Motors - Evolution in design, revolution in efficiency06 December 2004

Managing your motors for efficiency and reliability can be a headache at the best of times. And with discounted energy for industry perhaps set to disappear, against a backdrop of emissions regulations, then clearly it's not becoming any easier.

It is a fact that some 65% of energy used throughout Europe is consumed by the operation of electric motors. Although recent years have not seen any revolutionary changes in the way electric motors - be they AC or DC - are designed and built, there have been important changes that have reduced the costs of operating this machinery. These evolutionary developments have included the introduction of new materials for frames, coatings, bearings, enclosure protection and conductor insulation - and new ways of using these materials - alongside improved starters and drives.

In some of the more extreme operating environments, from quarrying through to energy production or transport, increased efficiency of motor operation has been achieved by improvements in design and construction, choice of materials, or cooling arrangements. But equally, better motor designs together with better drive technology have provided flexibility, longer service life and significant reductions in energy consumption.

Electric motors consist of a basic platform constructed from a cast iron or aluminium alloy frame, which houses stator cores built from stacks of thin, steel laminations insulated by heat-resistant inorganic resins. Radial ducts provide cooling and eliminate hot spots. The stator assemblies are vacuum-pressure impregnated with epoxy and polymer; the windings, in solid annealed copper, are secured in position with insulation material.

The whole assembly is then pressure impregnated with epoxy resin. Rotors are generally built from cylindrical or spider shafts in steel, with the cores shrunk, or as an interference fit onto the shafts, again with resin impregnated copper windings.

How should a motor that is energy efficient and meets the demands of the process, be characterised? It should
- deliver excellent starting characteristics
- have low noise and vibration levels
- have IP protection that is fit for the purpose
- provide reliable bearings and windings, leading to long maintenance intervals
- deliver a long lifecycle
- offer reduced operation and servicing costs
- be flexible in process operations
- provide significant cost savings on the company energy bill
- meet and exceed all current environmental impact legislation in manufacture and use.

One aspect of the design of a modern AC motor that has improved the machine's characteristics significantly, added to its service life and reduced its operating costs is the insulation for stator and rotor copper conductors. Typical of the materials used today is that of ABB's Micadur product. The MCI insulation system is applied to the entire range of the company's form-wound low and high-voltage AC products, with stator coils built up from insulated conductor wires and Corona resistant polymer/mica insulation tape combinations. Each assembled coil is wrapped in a porous mica tape. Complete with fibreglass cords for coil overhang support, the whole assembly is then epoxy resin coated using a vacuum-pressure impregnation (VPI) process.

All industrial plant uses a wide range of motors, from the very specialist linear motors, steppers and servomotors, typical of production-line installation, to the heavier, 'back office' equipment essential to the operation of the factory, and the business. For primary and secondary industries, from quarries and mines to steel production, heavier, more powerful machinery is a fundamental part of the production process.

Of course, all industries have budgets that reflect targets for expenditure on energy and plant, while an effective motor management policy can deliver savings on both capital and operational expenditure, by selecting the right tools for the job.

In marine service, electric motors drive a wide range of machinery, from 'hotel side' systems covering air-conditioning plant, boiler pumps, ballast and fuel pumps, through to fans, winches and windlasses. The most innovative uses of AC and permanent magnet induction motors, and the solution of choice for cruise and ferry operators, are the podded propulsors.

These systems consist of a propulsion module (motor, shaft and propeller assembly) and azimuth module/strut, connecting through the hull into a propulsor room. The majority have a single fixed pitch propeller, mounted at the end of the drive motor's armature shaft. Almost all the components are standard.

Characteristics of motors used for ship propulsion include low noise and vibration signatures, high availability and reliability, redundancy and optimal lifecycle costs. This latter aspect comes from the use of high-power electronics, combined with digital control systems, where algorithms developed within the software are capable of responding to rapid changes in power supply demands.

Less complexity, more power

The use of synchronous motors, particularly in cruise ship operations, has reduced complexity in drive motor design, and state-of-the-art induction motors are increasing power density. This is where the future lies: reduced machinery space occupancy, and more power from less weight. Newer drive motor designs have produced a range of permanent magnet motors, with axial, radial and transverse flux machines moving the technology forward, further increasing the power density. With its advanced AC induction motor, Alstom delivered a compact, high-power machine that met all of the criteria for the Electric Ship Technology Demonstrator project in the UK. This 20MW motor occupies a space of no more than 3m square, and at 100% of its rated speed is 97% efficient. As in other industries, the use in marine service of variable speed motors and drives provides examples of machinery that operate at optimum conditions, and where maximum torque is always available.

Rail transport has a long tradition of innovation in the use of electric motors and drives. The initial standard DC traction motors with armature and field coils wound in series have been replaced by DC motors, where field and armature coils are connected separately through thyristor circuits to the transformer, to provide separate excitation ('Sep-Ex') of the traction motors. Finally, AC drives and motors arrived, where lightweight/high-power induction motors are fed through a variable voltage, variable frequency source - the converter/inverter.

But it's not just the electric traction motors used to propel the train that are subject to some of the harshest operating conditions. The auxiliary equipment needs to be designed to withstand impact, dust, weather, and the excessive vibrations of a train that is constantly on the move, whether accelerating, braking, or being shunted around. Shock loadings of from 3 to 10 g-force are not uncommon in railway service. Typical applications for these auxiliary motors include cooling systems for the engine and brakes, and air conditioning for the passenger compartments.

Traction motors are generally force-ventilated machines, while for auxiliary applications they can be self-ventilated and are constructed wherever possible from aluminium alloy instead of cast iron to minimise weight. These auxiliary motors are expected to operate under variable power supply conditions while maintaining high efficiency and power factor levels - to satisfy the increasing demands from train operators to reduce overall loading and improve locomotive efficiency, without sacrificing flexibility. Variability in voltage and different traction power systems can have a dramatic effect on the motor operating conditions; converter/inverter supplied motors can experience switching spikes of up to 1kV and a dV/dt up to 1000V/µs as result of consequent power surges.

Perhaps one of the most interesting developments of late is the new 'go anywhere' motor from ABB. This Global Motor can be specified and used anywhere, regardless of local voltage standards, efficiency demands or safety requirements. The new motor offering covers aluminium IEC frame motors from fractional through to 75kW and it is rated for use on both 50Hz and 60Hz systems, with voltage ratings covering the world.

Designed to meet all mandatory requirements, the Global Motor simplifies the purchasing process and allows companies to standardise even further in a global marketplace. The range of approvals for the design is impressive, including IEC-CENELEC power and frame size standards, multi-labelled to meet UR (Underwriters Laboratories Recognised Component), EPAct (US federal efficiency requirements), CE (European Union safety directives), EFF 1 (the European Commission's highest efficiency value), CSA (Canadian safety standards), and EEV (CSA energy efficiency verification).

So, in addition to being highly efficient and energy-saving, this durable, cost-effective and efficient product set is reducing the potential impact of differing standards around the world. Perhaps this is the revolution - one driven by economics rather than technology.

SOE

This material is protected by MA Business copyright
See Terms and Conditions.
One-off usage is permitted but bulk copying is not.
For multiple copies contact the sales team.