3. Energy Saving Technologies: in industry

Leonid Ribitskis, professor

Anastasia Zhiravetska, associated professor

Nadezhda Kunicina, associated professor

Energy Saving Technologies

Chapter 3: Energy Saving Technologies: in industry. The subchapter is an overview of Electric drives as Energy Saving Technologies in particular: optimal selection of electric motor power and application of frequency-regulated electric drives are described)

3.1. Electric Motors

The systems containing electric motors of different types are applied a lot in all sectors of industry, building and household. Nowadays the electric motors systems consume about 40% of the total electrical energy. Electric motors applied in industry consume about 65% of the electricity. Due to their high operational characteristics such as robustness, free of maintenance, flexibility in parameters and efficiency and cost-effective production the electric motors are widely used in such applications like heating, cooling facilities, elevators, escalators, transportation. Traditionally most attention and efforts concerning energy efficiency were focused on IMs. But an interest to the other types of motors for the same applications is increasing for an alternative use because of the energy and materials costs, mandatory requirements to the efficiency, etc. The most attention is paid to such motors like permanent magnet synchronous motors (PMSM) and the opportunity of their application for low power and specific systems. Regarding these applications they are expected to reach much higher requirements.

For better realizing of energy efficiency and improvement of electric motors and drives in these characteristics the whole components of losses during the motors operation should be considered first of all. As IMs are the most widely used motors the losses components are described regarding to this type especially the squirrel cage induction motors, but generally everything could be turned to the other types of motors as well. The total efficiency depends on different factors of the system: motor efficiency, temperature and sizing, motor control, mechanical transmission, maintenance, mechanical efficiency, losses in the supply system, supply quality, etc.

El.motor classification scheme

Losses

The losses in an IM consist of joule losses in the stator winding P_s and the rotor cage P_r, iron losses P_FE due to hysteresis effects and eddy currents in the laminated steel core, mechanical losses Pfr,w due to friction in the bearings and additional or stray load losses P_SLL representing a number of losses not relating to the previous items (Fig.1) [1]. The testing methods to determine efficiency can be categorized as either direct or indirect. For the direct method, the electrical and mechanical powers have to be measured. The indirect method is based on the separate determination of the individual loss components [1]. The indirect method has such advantages if to compare with direct methods as the higher overall accuracy and correction of the different loss components to a specified environment and reference motor temperature and lower errors of the measurements. The largest part of IM losses are mostly returned into the other types, in different order of magnitude and mutual distribution.

Fig.1. The losses in an induction motors

The electromagnetic losses consist of winding and core losses. These losses are divided into fundamental, space harmonic and time harmonic losses. All harmonic components influence the increasing of additional losses in stator and rotor windings. The time harmonic components are of low importance in constant speed applications with high power quality. However, they should be obligatory taken into account when the considered IM is supplied from power electronic converters. The space harmonics of the stator and rotor MMF are related to practical limitations.

The other approach of the losses classification divides them into constant losses and those load dependent. The constant loss components are friction, windage and iron losses. The calculation of theses losses is based on the motor geometry and parameters of the material. The load dependent losses are the cooper losses in stator winding (P_s), in rotor conductors (P_r) and stray load losses (PSLL). The stator copper losses are calculated according to the current corresponding to the load and the winding resistance. The winding resistance is measured and corrected for a specific reference temperature in accordance with standards.

High Efficiency Motor Technology

The wide improvement of the electric motor systems efficiency can be developed in two directions. The first one is the standardization, classification and legislation aspects stimulating the implementation of high efficiency motor systems according to the international classification and measurement standards. The second is the technologic improvement for the losses reducing. The highest savings can be obtained by matching the motor output to the speed and torque demand of the load by using a variable-speed drive. The consumption of the electric motor should be reduced by replacing it with so-called high efficiency motor (HEM). It is of high importance to determine in which aspects they differ from standard motors. There are many possible approaches to improving the motor efficiency with better materials, design and manufacturing technologies.

Fig.2. Elements of an induction motor

The design of a HEM requires an optimization of both efficiency and product cost. A well-designed HEM is simultaneously a machine designed with better energy effective active materials having a better effective geometry and components for reducing the cost. The three main categories of technological issues concerning motor efficiency - motor materials, motor design and motor manufacturing - are interrelated.

The materials improving the efficiency of the motors cover a wide spectrum of the motor parts: materials for core production, rotor conductor and permanent magnets. The basic features of the core materials are considered in order to reduce the iron and copper losses that are the related quantities. Mostly these materials are soft magnetic composites and amorphous magnetic materials. An optimal balance between flux (iron losses) and active torque producing current (copper losses) can be found by design optimization. High-grade core materials, such as cobalt-iron and nickel-iron, have the following properties:

• high magnetic flux density (permeability): to reduce the stator copper losses;

• low conductivity: to reduce eddy-current loss;

• soft-magnetic, low coercivity: to reduce hysteresis loss.

Silicon application for the production of electrical steel sheets is the most common and effective to decrease the conductivity and eddy-current loss. But for the high efficiency motors this technology is limited because it reduces the non-saturated magnetic flux density and increases mechanical hardness. Insulating coatings on the lamination reduce iron losses restricting eddy-currents to flow in a lamination of the core. In high-efficiency motors inorganic and hybrid coating are used for excellent insulation consistency and thermal resistance. Thinner lamination results in less eddy-current losses but increases the cost of the motor.

The efforts for the increasing efficiency in terms of rotor construction are directed to the reducing of joule losses in the rotor cage that is the second large portion of loss in an induction motor. Squirrel-cage material conductivity, cross-section and length determine the resistance and the joule losses in the rotor. The use of copper for the rotor cage gained an interest within last decades in the production of motors. Copper rotor technology has a considerable energy-saving potential, but the disadvantage of this is a large increase of the cost of copper. The greater results in the reducing joule losses are expected in the combination of copper application for rotor manufacturing with rotor slot geometry optimization that can change the behavior of the motor in different ways.

A significant improvement in efficiency can be also reached replacing the rotor squirrel-cage of an induction motor with permanent magnets. According to the stator winding structure and supply voltage shape it is named a brushless DC or a permanent magnet synchronous motor correspondingly. Different materials are applied for the production of permanent magnets, but more compact and lighter constructions are obtained applying NdFeB-magnets.

PMSM are synchronous motors with magnets in the rotor instead of windings. PMSM are more efficient in comparison with IM because they do not require magnetizing currents. Because of absence of the rotor winding there are no rotor losses, so the thermal design of the rotor is much simpler than that of the induction motor. This motor is mostly applied in such dynamic applications like servos and robotics because its rotor can be made with lower inertia increasing so the ratio of torque to inertia.

Fig. …. Profile of PMSM motor

The other technological issue concerning motor efficiency is motor design. It concerns the following efforts:

• Stator, rotor and air-gap geometry optimization including: core length and diameter, rotor cage geometry and stator slot geometry that determine the rotor slip losses and decreasing of ripple torque, air-gap length that can make a considerable reduction in magnetizing current;

• Stator winding layout directly influencing the stator copper losses and harmonic (stray) losses;

• Cooling and thermal design;

• Bearing.

Fig. … SRM

Alternative motor technology – a significant efficiency improvement at reasonable costs is possible with other technologies like permanent magnets (PMSM) or switched reluctance motors (SRM) production and application. PMSMs have a number of benefits over the IM.

1. Higher efficiency - the PMs in the rotor supply the flux. Practically no magnetizing current flows in the stator winding and no current is induced in the rotor reducing the corresponding copper losses. Due to the synchronous operation, the flux variations causing the iron losses in the rotor core is zero.

2. Higher power density – the reduction of losses and the corresponding lower heating allows an increasing of the stator current with the same operating temperature.

3. Wider speed range with constant torque – the cooling system efficiency reduction at lower speed is compensated for by the loss reduction allowing thus a constant torque in the entire speed range.

4. Maintenance -the bearing cooling prolongs the lifetime.

5. Inertia – reduced rotor weight is an advantage in dynamic applications.

A drawback of PMSM is that they are of relatively high cost, need for position sensor and cannot be directly connected to the grid even for constant-speed applications. It needs an inverter with field-oriented control that increases the cost and adds to the losses.

Besides the permanent improvement of modern IM their basic disadvantage is the reactive current consumption from the network (cosφ<1) resulting in electrical energy losses. There are already compensated electrical motors having cosφ practically close to 1. Such electrical motor consists of typical IM with additional terminals from the stator windings and embedded device of the reactive power compensation. In comparison with traditional IM the cost of these compensated motors is not higher than for 10%. The power of compensated IM is up to 1000kW and supply voltage from 220V to 10kV.

The integrated compensation results in increasing of the output power for 3...3.5%, increasing of starting torque and overload capacity for 15...20%. the priority areas of application are pumps, fans, compressors.

Besides the sharp development of frequency-controlled electrical drive sometimes it is still efficient to apply the methods of scalar control of IM rotation speed realizing soft changing of the supply voltage. The mechanical characteristics are obtained from the set of those with different values of the supply voltage. For the obtaining of high efficient characteristics special IMs with the scalar control realization are produced.

The wide application of FC electrical drives resulted in the development and realization of special IM with specific features:

• strengthened insulation of stator windings;

• better ability of rotor to operate at increased frequencies;

• additional forced cooling;

• thermal limiter built-in into the stator winding;

• feedback sensors;

• opportunity of application in a wide range of frequency.

In a non-controlled drives some decreasing of energy consumption can be achieved realizing the operation in specific regimes. Generally it relates to:

• control/monitoring of the consumed power and network current;

• regulation of the cage and winding temperature;

• measuring of the output power on the shaft;

• determining of the load on the shaft;

• information on the technology and operation cycles of the installation;

• application of means of technology optimizing the starting and breaking regimes of operation;

• information of rated operation conditions and factors of reliability of electrical motors

• automatic selection of the optimal parameters of operation regimes: consumed energy, network current, value of slip, etc.