Cyndi Nyberg
Former EASA Technical Support Specialist
Most motors are run continuously with little variation in load. A continuous duty motor is energized and loaded for an extended period of time. When the motor is started, the temperature increases, and then the temperature stabilizes after some time.
If the motor was designed with a service factor, it is possible to run the motor at a higher-than-rated load for short periods of time without significant thermal damage to the windings, rotor or bearings. A motor to be used with a continuous load is sized based on that load rating.
There are, however, many applications where a motor is not loaded consistently throughout its duty cycle, or is energized intermittently.
Some motors are started and stopped often, while others are loaded lightly for some time, then more heavily for some time, and the applied load can vary greatly.
If there are periods of time where the motor is operating at less than full load, then it may be possible to size the motor smaller than the maximum load level.
Efficiency considerations
If it is possible to use a motor that is rated below the maximum horsepower level required, the obvious advantage is the initial cost. How- ever, the lifetime cost of the motor will also be lower, since the overall efficiency will be because of the heat generated, or the stator winding may fail prematurely.
If the duty cycle consists of periods of load and no load, then heating is not as much of a concern because there is still airflow as long as the rotor is turning (assuming an internal or external fan is present). Typical time ratings for intermittent duty motors include 5, 15, 30 or 60 minutes.
Load variations
For applications with a repetitive duty cycle, the load varies at specific intervals of time. These intervals generally repeat and do not change during the duty cycle of the machine. However, the actual loads can vary widely, from almost no load to more than full load of the motor used in the application. An example of this type of application would be an injection molding machine.
The root-mean-square (RMS) value of the horsepower over one cycle can be calculated to estimate the possible heating effect on the motor. The RMS horsepower is the square root of the sums of the horsepower squared, times the time per horsepower; divided by the sums of all the time intervals. To determine the RMS load on the motor, use the following equation:
As long as the RMS horsepower does not exceed the full load horsepower of the motor used in the application, the motor should not overheat. This, of course, is only true as long as there is adequate ventilation during the entire cycle. To keep it simple, we have disregarded the effect of acceleration time on a self-ventilated motor.
Example:
To properly size a motor for varying, repetitive duty, you will need to know the duration and horsepower load for each. It is helpful to draw a graph showing the required horsepower vs. time, shown in Figure 1, as well as a table that lists each time and horsepower, shown in Table 1.
In this case, the RMS horsepower is 39.3. To allow for voltage variations, and to provide a little extra margin of safety, the motor for this application can be sized as a 40 hp motor with a
1.15 service factor or a 50 hp motor with a 1.0 service factor. Neither of these motors would overheat in this application.
One thing to watch is to make sure that the motor can actually deliver the maximum required torque. This means that the breakdown torque (BDT) of the motor must be higher than the highest horsepower load torque throughout the duty cycle. If the motor cannot deliver this torque, the motor may stall.
Using our example, a 40 hp, 4-pole, Design B motor will have a minimum breakdown torque of 200% of the full load torque (from NEMA MG-1 requirements). To determine the percent breakdown torque needed for the peak horsepower, use the following equation:
Since our maximum required horsepower only requires 150% of the full load motor torque of the 40 hp motor, it can be used in this application.
Also, keep in mind that this type of analysis only works for applications where the duty cycle is a relatively short period of time. Any complete cycle that is longer than about 5 minutes will require a more involved study of the load and duty cycle. However, there are many applications where the repetitive load cycle is much shorter and the RMS horsepower can be used to size the motor.
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