AC inverters. The resultant design itself shows little compromise. At the heart of the new drive is a radical new core technology that Yaskawa calls Three Level Control. This fundamental electronic development has enabled a range of new features that not only significantly improve the performance of the drive, but also overcome many of the previous drawbacks of AC inverter technology.

The new features of the Varispeed G7 are considerable. The speed control range has been doubled from 100:1 to 200:1. Performance at extremely low speeds is both smooth and stable - there is full torque at zero speed (150% at 0.3Hz) - which means there is open loop zero speed control and better torque control. At the problematic low switching frequencies there is a dramatically reduced noise. There is also low radiated noise. Motor bearing currents are reduced to safe levels or even eliminated. Damaging micro surges due to reflected voltage waves have been eliminated. Leakage currents have been reduced. RFI, EMI and audible noise have all been reduced.

How it has been done
Most conventional modern inverters have six output transistors. Yaskawa has designed its own output transistors and there are 12 in the Three Level Control design. The voltage of the main circuit is divided by capacitors and the neutral point (0 voltage) has been introduced. The PWM waveform therefore has a three level electrical potential between Positive (P), 0 and Neutral (N) compared with the two level electrical potential between P and N of conventional designs.

This has the effect of reducing the PWM waveform dv/dt by 50%, which means the output voltage to the motor is always reduced by half. This serves to prevent reflected voltage waves and bearing corrosion. It also means longer cable runs are acceptable and require no additional hardware in the form of chokes. The neutral point (0) in Three Level Control is 400V peak-to-peak, which always maintains the shaft current below the peak danger level of 0.3A.

To apply its new Three Level Control, Yaskawa has developed a new control method and a unique power circuit, that can stabilise the electrical potential at 0 using a clever but simple hardware configuration. The result of this hardware development has been to simplify the main circuit and allow an even more compact design than found in conventional drives.

Each inverter phase creates a PWM pulse with a different switching frequency, resulting in the vastly reduced noise caused by the switching. Indeed, total noise levels have reduced by an impressive 5dB - a 3dB noise reduction has the effect of halving the perceived noise level.

Superior motor control performance
Yaskawa has developed alongside its Three Level Control, a new dual-observer vector control method. In conventional sensorless flux vector drives the electrical characteristics of the motor, detected at the drive, are compared with a motor model for the current and voltage. Yaskawa has added a further, direct, comparison of the magnetic flux that is compared with a flux vector model. The two observers are switched independently and automatically according to the operating speed range. Hence, the doubling of the speed range is achieved.

The dual observer detects accurately the secondary magnetic flux vector (j2) of an induction motor. The current control is performed using the j2 value as the reference. Therefore, highly accurate torque control, equivalent to that for DC machines, is achieved. This creates the 150% torque observed at just 0.3Hz and also enables recovery from impact loads at low speeds to be attained more rapidly than with previous technology. This control method can effect improved torque control by applying a limit directly to the torque current, based on the torque limit reference.

Extremely fast processing of the data from the dual observers is achieved using a high performance Central Processor Unit (CPU) and an Application Specific Integrated Circuit (ASIC). This means that torque characteristics and speed response are exceptional and even at very low speeds, the performance of the Varispeed G7 in open loop mode is equivalent to a system using encoder feedback from the motor.

Better motor protection
Compared with existing inverter models, the Yaskawa Varispeed G7's Three Level Control delivers great benefits for motor protection. The line voltage to the first motor coil is reduced by 35% below the safe working voltage of 690V, the phase-to-phase voltage is 22% below the safe working voltage of the motor insulation and 55% below the earth voltage potential. The Varispeed is the most motor friendly inverter drive ever.

There is a price
The new Yaskawa technology is not cheap. In fact, the company boasts that the G7 is the most expensive drive it has ever produced! However, for high performance applications, it is genuinely possible to replace a closed loop system for a vastly simpler open loop control (saving substantial encoder and cabling costs). Reduced EMI/RFI reduces noise filtering requirements. Harmonic reduction means the drive is less prone to contravene the G5/4 recommendation. There is also no need for chokes in installations with long cable runs. Finally, motors are protected and electrolytic bearing corrosion is eliminated. The drives high initial cost is more than made up for by its substantially lower operating cost.

Released May 2002