When an interviewer asks these C-terms in the electrical power system, some candidates get confused for a while before answering. This article would definitely make you understand these terms thoroughly and let you answer without confusions!!
Crawling is a phenomenon which is associated with induction motors. The synchronous speed of induction motor is defined as the speed of rotation of the magnetic field. This synchronous speed is determined by the frequency of the power supply and the number of poles in the motor winding.
Crawling is a characteristic experienced by squirrel cage induction motors. It is observed that the squirrel cage induction motors tend to run stable at a very low speed nearly to one-seventh of its synchronous speed. Thereby the actual speed of motor becomes very low when compared with the synchronous speed. This phenomenon is termed as crawling. The stator of a motor is made to rotate by the rotating flux produced by the motor winding. The non-sinusoidal wave (harmonics) nature of this flux reduces the speed of the motor. By providing the rotor with skewed slots and also by the proper selection of rotor and stator slots the process of crawling can be eliminated.
COGGING
The process of cogging is also associated with induction motors. It is also called magnetic locking. It actually refers to the phenomenon by which the rotor slots get magnetically locked with stator slots and thereby the motor refuses to start. Improper selection of rotor slots, stator slots and harmonics present in the power supply may lead to cogging. When the number of the rotor slots are the equal or integral multiple of stator slots, cogging is likely to occur. When the number of these slots are equal, the reluctance will be minimum and the motor refuses to start. Optimum selection of rotor and stator slots reduces the effect of cogging. And also by providing rotor skews such that each rotor slot is facing more than one stator slot at a time, the effect of cogging can be eliminated.
CREEPING
Unlike cogging and crawling, creeping is a phenomenon that is associated with energy meters. An energy meter is a device which measures the electrical energy consumed by any electrical loads. Based on the rotation of the disc in the energy meter, the unit of energy consumption is calculated. When electrical loads of a building are live in nature the disc of the connected energy meter of the building rotates according to the energy consumption. But in some cases, this disc tend to rotate even when there are no electrical loads connected to it. This process is called creeping.
Creeping causes inconvenience to the consumers since they are billed for the non-consumption of electricity too. By drilling diametrically opposite holes on the disc this unwanted rotation of the disc can be reduced. The latest technology has a feature of "No load detection" on the energy meter. This feature measures the zero load power consumption of the energy meter. By analyzing this, the actual tariff can be billed to the electrical consumers.
