Induction motor, load, current

Here it is simple and quick from an electrician. There are two types of current for motors, full load amperes and nameplate amperes. Full load current (FLA) is found in the National Electrical Code Tables located in Article 430. There are tables for single phase, three phase, direct current, and wound rotor motors. Nameplate current is found on the motor nameplate. Nameplate current is used to size motor overload current protective devices. FLA or nameplate horsepower is used to size conductors, motor short circuit and ground fault protective devices (SCGFPD) , controllers, and disconnects. A motor circuit generally consists of a short circuit and ground fault protective device, disconnect, conductors, controller, and overload devices. A motor circuit is unique because the motor starting current is high generally 6 times the FLA so the SCGFPD has to handle this high current for a short time duration and therefore is sized several times larger than the FLA. This device does not protect the motor conductors or the motor from overload because it is sized too high, but it does protect from ground faults and short circuits that cause very high currents for a short duration of time. Protection from long term over current and over heating is the function of the overload devices that are generally set just above the nameplate current. The overloads are slow acting and do not trip during the starting of the motor but serve as long term protective devices for over current. There are rules in the NEC for sizing these devices for safety purposes. For the USA, the NEC is not an engineering standard, but a safety standard that establishes minimum safety rules for electrical design, installation and manufacturing. Now about induction. The stator winding is wired to the supply. When the stator is energized it induces a magnetic field into the rotor. This induced magnetic field and resulting counter electromotive force (EMF) acts as part of load and limts the current in the stator. Consequently, if the applied voltage is too low the current will increase in the Stator because of too low of a counter EMF and cause the motor overloads to trip. This problem can be caused by too high of a voltage drop in the conductors as happened at Pump Station 8 on the trans Alaska pipeline. We overcome this problem simply by increasing the size of the overloads. The opposing magnetic poles cause the rotor to rotate. This is very brief and to the point and serves for newsgroup discussion purposes. There are many sources of information on the Internet. Try