Motor capacitor | EPFL Graph Search

A motor capacitor is an electrical capacitor that alters the current to one or more windings of a single-phase alternating-current induction motor to create a rotating magnetic field. There are two common types of motor capacitors, start capacitor and run capacitor (including a dual run capacitor). Motor capacitors are used with single-phase electric motors that are in turn used to drive air conditioners, hot tub/jacuzzi spa pumps, powered gates, large fans or forced-air heat furnaces for example. A "dual run capacitor" is used in some air conditioner compressor units, to boost both the fan and compressor motors. Start capacitors briefly increase motor starting torque and allow a motor to be cycled on and off rapidly. A start capacitor stays in the circuit long enough to rapidly bring the motor up to a predetermined speed, which is usually about 75% of the full speed, and is then taken out of the circuit, often by a centrifugal switch that releases at that speed. Afterward the motor works more efficiently with a run capacitor. Start capacitors usually have ratings above 70 μF, with four major voltage classifications: 125 V, 165 V, 250 V, and 330 V. Start capacitors above 20 μF are always non-polarized aluminium electrolytic capacitors with non solid electrolyte and therefore they are only applicable for the short motor starting time. The motor will not work properly if the centrifugal switch is broken. If the switch is always open, the start capacitor is not part of the circuit, so the motor does not start. If the switch is always closed, the start capacitor is always in the circuit, so the motor windings will likely burn out. If a motor does not start, the capacitor is far more likely the problem than the switch. Some single-phase AC electric motors require a "run capacitor" to energize the second-phase winding (auxiliary coil) to create a rotating magnetic field while the motor is running. Run capacitors are designed for continuous duty while the motor is powered, which is why electrolytic capacitors are avoided, and low-loss polymer capacitors are used.

A contribution to energy saving in induction motors

Electric motors consume over half of the electrical energy produced by power stations, almost the three-quarters of the electrical consumption in industry and almost the half of commercial electrical consumption in developed countries. Motors are by far the most important type of electric charges, and so constitute the main targets to achieve energy saving. Owing to their simple and robust construction, the asynchronous motors and especially those of squirrel-cage types, represent about 90-95% of the electrical energy consumption of electric motors, which is equivalent to about 53% of total electrical energy consumption. They are widely used as electrical drives in industrial, commercial, public service, traction and domestic applications. Owing to the importance of induction motors, this thesis is aimed at contributing to energy saving efforts, more specifically in the field of low power induction motors. A contribution is kept in perspective by taking into consideration the energy saving potential during the motor design stage as well as during its operation. Every effort to save energy in motor application can be made by always attempting to use energy only as much as what needed during its operation. The best way is to exploit the saving potential during motor design, while at the same time taking into account its intended application. It can be achieved either through the improvement of motor design or through the reduction of its input electrical energy when the motor has already been built. These two efforts are studied, elaborated and worked out thoroughly in this thesis. To attain this objective, a synthesis has been started with the description of how to model an induction motor. To obtain a better model, an improvement is proposed by using the Schwarz-Christoffel mapping to calculate the slot leakage inductance in induction motor. With such method, slot-leakage inductance can be determined more precisely, resulting in more accurate prediction of motor characteristics. It is based on the stored magnetic energy calculation using two-directional field distribution in the slot. The air gap influence can be observed easily, so that a reasonable slot leakage definition can be adopted. Unlike the conventional method, which is only suitable for rectangular slots (otherwise empirical corrections are required), the proposed general slot form can be extended to any desired polygonal slot form. Consideration of saturation is also indispensable because ignoring it could result in inaccuracy in motor performance prediction. Considering the saturation is essential owing to its important role in self-excitation phenomenon to establish voltage build-up in induction generator. However, the self-excitation phenomenon is undesirable in certain group of capacitor motors as it may hinder the switching-off process and mechanical braking at a desired moment. The undesirable switching-off failure condition is to be avoided by properly designing the capacitor motor. Like in this capacitor motor special application, where a proper design is useful from the point of view of operation safety, designing properly a motor is also very important in energy saving efforts. Motor design and optimization to minimize losses as well as to make possible wide speed-range motor operation are some of the efforts. However, when induction motor has already been built, saving energy is only possible by managing its supplying electrical energy. Various strategies are possible and a particular emphasis on the use of triac to reduce motor input voltage is presented. Besides, a brief economic saving evaluation is given to draw attention to the energy saving potential.

EPFL2005

A contribution to energy saving in induction motors

EPFL2005