Motors and drives are critical components of the power generation process and can significantly reduce the internal energy consumption of power plants. A thermal power plant typically uses 8-15 per cent of the electricity it produces. In recent years, this share has risen with the increasing use of devices that reduce emissions. The use of flexible and efficient motors and drives can help power plants save 30-60 per cent of the energy consumed to adjust the flow of air and water. Instead of running systems continuously at a power plant’s rated maximum capacity, a control system can automatically adapt its production to fluctuations in cooling and energy demands. Motors equipped with drives further enhance the control process. This improves the operational stability, allowing the plant to operate closer to its safety margins, thus increasing its rated output power.
Motors and drives are used in a wide variety of power plants and processes such as boiler feed water pumps, coal mills, fans, condensate extraction pumps, cooling and circulation water pumps, cooling tower fans, conveyors, and air compressors.
Energy savings can be achieved in plant processes in three ways – through the use of properly sized and energy-efficient motors (EEMs), deployment of variable frequency drives (VFDs), and optimisation of the complete system through correctly sized motors, pipes and ducts, efficient gears and end-use equipment such as fans, pumps, compressors, traction, and industrial handling and processing systems.
EEMs owe their enhanced performance to key design improvements, with more accurate manufacturing tolerances. EEMs produce the same shaft output power but use less input power than a standard efficiency motor. Although EEMs and standard motors are manufactured using the same frame, they have some significant differences. For instance, EEMs have better quality and thinner steel laminations in the stator, more copper in the winding, optimised air gap between the rotor and the stator, reduced fan losses and a greater length, and use high quality aluminium in the rotor frame.
In addition, EEMs have greater efficiency and lower operations and maintenance (O&M) costs. They have a lower slip, which results in higher speed. These motors also improve operations in the industry with their robustness and reliability. Further, they address one of the key environmental concerns of greenhouse gas emissions. While standard motors operate efficiently, with typical efficiencies ranging from 83 per cent to 92 per cent, EEMs perform significantly better. In EEMs, efficiencies range from 92 per cent to 94 per cent, which results in a significant loss reduction of about 25 per cent. Since motor losses increase the heat rejected into the atmosphere, reducing these losses can significantly bring down the cooling load in air-conditioning systems.
The microeconomic benefits of EEMs include better process control, reduced disruption and improved product quality, whereas the macro benefits include increased competitiveness and reduced dependence on fossil fuels.
EEMs can be considered for all new installations, major modifications in existing facilities or processes, as well as while purchasing new equipment packages and spares for replacing failed motors, instead of rewinding old standard motors. These replacements can lead to considerable energy savings depending on the age and efficiency of the motor . Motor replacements can be very profitable for applications that have high annual running hours, for example in firms with multi-shift operations.
Motor starting methods
The method used for starting motors can also contribute to energy savings. This is for the reason that motors draw more current at the start-up stage. The level of current at this stage can stress motor components and cause power quality issues on the power plant’s electrical systems. While selecting the starting equipment or any protective devices, the key factors that need to be taken into account are the voltage drop in the supply network when starting the motor, the required load torque and the required starting time.
Some of the important motor starting methods are direct-on-line (DOL), star/delta (Y/D) and soft starters. DOL is the simplest and the most commonly used cost-effective method of starting induction motors. This type of start-up gives the highest possible torque and is suitable for stable supplies as well as for mechanically stiff and well-dimensioned shaft systems. Large motors, motors that start and stop frequently, and those that have some kind of control system use a DOL starter. The star/delta connection generates a low starting current, which is about one-third of the starting current generated by DOL, although this also reduces the starting torque to about 25 per cent. The motor is started with Y-connection and accelerated as far as possible, then switched to D-connection. Soft starters are based on semiconductors, which initially reduce the motor voltage, resulting in a lower motor torque via a power circuit and a control circuit. The idea behind a soft start is to gradually allow the motor current to rise until the motor reaches its steady state.
Many industrial processes such as assembly lines operate at different speeds for different products and these processes demand the adjustment of flow from a pump or a fan. Varying the speed of the drive decreases the energy consumption, even by 50 per cent in some applications. A drive is a power electronic device that takes alternating current (AC) power and manipulates it to control the speed and/or torque of an AC motor.
A VFD is a type of motor controller that drives an electric motor by varying the frequency and voltage supplied to it. VFDs have the highest energy saving potential in flow systems such as pumping or ventilation systems with high output variations. Pumping systems are traditionally controlled by valves that reduce the output flow while the motor is still running at full load, thereby wasting an enormous amount of energy, which is released as friction. VFDs control the motor input frequency and voltage in order to adjust the motor rotation speed as per the requirement. As a result, the pump load/water flow is adjusted without the use of a valve. The efficiency improvement in these cases can be higher than 30 per cent, depending on the system design.
Industrial firms today use VFDs for various applications such as standard pumps, fans and blowers, conveyors, machine tools, film lines, extruders, and textile-fibre spinning machines. New-generation VFDs can now be deployed in AC motors regardless of motor horsepower or their location within a facility and can be used to drive almost all types of motorised equipment. VFDs are also known as variable speed drives, adjustable speed drives and adjustable frequency drives.
VFDs consist of three basic parts – a rectifier, an inverter and a DC link. The rectifier converts the fixed frequency AC input voltage into DC, the inverter switches the rectified DC voltage to an adjustable frequency AC output voltage and the DC link connects the rectifier to the inverter. A set of controls direct the rectifier and the inverter to produce the desired voltage in order to meet the needs of the VFD system.
VFDs are most commonly used in the industry given their fixed DC bus voltage, no motor cogging, higher efficiencies and lower cost.
Going forward, the cost of energy is likely to increase given the limited resources and environmental issues. In this scenario, installing EEMs is a viable option as they can help save a significant amount of electricity in industrial motor systems. As a result, industrial facilities will be able to significantly reduce their energy consumption as well as O&M costs by using new-generation electronic VFDs and EEMs.