TechArchive

Original article date: June 1998

With the drives market seeing massive advances over the last decade what will the next ten years have in store? Stuart Harvey of HID discusses the future for the drives market.

During the last five years inverters available on the European drives market have undergone a remarkable metamorphosis. Not only has the development of new technologies enabled better control algorithms faster processing speeds and superior motor modelling but the European Directives for EMC and Low Voltage have also placed new demands on the manufacturers.

The latest inverters incorporate V/F modes previously standard as options solely for the control of fans and pumps In this instance the drives are normally dual rated. For example an 11kW constant torque rated drive will deliver 15kW when the fan/pump mode is selected. The carrier frequency and thermal limits will all adopt their new characteristics automatically.

Modern drives also incorporate features such as automatic energy savings. By continuously scanning the reference speed requirement and comparing it to the actual speed estimation the drive reduces the output voltage to its lowest possible point without altering the motor speed.

Fuzzy logic is used by some intelligent drives in auto-tuning to measure load inertia but may also be adopted in the V/F modes to give tripless operation. It does this by negating the need to set acceleration or deceleration ramps regardless of load or inertia variations.

But by far the most revolutionary feature of virtually all modern drives however is the incorporation of sensorless vector control as standard. Although manufacturers have adopted different buzzwords to describe vector control they are all working towards a common goal – to achieve accurate control of the motor flux vector giving optimum rotor speed and torque. The higher the level of control the more accurate speed regulation and dynamic response to load will be resulting in overall efficiency increases.

The option to add or incorporate a feedback card to close the loop such as encoders or resolvers has become the norm in drive design. It is worth noting that closed loop vector drives were widely available long before commercially viable low-cost sensorless vector versions entered the market and resolved all the inherent problems created by their V/F predecessors.

With most modern inverters serial communications come as standard. For inverters with baud rates typically from 4800 to 19600 the typical on-board communications were two-wire RS232 or RS485. Over the last decade however higher speed communications with more nodes and longer distances for transmission became necessary. This requirement for higher speed communications forced the subject of bus support into the spotlight. This small but growing area of interest will quickly become a priority in drives design.

Far-sighted manufacturers were quick to develop either dedicated bus cards for the more commonly used European norms (such as Profibus Interbus and Modbus) or developed “ANYBUS” cards to complement their drive products. These cards are simple to install with the IC for handling system protocol simply plugging into the mains.

The next development expected in the market is achieving full torque at zero speed without use of a sensor – new control algorithms for estimation of slip frequency have already been developed. At Yokohama National University Professor Kawamura has already achieved accurate rotor speed control as low as 5rpm but below this level there are still a number of errors still to be resolved. These are the stator resistance variations the rotor resistance variations the spatial harmonics at very low speed due to the stator slots and the unbalanced parameters in the D-Q coordinates.

Reduced or zero harmonics is another key area almost certain to be realised within the next generation of drives. Starting with 12-pulse bridge technology as standard on all drives automatic phase displacement will lead to improved control of high inertia loads such as the use of mains regeneration as opposed to the current norm dynamic braking.

The AC asynchronous motor has long been recognised as the industry workhorse. It requires little or no maintenance as it has no brushes to wear has high protection integrity as standard (typically IP54) and is available worldwide with standardised speeds and motor frame sizes. In AC drive and motor packages below 75 it compares favourably in price to its DC equivalent. It is true that almost every AC motor would benefit from having an inverter installed on it even a fixed-speed 50Hz machine. All AC asynchronous motors are wound for an optimum flux level to produce the highest torque and efficiency even at mains frequency supply.

The effects of voltage fluctuation are dramatic affecting motor characteristics such as speed torque current efficiency power factor and temperature rise. These characteristics are all also affected by load variations. Electrical design aims to obtain optimum figures at the rated voltage. Most motors are wound for a limited range such as 400/440V or 380/420V three-phase and 200/220V or 230/250V single-phase.

Just as voltage changes affect motor characteristics so does the load imposed on the motor. The no-load speed of the motor is not affected and may be taken as the synchronous speed. The starting torque varies directly as the square of the voltage. Therefore a modern inverter with automatic voltage regulation can have improve the performance of a fixed speed 50Hz motor. AVR (automatic voltage regulation) is essential in modern vector drives to optimise the motor flux for changing power supply characteristics.

Even more apparent arguments for including an inverter on every motor include its inherent soft-start soft-stop capability which reduces mechanical wear and the energy savings offered through optimised control even for constant torque applications through vector control.

Another benefit of adopting inverter control is that it makes inefficient mechanical devices such as dampers in air handling redundant. In fact it could be argued the only real drawback to fitting an inverter on every AC motor would be its cost. Energy-saving legislation (see our feature earlier this year in February) will also play a significant part.

The key to the new products is the development of control architecture and hardware devices the ‘brains’ and ‘heart’ of the product. These developments should lead to a miniature encapsulated hybrid containing all of the building blocks for any AC drive system. This enables the manufacturer to decide whether this particular brain is for a servo drive closed loop or open loop vector drive or simply a V/F fan pump drive.

However at this stage one may also choose to load a custom ASIC because the brain is capable of being assembled in modular form and coupled to its heart (power modules) lungs (DC link and EMC filtering) and blood supply (input/output modules and communication modules). In this way any AC drive variant can be created from one universal hybrid. These modules will be made in production quantities only dreamed of today and incorporated into the drives widely used for current industrial markets as well as new markets at significant price reductions. And when this modular ‘brain’ unit becomes available an OEM will be able to construct its own inverter system at a fraction of the cost of an off-the-shelf complete drive module.

• HID Hitachi
• 01493 452525
• Stephen Scales

June 1998