Inverters turn it around

Written by: Paul Fanning | Published:

Inverters and VSDs can make a huge difference to the energy efficiency of virtually any company using electric motors. Paul Fanning looks at the latest developments.

Variable-speed inverter drives have done much to improve machine design since they became commonplace about 30 years ago. They have allowed operational optimisation through speed trimming and enhanced controllability, and contributed massively to saving energy.

With this in mind, it is clearly important to keep abreast of the latest innovations and developments in this fast-moving technology.

One of the newest entrants to the field is Atlas Copco, which has developed its first in-house designed inverter drive, called Neos. All of the company's GA 37-90 VSD range compressors will now be equipped with the new Neos inverter units, which will also be available as spare parts for retrofitting existing GA models in the field.

The new Neos inverter, developed and manufactured in Atlas Copco's plant in Antwerp has been tailored precisely to meet the current and predicted requirements of its compressors in terms of size, simplicity, robustness and reliability.

The launch is a response to the fact that only 5% of the off-the-shelf electric motors and inverters available to OEMs are used in compressors. The majority of these inverters are designed for a broad variety of applications, including pumps and fans, with less demanding torque requirements than those of heavy duty compressor operations. Atlas Copco has therefore designed and developed the first dedicated unit for this function, based on 20 years of experience in manufacturing VSD compressors.

During the development of the inverter, an extensive programme of rigorous endurance field testing was conducted in a variety of hot, cold and dusty industrial conditions and involved running units up to 10,000 hours at maximum load. The performance of 16 drive units, installed on a mix of 55, 75 and 90kw standard and full-feature compressors, were compared in application sites as varied as Finland, Bahrain, Spain, Sweden, Germany and Belgium. These tests proved conclusively that the Neos unit is capable of operating in the the harshest working conditions at continuous full load from -20°C up to 50°C ambient temperatures, and is fully protected against dust ingress by its IP 5X rated enclosure.

70kg lighter and less than half the size of the largest generic inverter, the compact Neos design eliminates the need for a control panel. All communication is achieved via CAN connection with the compressor's Elektronikon Mk 5 controller or, in the unlikely event of trouble shooting, via Ethernet connection to a PC. The only other inputs to the unit's control board comprise the motor temperature sensor input, relay output for the cubicle fan control, two digital inputs for the run-enable function and service switch, and the safe torque off function.

The motor control platform, direct torque control (DTC) used extensively in ABB's variable-speed drives (VSDs) since 1996 are not of course new. However, they have been greatly improved to bring even higher accuracy in motor speed and torque control, as well as the ability to control more motor types.

This fourth generation DTC provides enhanced performance in open and closed loop and a higher switching frequency. Speed and torque control accuracy have been improved, for both dynamic and static values.

Support for high speed motors up to 500 Hz (as standard) and absolute encoder and resolver support are provided, while other improvements include more choice of identification (ID) set-up runs.
These include highly accurate standstill ID runs, allowing the drive to identify the motor from its performance characteristics without the need to uncouple the motor from the load.

Drives equipped with the new DTC can now respond to changes in the customer speed reference much more quickly, with the motor shaft responding within 2 ms of a reference change instead of 10 ms.

DTC lets users of the latest ABB industrial drives, ACS880, control any motor type without a change to the firmware, making it practical to use squirrel cage, permanent magnet and ABB's advanced synchronous reluctance (SynRM) motors, the latest motor control technology offering high efficiency IE4 or high output compact motor variants. This widens the scope for engineers to use almost any motor to suit their needs.

The ACS880 drive offers fully functioning scalar control with its own ramps, fixed- and skip- speeds. Scalar is useful in multi-motor applications and can be used to test installations before switching to DTC; particularly useful when the test motor is not the same size as the motor to be used on site.

With DTC, field orientation is achieved without feedback, using advanced motor theory to calculate the motor torque. DTC uses the fastest digital signal processing hardware available and a more advanced mathematical understanding of how a motor works. The result is a drive with a torque response that is typically ten times faster than any AC or DC drive. This gives much improved process control and a more consistent product quality. The dynamic speed accuracy of DTC drives will be eight times better than any open loop AC drives and comparable to a DC drive that is using feedback.

Some applications require particular feedback devices, for either position or speed feedback. Although DTC negates the need for speed feedback devices, accuracy can be further improved with an extensive range of encoders, resolvers and tachometers. The ACS880 drives have option slots to allow these items to be fitted internally, making installation convenient and cheaper.

DTC was developed to bring the best motor control of induction motors and is now extended to control other industrial motor platforms, being one of the few to generate full torque at zero speed on standard induction motors. It can be applied to high dynamic or highly arduous applications and can tackle any industrial application.

High dynamic torque control means applications such as grinders and crushers can be managed effectively, whilst the ability to generate torque at low speeds means DTC can effectively handle mixers or start fully loaded conveyors. DTC can be operated in master follower modes, making winders and strip lines easier to control, as well as cross travel or long travel crane control.

WEG, meanwhile, has added the CFW100 mini frequency inverter to its line of frequency inverters. This compact inverter is available in three sizes (A, B and C) for rated motor power from 0,18 to 0,75 kW and rated current from 1.6 to 4.2 A. With a height of 100 to 126 mm, a width of 55 mm and a depth of 129 mm, the CFW100 units are among the smallest frequency inverters currently available.

The mini frequency inverters with integrated micro-PLC are particularly suited to simple technical applications in the commercial and consumer sector, such as swimming pool pumps, motorised hospital beds, lift doors and fitness equipment, as well as small fans or mixing machines and special-purpose machines. With its combination of extensive functionality and extremely small size, the CFW100 is easy to integrate into electrical cabinets and machines. This gives users an especially compact and cost-effective solution for controlling the speed of three-phase induction motors.

The CFW100 operates three-phase motors from single-phase 200 to 240 V supply (50/60 Hz). It has an overload capacity of 150% of rated current for an interval of 60 seconds every 10 minutes. Voltage vector control (VVC) and V/f characteristic curve control are supported for driving three-phase induction motors. The CFW100 has electronic thermal protection capability to prevent motor overheating and four digital inputs as standard, with possibility to increase by expansion plug-in module. The enclosure protection rating is IP20, and the mini frequency inverter can be used at ambient temperatures up to 50°C without derating.

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