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Improving system efficiency

16 February 2018

Many motor-driven systems in operation today may be far less efficient than they should be. Dan Welch explains why – and outlines steps that can be taken to bring efficiency back on track. 

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Every aspect of food and beverage manufacturing needs to run efficiently. That includes every electric motor – from the units powering grinders, mixers, pumps, fans and extruders, to those responsible for the delivery of essential building services, such as heating, ventilation and air conditioning.

Installing a variable speed drive (VSD) to control the motor speed of any variable torque application is proven to offer energy savings – typically between 20% and 60%. Even in systems where a VSD is installed, ABB engineers often discover efficiency is lower than it should be. In some cases, efficiency levels may be just a few percent points lower than expected, but over time this can equate to a considerable amount of lost energy and money - especially in larger facilities which may have an installed base of several hundred motors.

The cause can often be traced back to the electrical system supplying power to the motor. The system typically comprises a transformer, VSD, electric motor, switchgear and cabling. Where system efficiency is lower than expected, it is usually due to one or more of the following:

• System efficiency is calculated using manufacturers’ efficiency figures for individual products, rather than actual operating conditions.
• The interaction of individual components is having a negative impact on the overall system efficiency.
• Individual products are not optimised.

The following steps can help tackle these issues:

Check the figures: The opportunity to improve electrical system efficiency starts at the design stage. Don't be dazzled by manufacturers’ efficiency figures for their products. In most cases they are based on ideal laboratory conditions. For example, motor direct-on-line (DOL) efficiencies are reported on a pure sine wave. No public power supply is a pure sine wave, so straight away the motor will be 2% less efficient in operation than published efficiency data may suggest.

Furthermore, EN50598-2, a relatively new standard for VSD efficiency, defines that manufacturers should publish their figures based on their default factory setting. However, manufacturers use different default settings. For example, ABB uses 4 kHz as a default switching frequency, whereas other manufacturers chose to use 2 kHz. The lower switching frequency improves the figures the manufacturer can publish for the VSD efficiency. But, in practice it will reduce overall system efficiency. Catalogue data for drives will not tell you which switching frequency is used, making it impossible to compare products on a like-for-like basis.

Don’t argue with physics: Consider how the individual components in an electrical system will interact with each other. Simply connecting products together overlooks some fundamental lessons of physics which causes the components to react with each other in ways you may not expect. The net system efficiency depends on many factors. Adding a VSD can impact on system efficiency, as can adding a passive filter or a low harmonic drive. Likewise removing components from a VSD, such as chokes, may improve the efficiency of the drive but has a negative impact on the transformer, supply and motor.

Let each component shine: Even greater efficiencies can be achieved by optimising the individual components in the system. For example, selecting the most efficient motor for the duty could increase efficiency by up to 5%.

Looking specifically at pumps, the greatest efficiencies are achieved by reducing the speed of the motor driving the pump. A 2 Hz reduction in speed is a 12% energy saving typically on a pump system. One method to reduce pump speed is to switch to parallel pumping – running two pumps at slower speeds rather than one pump at full speed.

Efficiencies can also be realised by utilising the smart functions and advanced motor control features that exist within many of today’s VSDs. Examples include energy optimisation which helps ensure that motors are controlled using the least amount of energy.

Organisations operating large motors could also consider recycling the heat they produce. Traditionally, motors are cooled through a process of liquid cooling. However, opportunities exist to harness this unwanted heat and use it for other purposes, such as providing space heating. There are also non-electrical opportunities to improve overall efficiency. For example, removing ageing or unused orifice plate or pipework can improve total system efficiency by 2%.

Stripping out any unnecessary costs and streamlining processes wherever possible is crucial. The steps outlined in this feature can help motor users to identify significant opportunities to improve efficiency. It’s just a question of knowing where to look.

Dan Welch is food & beverage industry lead, drives & controls at ABB.

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