When most engineers think about energy efficient motors and gearmotors, they mainly think about how it relates to cost. While this relationship is important, efficiency is actually intertwined with many other critical design considerations such as cost, power source, size etc.
Typically when you’re designing an application you calculate the motor’s output power based on what you need it to “do” in terms of output power. Unfortunately, many designers fail to take the next step, which is to calculate the electrical energy needed to drive the motor. You’ll want to do this to ensure that your power supply is sufficient for your application. This will also give you an idea of how energy efficient you need your motor to be. If you have an application where you think you’ll have just barely enough power to supply your motor, then you know you’ll need to pay special attention to the motor’s efficiency to ensure a successful project.
For example, there are many applications where it is advantageous to minimize the size and weight of the motor, but how often does an engineer stop to consider how efficiency affects weight? One of the primary factors driving motor size is heat dissipation. A motor’s ability to dissipate enough heat is dependent in part on its surface area. Since heat is the primary form of energy loss in motors, we can see that increasing efficiency will decrease heat and therefore allow the use of a smaller motor.
From an environmental standpoint, in terms of energy usage, there actually aren’t many regulations that pertain directly to fractional horsepower motors. On the contrary, there are many different regulations that apply to various end products into which fractional horsepower motors are applied.
Motor efficiency is an interesting topic, and is a bit more complex than most people realize. Contrary to most published ratings, a motor’s efficiency isn’t static—it varies with load. By definition, efficiency starts at zero with no load, and ends with zero at its stall point and reaches a peak somewhere in the middle.
Motor Efficiency vs. Torque Curve
Take a look at this curve of motor efficiency versus torque. We have separated the curve into three areas. In a well-designed motor, the continuous duty rating should occur at or near peak efficiency. Most motors are designed to operate in this area, and published nameplate data is found within this region. As you can see from the curve, the motor’s efficiency tails off gradually as the motor is overloaded (the dark green “intermittent duty” area) and drops off sharply when the motor is under-loaded (the light green “motor too large” area).
The most interesting part of the curve is the dark green shaded intermittent duty region of the graph. Motors can be run in the overloaded condition if the duty cycle is adjusted. The advantage of doing this is that it allows the use of a smaller motor. The loss of efficiency can be an issue, but the shorter duty cycle often means that the total energy cost is minimal.
The opposite case is shown on the left side of the graph in the light green shaded “motor too large” region. It’s interesting to see how quickly the efficiency drops off after you reach a certain point. Operating in this “motor too large” region should be avoided whenever possible because the motor will be running under-loaded. Not only is the efficiency reduced and energy cost increased, the motor is also larger than it needs to be. However, for applications with a variable load, it may not be possible to avoid this condition during part of the cycle. In this case, care must be taken to ensure that the motor doesn’t overheat, as some types of motors will actually draw more current at light load than at full load.
Again, because efficiency is not constant through the operating range, there are trade-offs when using a motor that is under-sized or over-sized.
>> Miss the first post in the series on energy efficient motors? Read it now
