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Downtime should be classified as either PM (Preventative Maintenance) or Unplanned Downtime (Emergencies). The first allows more time to respond to a need, the second forces one to react to a need. As many plants run 24/5 and 24/7 with continuous production, it is common to hear companies with DC describe, "Planned Downtime". This is described as the time necessary to maintain the equipment during normal operation. In the case of DC motors, this downtime is significantly greater than AC motors which typically only require bearing maintenance. Continuation of operating with DC motors will always result in more downtime!
Did you know that the cost of a brand new DC motor is more than an entire AC Drive Package with a Vector Motor? As a matter of fact, the average is to buy new, you would typically pay 10-20% more for the DC motor...and that is why people continue to rewind their old DC motors. Unfortunately, if you have a great relationship with a motor rewind shop, you actually have a real problem! In most cases, companies actually spend as much in repair cost on DC motors over their lifetime, to have bought an entire AC package. Combine this with the energy savings and the results are a little more like having purchased two or three systems!
There is a reason they put the window on the side of DC Motors, it is to look for that flash that says "I am about to get really dirty today". While DC Motors are easy to work on, it is because the basic design is over 100 years old and it has not been improved much since. When they were originally conceived, no one even thought about utility savings or clean power, and apparently back then, everyone liked getting dirty changing brushes. By design, the DC Motor needs the brushes changed regularly, the commutator turned occasionally, and will ultimately be rebuilt. Aside from the cost, each time requires someone to stop doing something and work on this problem. The more DC motors you have, and the larger you are, the bigger the waste of time and labor you experience.
So....Does it make sense to save 10-15% on your second biggest expense?
Lastly, the elimination of large isolation transformers reduces I/R losses where power is basically converted as heat...helping to keep your plant nice and warm in the summer.
The first thing to understand is, the cost of energy over time is significantly morethan the cost of a motor, so trying to save money on a motor by rewinding it, is not actually saving money, it is costing you more! Instead, move away from DC to AC technology where the energy efficiencies are 10% new out of the box! This gap typically widens as DC motors are "repaired"
Utility Savings for AC Motors are gained through producing the same amount of work, with less overall Current (Reduced KVA). The utilities get you here twice, once for the current you use, and again for when you use it, that is called Peak Demand. This is in addition to the Power Factor of a DC motor being poor, meaning that the utility companies have to produce more power than you use because it "looks" like the motor requires more power, so they add penalties to your bill! (Reduced KVAR).
"Planned Downtime"...Really? You plan for it?
10% or more can really add up over time!
Source: Switch to AC Drives & Save Energy, Maintenance Costs – Plastic Technology, Callari, J. (2014)
Repair costs just add cost to the problem!
Nobody likes changing the brushes...Nobody!