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High efficiency AC drive

Variable frequency drives (VFDs) started to appear in the market in the early 70’s. Since that time the basic design has remained the same with only minor improvements based mostly on advances in semiconductor components.

VFDs are used to control the speed of electric motors and are also often known as variable speed drives or VSD’s. Electric motors consume approximately 25% of the world’s electrical energy and control of the motor speed rather than fixed, across-the-line allows more efficient motor operation. The ability to vary the speed and control the motor acceleration also reduces wear and tear on driven mechanical components. Large efficiency gains can be made in pumping and compressing applications by using a PID loop control to run the motor at real application requirements and the PID loop control can often be implemented within modern drives themselves.


Conventional VFDs


A conventional drive rectifies the supply AC to charge capacitors and runs an internal DC bus at supply voltage. The drive then uses a series of transistors on the output side to generate a wave form to control the motor speed. The process of changing AC to DC and back to AC is why they are often also called inverters. A schematic of this design is shown below.

It is import to understand the output signal is not a pure sign wave form and the drive uses a combination of a carrier frequency and pulse width modulation (PWM) to control the motor. As a transistor is a digital and not analogue device it can only switch on and off. The drive varies the length of on or off time and the inductance of the motor effectively averages the voltage level at the motor. A schematic of this process is shown below.

The rapid switching from zero to DC bus voltage is the cause of the electric noise associated with VFDs and why EMI control methods such as cable shielding and earthing is importing in many VFD installations. As the DC bus is supplying this energy it must also draw down the charge from the incoming AC supply. In practice this means the drive will appear as a non-linear load to the power supply and significant distortion from a pure sign wave will need to be drawn from the power supply.


VFD’s can also cause an imbalance across the phases which results in leakage current at the motor to earth. This leakage current causes currently to flow through the motor bearings which can often lead to premature failure of the motor bearings.


Mathematically any waveform can be deconstructed to a series of sign waves of different harmonics known as a Fourier series. Some of these harmonics can be especially problematic to power supply system’s causing cables to overhead, circuit breakers to trip, voltage distortion and wasted energy. Total harmonic distortion, or THD, is a single metric often used to measure the distortion from a pure sign wave.


A different VFD design

A relatively new VFD design has been developed by Yaskawa which does away with the DC bus entirely. This new design uses 18 transistors arranged in a matrix which allows the drive to output a variable frequency without inverting the AC supply. This layout is shown in figure 3 and greatly reduces the harmonics in the system and the associated wasted power.


In this design any input phase can be connected to any output phase at any time with correct timing and sequencing used to generate the required output to the motor. In the convention design the drive can only switch between 0V and the DC bus voltage but in a matrix drive the drive can select between any of the voltage differences across the three phases. The drive still uses PWM output but only needs to switch between the two closest potential differences of the three supply phases. This in turn greatly reduces the magnitude of the pulses sent to the motor and sourced from the power supply. The design features ultra-low harmonic, near unit power factor and minimal leakage currents.


Integral to the design is the bi-direction layout of the transistor matrix. This means the drive is fully regenerative without any external devices. Power returned from the motor is feed straight back to the power supply rather than wasted as heat through a breaking resistor. This is ideal for high dynamic, back driven or lifting applications. The design also allows for the appropriate transistors in the matrix to be held closed in a bypass mode at near 100% efficiency.

There are alternative solutions to mitigate harmonics in drives. Such devices as active front ends, regeneration units and multi-pulse transformers are all available. These devices add considerable complexity and physical size of the installation. The matrix drive accomplishes all this in a simple three wire in, three wire out installation. The additional devices in alternative solutions also have their own wasted power and inefficiency. In real word, side by side testing, of a matrix drive against an active front end power savings have been observed of 15-20%.


The matrix drive is a true high efficiency, green solution.


For more information visit www.yaskawa.com or this page on our website.