Motors | My Business | Mississippi Power | A Southern Company
  • Katrina 2005-2015
  • Katrina 2005-2015
  • Motors

    Be aware that there are benefits to "soft starting," but energy conservation is not one of them

    "Soft starting" refers to reducing the current in the power supply during motor acceleration. Benefits of soft starting include preventing a severe voltage dip when then motor starts and lessening mechanical shock to equipment. Despite the claims of starter suppliers to the contrary, soft starting does not result in reduced energy costs.

    Carefully evaluate whether to replace a functioning motor with a more energy efficient one

    Carefully evaluate whether to replace a functioning motor with a more energy efficient one. It is not a good idea to replace motors that run only a few hours per day. Continuously running motors such as those used in the papermaking process are better candidates for replacement. It takes too long to recover the cost of replacement of motors that run less than 4,400 hours per year.

    Choose a motor with a sufficient horsepower rate

    Choose a motor with a sufficient horsepower rate. In general, a motor operates at three fourths of the horsepower shown on its nameplate. For this reason, an oversized motor is often a more efficient way to drive a the load than a smaller motor.

    Compare brands and prices of motors based on NEMA value rather than hype

    Compare brands and prices of motors based on NEMA value rather than hype. Ignore advertisements and sales promotions that contrast "high efficiency," "premium efficiency," "extra efficient," or "super efficient" motors with "standard efficiency" or "low efficient" ones. Instead, compare the efficiency value stamped on a motor's nameplate to standards published by the National Electrical Manufacturers Association (NEMA).

    Consider certain factors when troubleshooting problems with motors

    When troubleshooting problems with a motor, consider what the symptoms are and how long they have been present. Note the changes that occurred in the equipment once the problem began, and evaluate how accurate the evidence of the problem is. Also consider the motor's ratings and its circuitry. Keep track of the specific actions taken to remedy the problem.

    Consider energy efficient motors

    What does all the hype mean? You'll see all kinds of ads, sales promotions and technical literature about motors having "high efficiency." These motors may be called "premium efficiency," "extra efficient," even "super efficient." Labels on the motors themselves may use similar wording. Those products are all contrasted with "standard efficiency" or "low efficient." How can a user make sense out of all that? By recognizing a few simple facts. Every National Electrical Manufacturers Association (NEMA) standard polyphase Design B motor below 150 hp (and many of them well above that) has an efficiency value stamped on its nameplate. That's a "nominal" value, representative of that design. For every such value, a "minimum" or guaranteed value is published in NEMA standards. Disregard the fancy labels and catchy slogans-get those numbers. Compare brands-and prices- based on them. Some extra-cost motor lines offer efficiencies far above the NEMA "energy efficient" value; some "standard" motor lines may also reach such high levels for some ratings. Each customer's economic analysis will point toward the best solution.

    Consider the average life of a motor to be at least 15 years

    Based on surveys by the Institute of Electrical and Electronics Engineers (IEEE), the average life of a motor is at least 15 years.

    Do not invest in "power factor controllers" to save energy

    A power factor controller adjusts motor voltage to suit motor load. This device only saves energy on smaller motors; such motors use so little power that the savings is insignificant.

    How long do motors last?

    How long should a motor last? Various Institute of Electrical and Electronics Engineers (IEEE) reliability surveys have shown two general life relationships for industrial motors. One is that annual repair/replacement rates for the overall motor population tend to be 5 to 6 percent. That implies a 15- to 20-year average life span. However, these surveys did not make clear what was accepted as "end of life," nor did they indicate what percentage of the "failed" motors went back into service after repair. The other IEEE survey result was that motor starters tended to fail more often than motors themselves. Based on information of that sort, a reasonable approach seems to be to consider average motor life at least 15 years.

    Motor change economics

    Do the savings from the lower motor losses of an energy-efficient motor justify the extra motor cost? The answer depends on the actual number of operating hours during which losses occur. In seeking candidates for improvement in motor efficiency, don't bother with motors that run only a few hours a day (particularly if they're not fully loaded). The payoff will not be there. Better yet are the continuous runners involved with a process such as papermaking. Although it's no magic number, 4,400 hours annually ("half time") is a useful starting point. Below that, payback might be too long. Replace existing motors that haven't failed just to get the high efficiency? That's seldom justified. You won't see the payoff for even 6,000 hours of operation.

    Nameplates

    The nameplate is a motor's single most important component. NEMA standards require certain information on all AC motor nameplates. The major items any nameplate must display are: 1. Manufacturer type and frame designation. 2. Horsepower output and time rating (e.g., "continuous"). 3. Phase, frequency, and full-load rpm. 4. Voltage and full-load amperes. 5. Code letter indicating locked-rotor kVA (usually a letter from D through K). 6. Design letter (usually B, C or D; unrelated to the code letter). 7. Service factor, if other than 1.0. 8. Maximum ambient temperature and insulation system designation. 9. Nominal full-load efficiency when applicable (required for 1-125 hp, singlespeed, low-voltage, etc.). What's the importance of all this? Without knowing the serial number, for example, the manufacturer probably can't identify the specific design and answer questions about it. Without the frame size designation, no dimensional questions can be answered. The rpm is a must when comparing two motors to drive the same load. Temperature rating defines suitability to different surroundings. The code letter permits prediction of motor starting capability.

    Selecting the motor size

    When the subject of efficiency comes up, motor users usually get this advice: "Get rid of those oversized motors. Replace them with motors rated closer to the actual load horsepower, and you'll save energy." On the surface, that seems to make sense. We're used to thinking of a "mismatch" as bad. Suiting components to each other, having their ratings alike, must be better. But motor efficiency doesn't behave that way. In general, it has a maximum or peak value for any motor (whether standard or "energy efficient") at three-fourths of the nameplate horsepower rating. Because of that relationship, an oversized motor is often a more efficient way to drive the load than a smaller motor more nearly matching the required horsepower.

    Should you install power factor controllers?

    Do "power factor controllers" save energy? "Power factor controller" is a confusing term. Often called a Nola device, it isn't really intended to govern power factor as such. Rather, it electronically adjusts motor voltage to suit motor load, using the motor power factor as an indicator of that load. The device works, but it seldom pays off. When a motor runs fully loaded, its internal power loss is mostly "copper loss" caused by current flow through the windings. If motor voltage is lowered, current must go up to supply the same shaft output; copper loss goes up too, and efficiency tends to drop. In contrast, a lightly loaded motor's internal loss is mostly magnetic loss in the core iron. That drops rapidly when voltage is lowered. Little shaft output is needed, so the reduction does no harm. Net losses go down, and efficiency rises. Total loss can be cut 10, 20, even 50 percent-but the actual watts involved are quite low compared to full-load operation. Unless shaft output averages one-third of rated power, or less, the Nola device normally isn't cost effective-repeated tests confirm just that for three-phase motors of all sizes. The loss picture is different, and the savings greater, for small single-phase motors. But such motors use so little power anyway that the total savings is also small.

    Soft starting opportunities

    For starters, what is motor soft starting? There's no industry standard. Many think of a "soft starter" as an electronic solid-state device for reducing motor voltage (and current draw) during startup. But there's no difference in principle between that and any other "reduced-voltage starting" method autotransformer, series reactor, and so forth. Each reduces the current (and therefore the voltage drop) in the power supply system during motor acceleration. Whatever the type of starter used, two benefits result. One is the mitigation of a severe voltage dip on the system when the motor starts. System limitations or utility rules may require this. A second benefit-though unimportant in some drives-can be the reduced mechanical shock to equipment resulting from lower accelerating torque. Starter suppliers sometimes say that a third benefit is reduced energy cost. Reduced starting current supposedly means the motor "consumes less power." This is untrue. Accelerating a load consisting only of pure inertia requires the expenditure of a fixed amount of energy within the motor. At reduced voltage and current, the acceleration takes longer- but the energy expended is the same.

    Study the nameplates on motors for important information

    Study the nameplate on a motor for important information required by the National Electrical Manufacturers Association (NEMA). Key items shown on the label include serial number, frame size designation, RPM, temperature rating, and code letter.

    Need some troubleshooting tips?

    • What exactly are the symptoms? Measured conditions-how much (or little), how often, what, when, where-are the details needed.
    • How long has this particular trouble been occurring-just started this week, or has been happening on and off for years?
    • No matter how seemingly insignificant, what changes in equipment, operations or maintenance practices took place about the same time the troubles began?
    • How accurate is the evidence of trouble? If meters were used, what kind were they, and how were they connected?
    • What are the ratings-nameplate voltage, current, etc.-of all components involved in the problem (e.g., motors, contactors, fuses, circuit breakers)? Don't assume that the equipment voltage rating matches the circuit.
    • What's the circuitry involved? Is a three-phase transformation open delta or open wye rather than a full set of three transformers? If a motor trips off unexpectedly, or won't start, just how is it controlled? Basic one-line and schematic diagrams should be available showing how things are today, not as they were long ago.
    • What specific actions have been taken to correct the trouble? What were the results?