Energy Efficiency: Battling Start-Up Spikes

This is the fourth article of a five-part series on industrial energy efficiency. This month we cover Part Four of the series: Start-Up Spikes. This occurs whenever energy-consuming equipment and systems are started simultaneously.

Start-up spikes are an all-too-common occurrence in most manufacturing and distribution facilities. When energy-hogging equipment is started up at the beginning of a shift, it can often lead to unintended peak-demand energy charges.

But these spikes can also be a problem for any commercial buildings where lighting and HVAC systems kick into high gear at the same time each day.

Hard Starts

In the manufacturing and warehouse environment, start-up spikes result when multiple mechanical systems are turned on simultaneously. These “hard starts” can result in additional energy costs. But they’re also rough on equipment, causing premature wear and tear.

That’s because the inrush current from a hard start is often five to six times a motor’s full-load running current. This massive current creates heat in the motor windings, and heat can kill a motor over time.

One solution to this problem is a “soft starter.”

Soft Starters

Soft starters ramp up the voltage gradually, thereby limiting the inrush current. Here’s how: Every time your compressor, pump, or machine starts, the soft starter limits the current for about the first five seconds. It then reverts back to normal running conditions. This results in a more gradual increase of current and eliminates the spike.

The gradual increase in voltage significantly reduces heat buildup. Which ultimately results in an extended lifetime of the motor – particularly of motors that are stopped and started frequently.

In fact, a soft starter will allow you to turn a motor on and off much more frequently without damaging the windings. And for motor applications that involve intermittent loads, a soft start may enable you to shut the motor down in between loads, rather than running it continuously.

Variable Frequency Drives

At the high end of the starter spectrum is the variable frequency drive (VFD). VFDs are typically used for motor speed control, but they can also be used on small motors where they function as motor starters only.

The benefits of using a VFD include:

  • Reduced current starting
  • Communications to a central building management system, and
  • Easy interface for automatic control.

But these benefits come at a cost: increased complexity, increased installation costs, and sensitivity to the environment in which the VFD is installed. Also, additional equipment is often required to support VFDs (such as filters and surge protectors), which further increases cost.

Staging the Start-Up

Another technique for eliminating start-up spikes in factories and warehouses is to stage equipment to come online just in time (that is, sequentially, rather than simultaneously). By gradually ramping up mechanical equipment in a staged manner, excessive energy charges can be avoided without compromising production output.

Equipment start-up can be sequentially staged any time power has been interrupted through a “load control system.”

This staging of load ensures that power quality is maintained and any on-site generators are not overloaded during start-up. In addition to the sequential start-up, the load control system would monitor on-site generators, removing power load from the system if the generators become overloaded.

A Case Study

Start-up spikes can sometimes go undetected unless you’re monitoring your energy data. The following situation, reported by Industrial IP Advantage, is a case in point:

A manufacturer’s energy consumption profile documented a significant spike in demand that occurred monthly, without fail, on the same day and at the same time. A submeter pinpointed the source of the spike. During lunch break on the same day of the month, the maintenance staff simultaneously started all of the production equipment for testing purposes.

Staging the start-up – achieving a steady state with one system before turning on the next – would avoid the spike. But the optimal energy management strategy also included scheduling the once-monthly testing at 6 a.m. during the power utility’s off-peak demand period. The bump in overtime costs is minimal relative to paying peak rates over the course of an entire year.

This example underscores the importance of routine energy monitoring, so that start-up spikes can be pinpointed and eliminated before they become a problem.

Next Up…

Up to 20% of total electrical use in certain industries comes from air compression systems. Our last article in this series will address how these systems are prime targets for energy efficiency measures.


Sources:

DS&O Electrical Cooperative

Cummins

Industrial IP Advantage

Consulting-Specifying Engineer