Inverter control taken to another level

In an industry laden by three-letter acronyms, you could be forgiven for the collective groan that greets the introduction of another one. However, TLC (Three Level Control), the latest offering from Yaskawa, could instead be greeted with applause. Forming the main operational basis for Yaskawa’s latest drive, the G7, TLC uses a new core technology which, according to the company, should alleviate many of the functional drawbacks inherent in many other drive architectures. Yaskawa is not being coy on the price. This technology will come at a premium – with the company openly boasting that it is the most expensive drive it has ever produced – but the operational advantages are deemed to be strong enough to offset this premium. So, what is TLC? TLC uses 12 (instead of the usual 6) output transistors. The voltage of the main circuit is divided by capacitors and, of major significance, a neutral point (0V) has been introduced. The pulse width modulated (PWM) waveform therefore has a three-level electrical potential between positive (P), 0 and negative (N) compared with the two levels (P and N) of conventional designs. It has the effect of reducing the PWM waveform dv/dt by 50% – reducing the output voltage to the motor by half. This, in turn, prevents reflected voltage waves and bearing corrosion and allows longer cable runs with no additional hardware in the form of chokes. TLC also keeps the shaft current below the 0.3A danger level. The company has had to create a new control method and a unique power circuit which can stabilise the electrical potential at 0 using a clever, but simple, hardware configuration. Each phase also creates pulses with different switching frequencies – this results in a reduction in the radiated noise. Total noise levels have fallen by 5dB. Yaskawa has also developed a new dual-observer vector control method that directly compares the magnetic flux of the motor with a flux vector model as well as the normal comparison of the electrical characteristics of the motor (detected at the drive) with a motor model for the current and voltage. The two ‘observers’ are switched independently and automatically depending on operating speed range, leading to a doubling of the speed range. Torque accuracy equivalent to that of a DC machine is also achieved, thanks to the dual observer detecting the secondary magnetic flux vector. The application base for this new drive is potentially huge. But it is up to specifiers to make a decision for purchase based on the additional benefits as opposed to the selling price. Short termism could result in a lower-cost drive installation costing more due to maintenance and corrective action needed. The specification of drive/motor combinations already makes a lot of sense; the choice of which drive to use is now that much wider. • Many drive limitations have been resolved including bearing currents, wire lengths and harmonics problems • The premium price of the unit will be offset by the savings offered by the operational characteristics of the unit • Noise levels have been reduced by 5dB