Motor-driven equipment, including pumps and fans, accounts for about 64% of electricity and about 30% of total energy consumed in the industrial sector.
There are two general tactics for saving electricity in motor systems:
- Developing a plan for upgrading your motors to premium efficiency motors when they require replacement, and
- Matching instantaneous motor power most efficiently to the needs of the task.
These tactics will save energy and reduce maintenance costs, and they may also improve productivity by preventing motor failures.
Motor Maintenance and Plan for Motor Replacement Decisions
Motor maintenance includes routine inspections of all motors (with emphasis on those critical to production), including the drive train, which should be realigned and lubricated as needed; measuring energy use; and identifying any overheating of mechanical and electrical components.
Before your existing motors fail, it is helpful to have a plan in place for whether to replace or repair the motors, and which replacement motors to choose in the former case. How do you decide whether to have the motor repaired (such as through rewinding) or to replace the motor with a new premium efficiency motor? In many cases the latter is actually the smartest choice. This is because rewinding can degrade the efficiency of a motor by 1 or 2 percentage points, while premium efficiency motors can be 3 or 4 percentage points more efficient than the original standard efficiency motor. For an average motor, the purchase cost is often less than 2 percent of a motor's total lifetime cost, and the motor will consume 50-60 times its initial purchase price in electricity within 10 years of service.
Performing this cost-effectiveness analysis is easy with the use of the Motor Master software available from the U.S. Department of Energy Advanced Manufacturing Office (AMO). This free software tool includes a catalogue of motors of various sizes, along with their costs and rated efficiencies. By inputting data such as your price of electricity, load factor, and hours of operation, the software provides the energy and cost savings, and the payback period and return on investment for the particular replacement choice.
Match Motors with Loads
Motors are often oversized for their loads, mainly due to conservative design practices, or subsequent modifications to processes and equipment. In addition many motors are sized to provide the maximum output required under the worst operating conditions, but during typical operation the motor system seldom requires this much output. The excess energy during other times is usually dissipated through some type of throttling device such as dampers or valves. There are two general strategies to save energy in these situations: 1) install a smaller motor to replace the oversized one, or 2) install an adjustable speed drive.
As a general rule of thumb, if the maximum loading on the motor is less than 40-50% of its rated capacity, it may be cost-effective to replace the motor with a smaller and more efficient one. The Motor Master software tool mentioned above is also very useful for evaluating these opportunities.
For applications in which the loads vary considerably, installing a variable frequency drive (VFD) can be a good investment. In general, installing a VFD is considerably more expensive than buying a new smaller and more efficient replacement motor, so if the load is consistently low (such as below 50% of the rated output), the motor replacement option is the smarter choice. Another alternative to installing a variable speed drive on a single motor is to install two smaller motors, bringing the second one on-line as needed to meet larger loads.
Determining if a motor system is a good candidate for variable speed operation requires knowledge of the loads and hours of operation per year. Good potential VFD applications have significant hours of operation at less than the rated (maximum) output. Motors driving pumps and fans should always be evaluated, and potential energy savings from these systems can exceed 50%. Motor and load systems that deliver rated output less than 40 percent of the time, or for which the average output is less than 60% of the rated output are good variable-speed prospects. To be economical, the motor system should also be in operation for many hours per year.
Generally, the payback period for a VFD installed on a pump or fan application operating more than 6000 hours per year will be less than two years when the average output is less than 70% of the rated load. DOE's Advanced Manufacturing Office (AMO) has a pumping system assessment tool and a fan system assessment tool to assist in evaluating the cost-effectiveness of VFDs for those systems. As mentioned above, installing multiple pumps, fans, or motors for other applications, and staging their operations to match loads is another practical energy and cost savings strategy.