Before even thinking about your motor, look at your system and current situation. How much power do you really need? Have you optimized your system for that power? Hydraulic design is a forgotten art in most systems these days. Plants typically only look at their system if they run out of pump capacity, but not too many probe the energy wasted due to inefficiency. Unfortunately for most plants, the systems were designed and built when electricity was relatively inexpensive. So, they aren't optimized to reduce motor energy expenses.
Pump size, excess pump capacity, hydraulic systems, recycle streams, minimum flows all have a price. If your electric bill is high, start questioning how you run your system. If the system isn't anywhere near design conditions, there's probably money to save. Most motors are oversized and that alone may cost you because the motor isn't running at peak efficiency. You also need to look at load and determine if variable frequency drives (VFD) would be a better option. (See "Watch Out with Variable Speed Pumping." First question your demand-side requirements (process) before trying to optimize motor systems.
Motors play an important role in our country's electrical energy policy. Sixty-three percent of all industrial power is used by motors. That's 23% of all electricity sold in the U.S. If you have a standard motor that's more than 15 years old you can on average achieve about 15% improved efficiency by replacing it. In older motors, about 33% of all losses occur in the stator, which is mostly winding losses. Modern motor systems typically contain 20% more winding material.
One of the biggest dilemmas with motors is deciding which to repair and which to replace. There's an old rule of thumb from the 1970s that said if the cost of repair was less than 57% of the cost to replace, then you should repair. Although that number appears high, with less than a two-year payback, when calculating all other factors, it proves very resilient. Under that guideline, only 20% of motors were replaced.
However, that rule died in 2001 when NEMA premium pumps arrived. They are generally 3% more efficient than today's standard pumps. Suddenly, cost savings associated with higher efficiency pumps became a huge factor. For example, if you had a 200-hp motor that wasn't NEMA premium and that ran at least 25% of the time, then you would save money by replacing the motor. The savings assumed the cost of the pump and energy savings but not installation costs or any changes in piping, foundations, or electric wiring. Of course, the numbers made more sense when motors need repair, since you have to pay for labor cost to physically remove the pump and send it for repair. In any continuous motor more than 15 years old, replacement made sense, even without tax incentives. But, almost a decade after introduction of NEMA premium, maintenance people have been asking the right question: What's the new rule of thumb for repairing or replacing a high efficiency motor?
The answer is a bit surprising. Most people expect the number to be lower because of energy improvements to the pumps. Instead, the new rule of thumb, call it the two-thirds rule, is 67%. The two-thirds rule assumes your maintenance department is up-to-date on repair technology.
Why did the number increase and result in less pump replacement? Besides improvement in new motor efficiency, repair practices and procedures got better. The method for repairing motors, coil burnouts at 800ºF -900ºF, actually reduced motor efficiency. A typical motor repair would reduce efficiency an average of 1% with efficiency reduction. A motor that has seen several repairs could drop overall efficiency by 5%. This also was replaced in the past 10 years with temperature controlled burnouts less than 700ºF. Instruments measuring balance and diagnosing problems also have improved, making repairs more reliable and better at returning motors to new pump efficiency. In fact, better winding techniques can return repaired motors into higher efficiency than their original efficiency. Improvement gains up to 1.5% are typical. They also reduce future frequency of repairs compared to 20 years ago.
For energy-efficient motors, premium or near premium, it's possible to see gains up to 0.5%. This may not seem like much, but efficiency improvements from repairs lead to a 10% change in the two-thirds rule.
Remember that we are talking about a rule of thumb. Each situation is different depending on repair history and expected service.
Gary Faagau is Chemical Processing's Energy Columnist. You can e-mail him at GFaagau@putman.net.