The National Electrical Code (NEC) covers non-utility electrical installations from the point of view of safety. One primary focus of NEC articles is over-current protection and the related concept of ampacity. NEC Chapter three contains extensive tables for sizing electrical conductors in many intended applications. The implication of these tables is that choosing the correct over-current protection device takes place in terms of trip-out rating.
These NEC specifications are applicable for most electrical installations and equipment with the profound exception of electric motors. Electric motors are a special case because of their special start-up characteristics so they demand a different kind of over-load protection. There is a lengthy NEC section (Article 430) dedicated to the design and installation of these systems.
Most electrical devices draw more current for a few seconds while starting than during sustained operation. This behavior is even greater in the case of electric motors. The way to think of it is that more energy is required to accelerate the turning rotor plus load up to speed than is necessary to keep it turning once it hits rated speed. The excess energy is stored in the form of angular momentum during operation.
When the motor powers down, this energy returns to the electrical system or dissipates in some other way as the rotating mass loses momentum and comes to a stop. Accordingly, an unusually large amount of current is needed during startup. If the over-current equipment is not designed with this large current in mind, it will trip out before the motor attains operating speed.
The natural question is how to design an over-current protection system that will allow this large current flow without endangering the supply conductors. To meet this challenge, electrical engineers long ago devised an ingenious method that enables large electric motor installations to operate safely. They have a unique two-stage over-current protection system.
The first stage is at the source such as an entrance panel or downstream load center supplied by a feeder. Here the motor supply conductors are protected in the conventional manner by a circuit breaker or fuse(s). The only difference is that the current rating is permitted to be much higher than specified by the ordinary non-motor ampacity tables. The intent is to provide ground-fault and line-to-line short-circuit protection only, not overload protection.
Overload protection comes farther downstream, usually in an enclosure adjacent to the motor. The rating of this over-current protection is chosen in accordance with tables in a later part of Article 430. Under limited conditions, if the motor will not reach operating speed without tripping out, it is permissible to substitute a higher rated (less sensitive) device.
This is how large motors are generally wired. Article 430 is lengthy and detailed, so the design of wiring systems for large motors should take place only in consultation with experienced professionals.
Of course, not all motors have a size big enough to be covered in the NEC. In 2011, researchers created an electric motor the size of a single molecule. This motor was powered by ambient electric fields, bringing new meaning to the term “fractional horsepower.”
In early 2014, a paper in Physical Review Letters described a motor the size of a single atom, one nanometer to be precise. Small electrodes captured a calcium-40 ion in a volume of high electromagnetic energy which made the ion cone shaped. Laser beams focused on each end of it. One heated and the other cooled, and this differential temperature altered the ion’s quantum characteristics to produce a vibration. The lasers could be adjusted so that the vibrations would resonate with the ion’s characteristic frequency, reinforcing the effect.
These are motors is the broad sense that electrical energy is converted into mechanical motion. What about actual rotary motors? It is hard to believe, but there are naturally occurring single cells that have freely turning and even reversible motors. The bacterial flagellar motor is embedded in the organism. Power comes from the flux of positive hydrogen or sodium ions. It exists across the cytoplasmic membrane and derives its energy from an electrochemical gradient. The organism has a helical filament that can attain speeds as high as several hundred revolutions per second, much greater than our rpm-rated motors. It is truly electrically powered.
This unusual bacterium was first reported in the 1970s, but a more complete description was not developed until 2008.
Then, in May 2015, Nanoscale, a peer-reviewed journal, published a paper describing a new type of human-made micromotor, truly electrical and rotary, sized less than 1 μm. It could rotate continuously for 15 hours, turning a total of 240,000 cycles. An improved model, based on observations of the prototype, went 1.1 million cycles in the course of an 80-hour operational lifetime.
These nanomotors may eventually play a role in conquering cancer. The thought is they would power submicroscopic robots that could be introduced into the human body to destroy malignant cells even after metastasis.
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