Capacitance modules offer machine builders an energy saving solution

30 January 2013

Andrew Davies, of Bosch Rexroth, argues that capacitance modules could offer a simple energy saving solution for machine builders.

Traditionally, energy efficiency has not been a high priority for machine builders. However, with escalating energy prices there is now increasing pressure from end users for energy saving options to be fitted to industrial machinery in order to help mitigate the impact of rising prices for end users.

One way of achieving this is to use simple capacitance modules which improve the energy balance in applications where machining cycles take place in rapid succession, such as roll feeds or cross cutting lines.

Effectively, the capacitance module acts like a kinetic energy recovery system (KERS) on a Formula 1 car, which recovers a moving vehicle's kinetic energy under braking and is then stored in a reservoir, such as a flywheel or a battery for later use under acceleration.

A capacitance module, connected to the DC bus, effectively gives the machine a temporary energy storage unit which can reduce the heat loss in the control cabinet by relieving the braking resistor.

The primary role of the capacitance module is, in the event of a power failure, to enable a controlled retraction motion. Rather than use the energy generated by braking to produce heat in a braking resistor, the energy is stored in a capacitance module and re-used on the next cycle of the machine.

However, an additional benefit of the capacitance module is that in storing energy the panel does not become overheated and there is no need for a cooling system to keep the panel cool.

In recent years, Bosch Rexroth has worked extensively with Henrob, a manufacturer of self-piercing riveting equipment, to introduce capacitance modules into a number of the company's machines. This has enabled both parties to analyse the impact of capacitance modules in actual production environments.

A breakdown of costs using 500 capacitance modules on a vehicle production line, which requires the driven load to stop very quickly results in 1.3 amps of energy being used during an average cycle with 400 volts input into the machine and each rivet taking 1.6 seconds. With a capacitance module in use this results in a 13.36% saving in current.

During the course of a typical production day, 500 cars will be manufactured, each with 3000 rivets, which equates to 1,500,000 rivets per day with a total KWH of energy per day of 14056.25 costing £1686.75.

The saving from the capacitance module equates to £225 per day which over a 12-month period resulted in a saving of more than £54,000. Ultimately, the use of a capacitance module in this automotive application had a payback period of 2.64 years.

This offers clear evidence that capacitance modules can play a significant role in helping to save energy and cut costs in machinery applications. Crucially, capacitance modules have a relatively low unit price and can therefore offer a short payback period on any investment.

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