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Energy-wise motor design
Here is a look at where to find energy savings when applying motors in a design
James Nanney, Industry Manager, Baldor Electric Co. -- Design News, June 18, 2001
Up until recently, the typical engineer has probably been more concerned with "parts per minute" than "watts per part." But given recent energy shortages, that may be changing—fast. In many systems, the largest consumer of energy is likely to be the electric motors and servos. Looking at the motor loads in the design stage can significantly reduce operating costs for years to come.
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Though the initial cost is higher, the payback on energy efficient motors, like this one from Baldor, can be less than one year. |
Servo and drive systems. Servo amplifiers and adjustable-speed drive regenerative resistor banks should be looked at first for energy savings. Some applications require the repetitive stopping of large inertial loads. That energy must be dissipated by the control's regen resistor bank, which transfers the heat to the plant air, adding to the heat load. For larger motors (10 hp and up), engineers should consider a line regen control, which puts the energy back on the line (run the meter backwards) and eliminates the additional heat load.
The extra heat from drive regen resistors can present substantial loads on plant cooling. One alternative is to use a multi-axis drive, to take energy from one servo and dump it into the common bus for use by other drives first. The regen resistor is only used by the control as a last resort.
In servo systems it is also common to see servo motors in a "hold" mode, even when it is not always necessary. A more energy conscious method would be to decelerate to a stop, then set a mechanical brake to hold motors that require zero shaft movement.
Power quality can be directly related to good system design. Often one transformer supplies all single-phase power to all the controls devices, PLCs, and even single-phase servos. As power grids get weaker, voltage imbalance can become an issue on three-phase circuits and increase every motor's power consumption.
There are many nonlinear loads connected to power sources that create less than perfect power, which is converted by electric motors to heat. By controlling harmonics with filters, proper wiring techniques can extend the life of the electronics, and reduce motor power consumption.
Performance issues. Higher efficiency motors do perform differently. Each increase in efficiency involves trade-offs, most of which are transparent to the application. First, with current, the locked rotor amps (LRA), or starting amps, increase. Typical pre-EPAct motor LRAs would have been approximately 6 times the full load amps (FLA). EPAct motors have an LRA at around 6.5–7× the FLA. In premium efficiency motors, LRA is about 8× FLA.
In a retrofit project, for example, a pre-EPAct motor would have a typical operating speed of 1,725 rpm for a 4-pole motor; higher efficiency motors can be 1,750 rpm and higher. This is a consideration for pumps and fan applications where speed increases load, so check performance data for exact speed at full load. The higher the efficiency, the cooler the motor operates.
If the project is an upgrade, engineers should consider replacing all older motors with higher efficiency designs. Payback calculations are available from the motor suppliers based on operating time.
Vendors should be able to supply energy calculations to determine energy costs for different technologies. Regen resistor banks are energy wasters, so try to eliminate or reduce their use to lower the plant's air conditioning heat load. Estimated wattage from calculations to size regen resistors can be used to estimate air conditioning costs and pay back. The plant's HVAC supplier can calculate the additional heat load cost or savings from reduced heat load. Consider ways to turn motors and controls off when idle. Cycle motors and controls "on" one at a time to reduce peak current and consider ASDs and soft starters to help reduce peak currents. After the installation is completed, make sure the voltage is balanced and the harmonic voltage is controlled to reach the "watts per part" goal of the project.
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Energy-efficient motors from Baldor: Enter 546
| More efficient motors yield quick payback | ||||||
|---|---|---|---|---|---|---|
| Motor type | Efficiency | Cost | Op. costs 6,000 hrs | Payback (months) | Op. costs 8,260 hrs | Payback (months) |
| Non-EPAct | 85.5 | $378 | $4,188 | — | $6,115 | — |
| EPAct | 89.5 | $420 | $4,001 | 2.7 | $5,841 | 1.8c |
| Premium Efficient | 91.7 | $560 | $3,905 | 7.7 | $5,701 | 5.3 |
| Footnote: Calculations based on a totally enclosed, fan-cooled 10-hp motor. Penny-wise, pound-foolish. Efficient motors, which cost more up front, can produce major savings in the long run. The calculations in the above table are based on 6¢/kW-hr power. Payback should be based on expected future electric rates. Electricity costs for a 10-hp motor running 3 shifts a day, 5 days a week, are approximately $3,000. If the cost of power increases just 1¢/kW-hr, the extra cost of one year approaches the cost of a 10-hp non-enclosed motor. |
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