DC drives and motors have a high cost of operation.  The most widely understood operating cost of a DC motor is the maintenance cost associated with the brush and commutator.  The brush and commutator is how electrical energy is transferred to the armature and they require regular maintenance in order to avoid unexpected downtime. Unfortunately, if you have a close relationship with a local motor repair shop, you have a problem!

The second and least understood operating cost is in the difference in the cost in energy consumption.  To help quantify the difference in energy between an AC drive and motor and a DC drive and motor on an extruder, we will divide energy costs into three different groups:

  1. Efficiency: Percent Efficiency is defined as Power Out divided by Power In.   It is required by law that the efficiency of an AC motor operate above 92% and many motors are at least 95% efficient.  Although there are no laws regulating the efficiency of a DC motor and consequentially they are 85% efficient or worse, especially if they are older motors.  This makes the difference in efficiency between the two systems as much as 10% annually.
  2. Power Factor:Power Factor is the relationship between True and Apparent Power.  True power is measured in kilowatts (KW) and apparent power is measured in  kilo-volt-amperes (KVA).  What is important to understand is the lower your power factor, the more current it takes to do the same amount of work.  Therefore, utilities penalize for power factors below 0.95.  In order to avoid such penalties many manufacturers have installed power factor correction capacitors.  (The slower a DC motor runs, the worse it's power factor.  A good rule of thumb is if you wish to determine the power factor of a DC motor, multiply it's percent speed by 0.85.  In many cases DC motors installed on extruders operate with a power factor below 0.70, while AC drives and motors have a great power factor operating above 0.90 which means you may not require power factor correction capacitors to avoid penalty charges.)
  3. I2R Losses: I2R losses are associated with energy losses that occur when there is a voltage drop in a circuit that is dissipated in heat.  Due to the higher current draw with a DC drive and motor as a result of its poor power factor, the I2R losses are also higher than AC.  These losses occur in wire, isolation transformers and the distribution system.  When retrofitting from DC to AC, it is recommended to remove the isolation transformer and by doing so, you reduce I2R by as much as 5 KW!


For these reasons, the return on investment (ROI) when converting from DC to AC on an extruder is as little as two years, with much larger implications to the bottom-line over the life of the system! Integrated Control Technologies performs energy studies on extruders and can help you quantify energy savings when converting to AC Drives.  The cost of the study is $2500 and includes up to five extruders.  If you company performs a retrofit within a year of the study, half of the cost of the study can be applied to the purchase of a new AC system.  Contact ICT for more information.

All extruder machinery manufacturers now use AC drives on their extruders. The two primary reasons driving this change from DC was cost and performance.  Although all new extruders use AC drives and motors, the vast majority of extruders operating in the United States are still using DC drives and motors.  This is because many of these extruders are ten years old or older and DC drives was what was offered by extruder machinery manufactures at that time.

DO YOU UNDERSTAND THE REAL COSTS ASSOCIATED WITH YOUR EXTRUDER?

EXTRUDER ENERGY STUDIES

RETURN ON INVESTMENT


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On the lines converted, we are seeing about a 15% savings - Cray (Heritage Bag)

SOURCE: Callari, J. (2014). Switch to AC Drives & Save Energy, Maintenance Costs. Plastic Technology, 16 - 18. 

CONTROLLING PROCESS WITH BETTER TECHNOLOGY

PRE/POST STUDY

SAMPLE RESULTANT

In a recent study where the KWH was $.075, ICT studied a 250HP DC extruder drive on a Pre and Post basis at the customer request.  The results are forecasted over the typical rated component life of 10 years to derive the total impact in a transition.  In this case, the net savings for the customer was over $130,000 or 4.56x on their investment.

The actual return for this customer was just over two year providing there was no failure on the DC system.  A failure of the drive and especially the motor can make the ROI less than one year, and possibly even immediately if considering a new DC motor since the cost of a new DC motor typically exceeds the cost of an entire AC system!

CONTROLLING PROCESS WITH BETTER TECHNOLOGY