Benefits of entire drive train optimisation

12 December 2011

Optimisation of the complete drive train, and consideration of how the individual components interact, can offer some impressive energy savings.

According to analysis conducted by the German Electrical and Electronic Manufacturers' Association, ZVEI, the energy saving potential by using high-efficiency motors is 10%, by using frequency converters it is 30% and by optimising the drive system it is 60%. Depending on the application, a different combination of various measures will offer the maximum effect by ensuring that all the associated drive train components are interacting in an optimal fashion.

The complete drive train comprises the mechanical interface to the driven machine (coupling and gearbox); the motor; a frequency converter that controls the motor speed; and a motion control system that supplies the frequency converter with a setpoint

Looking at these components in isolation, the motors and frequency converters represent the highest potential for saving energy. Using design measures and the use of certain material combinations, the efficiency of motors can be significantly increased, and has already been legislated for across the EU.

Moving on to frequency converters, the closed-speed control and energy recovery are of special significance. Closed-loop speed control represents a high energy-saving potential, especially for pumps, fans and compressors. Unlike mechanical closed-loop control systems, with closed-loop speed control, the power drawn in partial-load operation is continually adapted to address the actual demand. As a consequence, energy is no longer wasted; and energy consumption can be reduced by around 60%. As a result of this high energy-saving potential, legislation will also permit the use of an IE2 motor with frequency converter as an alternative after the introduction of legislation requiring the use of IE3 motors, from 2015.
The use of frequency converters also offers the potential for energy recovery. In conventional drive systems that use braking resistors, the braking energy is dissipated as heat – and wasted. The Sinamics G and S converters, for example, are capable of energy recovery, do not require a braking resistor and feed the braking energy back into the line supply. This is energy that can be used elsewhere in the plant or system. In hoisting applications, for example, this can result in energy costs being reduced by up to 60%.
The lower power loss also simplifies system cooling, allowing for a more compact design.
Using the ZVEI data as an example, with component-based measures - such as increasing the motor efficiency and using a frequency converter for variable-speed operation - only 40% of the energy-saving potential in the complete drive train can be addressed. The remaining 60% comes from optimisation of the overall system. To address this remaining 60%, integrated solutions for the complete drive train are required, where the focus is on the interaction between the components. The mechanical coupling to the driven load must also be taken into consideration.

The efficiency of the complete drive train can be improved by using high-efficiency helical and bevel helical gearboxes.

Geared motors are available from Siemens that comply with the new international efficiency classes IE2 and IE3 are available for the power range from 0.09 up to 200 kW. The combination of motor and gearbox are supplied combined as s a functional unit with optimised energy efficiency.

For higher power ratings completely integrated motor and gearbox units drive solutions have been developed as a single unit. One example of this is the Flender EMPP vertical mill drive, which is available for power ratings from 500 kW up to 15 MW. This consists of a permanent-magnet synchronous motor, directly integrated into the gear unit. As a consequence, one gearbox stage can be eliminated and the high forces are optimised, resulting in significantly lower losses being transferred. The number of mechanical components is simultaneously reduced.

In applications with very low speeds, but high torques and for speeds of over 10,000 direct drives offer a suitable energy saving solution. Here, the gearbox between the motor and the driven machine is completely eliminated. Whereas cascaded step-up or step-down gear stages for variable-speed drive systems will reduce the overall efficiency, a direct drive has an efficiency that is between 2 or 3% higher.

Utilising heat recovery
Drive components can be cooled using air or liquid coolant filled internal circuits. Liquid cooling offers an efficient solution as it allows heat to be recovered. The temperature of the water used for cooling increases and can then be used as pre-heated service and process water, improving energy efficiency of the complete plant as water is already required to cool the drive and the recovered heat is an associated advantage.

Assessing the drive environment in many plants and systems, it can be seen that not all axes operate in the motoring mode. Instead, many axes, particularly in specific process phases, operate in the regenerative mode. The energy that is taken from the process can be recovered and used. By using inverters with a DC link coupling such as the Sinamics S120 drive, regenerative energy can be made directly available to motoring drives via a common DC link. Depending on the buffer capability of the DC link, it is even possible to utilise the regenerative energy with a time delay. The direct energy exchange from inverter to inverter minimises the power loss in the system. This means that the power rating and/or size of the infeed can be smaller than the total power of the connected inverters. The energy-saving potential here can often in the double-digit percentage range.

Handling dynamic peak loads in drive groups can be covered using additional capacitors in the DC link. The regenerative energy that is available is not converted into heat, but instead is stored in the capacitors and output to the loads with a time delay. An alternative way of achieving this is to use flywheels as energy storage device, which then output their energy back to the system after the surplus energy phase.

In addition to saving energy, intermediate storage can be used to buffer brief disturbances in the local line supply as a result of dynamic load peaks – which in turn, improves the process reliability of the complete plant or system.

The condition of the plant and the drives can be identified from how the energy demand develops as a whole and individual parameters, for example, the peak current that is demanded. This means that diagnostic functions integrated in the drive train again help to reduce energy usage – and/or keep it permanently at an optimally low level. For example, for a compressor drive, a Sipart PS/2 electropneumatic actuation controller senses the system pressure and communicates this to a higher-level maintenance station. Here, the system pressure is evaluated, visualised and, if required, a message is output to notify maintenance personnel that maintenance is necessary. If the system pressure decreases more frequently and faster, then the compressor drive must be more frequently switched-on and for longer periods of time. As a consequence, more energy is used – which in turn means higher energy costs. Specific maintenance and optimisation measures can resolve this issue and reduce the energy requirement.

In conclusion
A lot of energy can be saved in the drive train - far more than the use of energy-efficient motors and frequency converters stipulated by legislation. However, the maximum possible energy efficiency and cost-effectiveness can only be achieved by taking a holistic approach regarding the possible interactions and synergies of individual measures and combining them on an application-specific basis. Seamlessly designed and optimised drive technology – extending from the line connection through the mechanical coupling to the driven load – is key to the development of energy efficient and cost-effective operation of industrial plants and systems.

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