By Walt Dryburg, Integrated Power Services
Too often, motors are selected only on the basis of horsepower or price, with the result that the goal of moving a load at minimal cost is not achieved.
At its simplest, a motor is a heat-management problem, with the objective of getting the most work out of it without heat buildup inside it exceeding design limits. Here is a basic checklist for applying induction motors that operate at constant (line) speed—by far the most common industrial application—and avoiding the major pitfalls that shorten their lives and increase the expense of the application.
Step 1: Know the load characteristics because torque results in heat
For line-operated motors, loads fall into three general categories: constant torque, torque that changes abruptly, and torque that changes gradually over time.
Constant torque: Bulk material conveyors, extruders, positive displacement pumps, and compressors without air unloaders run at relatively steady levels of torque. Once they accelerate, reach running speed and the load is adjusted, the torque required remains about the same until the process is shut down, Fig. 1.
Fig. 1—Typical speed/torque relationship for an induction motor. Breakdown is the point at which additional load will cause the motor to stall.
Sizing a motor for these applications is simple once the torque (or horsepower) for the application is known. Machine manufacturers for such components as pumps and extruders, for example, publish power requirements for their equipment. Motor efficiencies are highest when they operate between 75-100% of load. Thus, system efficiency is best when a motor that operates in this range is selected.
Rapidly changing loads: Load demands by elevators, compactors, punch presses, saws, and batch conveyors change abruptly from low to high in a short time, often in a fraction of a second. In these applications, a motor may run at full speed unloaded until a clutch is engaged or machine cycle initiated. Then the load torque increases dramatically. The most critical consideration for selecting a motor in these cases is to choose one whose speed-torque curve exceeds the load torque curve, particularly at the breakdown point. If a high, momentary load exceeds a motor’s breakdown torque, it will stall. But a motor that can handle the peak torque will be able to accelerate the load to full running speed and keep it there.
Gradually changing loads: Loads from centrifugal pumps, fans, blowers, compressors with unloaders, and similar equipment tend to be variable over time, Fig. 2. In choosing a motor for these conditions, consider the highest continuous load point, which typically occurs at the highest speed. It is critical to know the value of the peak because this is the value that will challenge the motor’s ability to turn the load. Also, just as important is the duration of the peak load, because the motor must be sized to manage the peak load—for its duration.
Fig. 2—Speed/torque relationship for a motor driving a blower. The breakdown torque is the point where the load line crosses the output of the motor.
Step 2: Get a handle on horsepower
The rule of thumb for motor horsepower is: Select only what you need, and avoid the temptation to oversize or undersize.
Constant load: This is the simplest case. Determine the load requirements (from the nameplate on the driven unit, or by empirical methods), or calculate the required horsepower from this formula:
Horsepower = Torque x Speed/5250
where torque is in lb-ft and speed is in rpm.
Choose a motor for which the load is 75 to 100% of the motor’s capacity. If the load is steady and the duty cycle long, it is safe to choose a motor where the load is close to the upper end of the motor’s capacity. This optimizes efficiency.
Variable load: You must know the entire load range—particularly the peak load and the duration of the peak. The load of a pump, for instance, may range from 20 to 100% over its operating cycle. Use peak load to calculate the maximum size of the motor needed for the application, because the motor must be able to turn it at peak without overheating.
Motors have service factors (SF)—essentially safety factors—that indicate how much the rated load can be exceeded for short periods without overheating. For instance, a standard Design B motor with a SF of 1.15 can operate briefly at 15% greater than its rated load without overheating. This is important where loads vary and peak at or slightly above the rated torque of the motor. But don’t get greedy with the service factor. Operating a motor at 1.15 SF on a continuous basis will shorten its life. For varying loads, calculate the RMS horsepower requirement and size the motor so that the load falls within 75 to 100% of the motor’s capacity.
Step 3: Getting started
Another consideration is inertia, particularly during startup. Every load represents some value of inertia, but punch presses, ball mills, crushers, gearboxes that drive large rolls, and certain types of pumps require high starting torques due to the huge mass of the rotating elements. Motors for these applications need to have special ratings so that the temperature rise at startup does not exceed the allowable temperature limit. A properly sized motor must be able to turn the load from a dead stop (locked-rotor torque), pull it up to operating speed (pull-up torque), and then maintain the operating speed. Fig. 1 shows that current during this acceleration period is typically 5-7 times full load current. The message here is that because current makes heat, startup can be the most critical part of the operating cycle. Motors are rated as one of four “design types” for their ability to endure the heat of that starting and pull up. In ascending order of their ability to start inertial loads, NEMA designates these as design type A, B, C, and D. Type B is the industry standard and is a good choice for most commercial and industrial applications, Fig. 3.
Fig. 3—Starting characteristics for the four basic motor designs. Design B is the general-purpose unit, and is usually less expensive. However, because starting torque is low, it may not be able to start a high-inertia load. In that case, one of the other designs that has adequate low-end torque must be chosen.
Getting started is not quite the end of this part of the problem, however. As speed increases, a motor’s ability to supply torque declines by at least 1/3. A motor that is loaded close to its limits may be unable to sustain the load as speed increases and stall back to zero. When selecting a motor, check its speed/torque profile to be sure that, once started, it can carry the load.
Step 4: Adjust for duty cycle
Duty cycle is the load that a motor must handle over the period when it starts, runs, and stops.
Continuous duty is—by far—the simplest and most efficient application. The duty cycle begins with startup, then long periods of steady operation where the heat in the motor can stabilize as it runs. A motor in continuous duty can be operated safely at or near its rated capacity because the temperature has a chance to stabilize.
Intermittent duty is more complicated. The life of commercial airplanes is measured by their number of landings; in the same way, the life of a motor is proportional to the number of starts it makes. Frequent starts shorten life because inrush current at startup heats the conductor rapidly. Because of this heat, motors have a limited number of starts and stops that they can make in an hour. NEMA standards for a range of motors is shown in the table.
Selecting a motor for intermittent duty begins with an educated choice. A rule of thumb is that for every 10° C cooler that a motor operates, it’s life doubles. Therefore, for maximum life, a motor must run at less than maximum temperature. If a motor is sized for peak load alone with no consideration for the number of starts it must make, it may burn out rapidly in intermittent duty. However, by choosing the next size larger motor, you gain greater capacity for coping with frequent starts.
Step 5: The last consideration, motor hypoxia
If your motor is going to operate at altitudes that are substantially above sea level, then it will be unable to operate at its full service factor because, at altitude, air is less dense and does not cool as well. Thus, for the motor to stay within safe limits of temperature rise, it must be derated on a sliding scale. Up to an altitude of 3,300 ft, SF = 1.15; at 9000 ft, it declines to 1.00. This is an important consideration for mining elevators, conveyors, blowers, and other equipment that operates at high altitudes.
Centrifugal pumps usually represent steady torque loads for motors. The motor can be run for most of its duty cycle at or near maximum capacity.
Putting the steps together
A double-action pump has been installed to boost flow requirements in a cooling system in a seaside refinery. The manufacturer’s specifications call for a nominal speed of 1760 rpm, with the following load profile: 4.8 hp for 30% of the cycle, 17.4 hp for 20%, 5.0 hp for 30%, and 17.4 hp for the final 20%. What size motor should be chosen?
This is a variable-load, continuous-duty application, with no requirement for altitude adjustment. Given the peak load and short duration of it, a “trial choice” would be a 15-hp motor with a service factor of 1.15. This motor would have to work slightly past its rated capacity during the brief moments of peak load, but perhaps it can cool enough in its off-peak moments to stay under its rated temperature limit. Can the 15-hp unit handle it, or should you opt for the more conservative choice of a 20-hp unit and its substantially bigger price tag and power appetite?
Loads on unwind/rewind equipment, such as this slitter, change with the weight of the roll. The motor should be sized for the rms value of the load, plus what it takes to start a full roll turning and maintain the intended operating speed.
Calculating the RMS hp for the application shows that the load that the motor will have to manage will be 11.6 hp, which is 77% of the 15-hp unit’s capacity. This falls in the ideal efficiency range for this unit. Thus, not only would the 15-hp unit suffice, it would handle the load more efficiently than the more expensive 20-hp motor.
These chop saws on a steel line represent about the worst load conditions possible. Motors go from no-load to extreme peak load in about a second. Inrush current is enormous. To size a motor for these conditions, you must consider the peak load and its duration in the duty cycle.
Filed Under: Motion control • motor controls, Motors • ac, Motors • dc
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