WorldWide Drilling Resource

Rating, Derating, and Duty Cycle Rating by Britt Storkson Owner, P2FlowLLC A rating is the evaluation or assessment of something in terms of quality (as with a critic rating a novel), quantity (as with an athlete being rated by his or her statistics), or some combination of both. (Wikipedia). In electrical/electronics, we use ratings extensively to determine if a part is suit- able for where we want to use it. For example: If we have a 50-amp electrical load to service, we would choose an alternator rated at 50 amps or greater to power the load. We don’t want to oversize the alternator because we would just be wasting money on capacity we wouldn’t use. Conversely, we don’t want to undersize the alternator because we would get too much in the way of voltage drop and could possibly damage the alternator in the process. The above assumes continuous duty at a certain temperature range. Often, product manufacturers recommend derating products if they will be used in an elevated-temperature environment. Since copper has a positive temperature coef- ficient - meaning it is less conductive (higher resistance) at higher temperatures - the current flow through the copper is restricted and more heat is generated. We get around the physical limitation by buying a larger part. Another way to deal with this limitation is to provide external cooling, but often it’s more complicated and expen- sive than simply buying a higher rated part. Another aspect of parts rating is duty cycle rating. Often, parts are rated at a certain percentage duty cycle. This means, if a part has a 20% duty cycle, it can be run for, say, 20 seconds and “rested” for 80 seconds. This is to allow the part to cool enough between on cycles to prevent damage to the part. Most of the time, the part manufacturer will provide recommended cycle on and off times, which will ensure good reliability. A good example of this is an electric solenoid which locks and unlocks car doors. It only needs to be on a couple of seconds or so to do the job. If you applied power to this solenoid for a minute or more, it would soon heat up and fail - and use a lot of power in the process. Why can’t we have door lock solenoids that can be powered continuously? The answer is cost. Yes, we can have door lock solenoids which would run continuously, but it would cost a lot more to make them, and we don’t need continuous duty in this application. Just about everything electrical can be overloaded if we keep the duty cycle low . . . meaning we don’t run it that way very long. A typical electric motor is overloaded on startup. It draws considerably more than its rated current (amperage) on start-up than it does during operation, and this is fine because it does it only for a second or two before going to nor- mal operation. This is all about “material efficiency”, which is using the minimum amount of material to obtain the maximum benefit possible from the material. This almost from the beginning, and it allows us to have the maximum benefit from the products we use - at a minimal cost. Britt Britt Storkson may be contacted via e-mail to michele@ worldwidedrillingresource.com 17 WorldWide Drilling Resource ® DECEMBER 2017