Posts Tagged ‘actuators’

Take control with better actuators

November 25, 2009

Neil Webster, General Sales Manager at ABB Instrumentation, explains how investing a little more in using the right actuator will end up saving users money in the long run, thanks to higher process efficiencies and reduced energy consumption and maintenance.

When it comes to working, many electric actuators are part-timers. They don’t offer continuous operation without overheating their motors, so they operate in cycles meaning that they’re only available for duty between 25% and 50% of the time.

This common mode of operation is known as S4 in the IEC 34-1 standard. It’s characterised by the maximum percentage of allowable “on time” in each cycle, as well as the maximum number of cycles permitted per hour (typically around 1200). The rest of the time is spent allowing the motor and gear train to cool down to make sure it does not reach the maximum permissible temperature specified by the manufacturer.

To avoid overheating, these cheaper S4 devices tend to have a relatively wide dead band. The dead band is a measure of the deviation between the actuator position demanded by the process and the actual position of the actuator. The actuator only starts to move when the difference between the two positions breaches the pre-set dead band.

Some S4 devices use an internal heat sensor to adjust the dead band and make sure they stay below the maximum permissible temperature. This strategy is known as thermal de-rating. It means that the default dead band is typically 0.5% but can reach 5% in some S4 actuators.

This kind of operation is simply not good enough for many companies. Fast, precise control demands an immediate response that carries the control element quickly and smoothly to where the process needs it.

S9 actuators are smooth operators

There are actuators available to provide uncompromising performance. These actuators offer a mode of operation known as S9, in which they are continually energised to offer the best possible response. For example, ABB’s Contrac electric actuators offer S9 operation, enabling them to operate with a dead band of just 0.05% without any thermal de-rating, even when operating in high ambient temperatures. In some cases, a wider dead band is desirable to maintain the stability of the process, but the point is that S9 operation allows the dead band to be optimised to suit the process, rather than the actuator.

ABB’s S9 actuators also offer a more immediate response because the motor constantly balances the forces from the process with the motor to maintain the control element in dynamic equilibrium.  This improved control performance may come at a higher upfront price but more accurate control will result in a more efficient process.

The advantages of S9 operation do not end with improved process control. Even without the boost to the bottom line that better control often delivers, many plant operators stand to save money in the long run by investing in higher-quality, S9 actuators. That’s because they offer a vastly increased service life and much lower maintenance costs.

Maintaining lower lifetime costs

Permanently energised actuator motors can increase or decrease their torque smoothly and proportionally, according to the deviation between the actuator position and the demand signal. This smooth operation avoids the shocks and acceleration associated with the constant starting and stopping of S4 mode operation, significantly increasing the life of all the mechanical components involved. Other factors, such as oil lubrication and the use of spur wheel gears, further reduces friction and internal mechanical wear.

S4 actuators may need to be stripped down and maintained after less than one year. Yet S9 actuators like ABB’s only need stripping down every 10 years, as long as the recommended checks are made in the mean time.

The ideal maintenance interval varies according to the duty, but some of the latest actuators now have an on-board diagnostic computer that monitors operations and can actually predict the optimum maintenance interval. Over a 10-year period, cumulative maintenance costs for S4 actuators on a medium duty are likely to be of the order of 15 times more than maintenance costs for S9 models.

Think ahead to save money

When budgets are tight, it’s always tempting to opt for the cheapest offer. But in life you get what you pay for and actuators are no exception. Investing a little more in improved control will impact on the efficiency and profitability of many processes, as well as reduced energy costs. In addition, “hidden” extras such as additional maintenance costs for cheaper actuators are soon likely to outweigh any initial savings. Even better, today’s fit-and-forget S9-mode electric actuators should give reliable service for over 20 years, giving hard-pressed engineers one less thing to think about.