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Cahier Technique No. 207 Collection Technique .......................................................................... Cahier technique no. 207 Electric motors ... and how to improve their control and protection E. Gaucheron Building a New Electric World "Cahiers Techniques" is a collection of documents intended for engineers and technicians, people in the industry who are looking for more in-depth information in order to complement that given in product catalogues. Furthermore, these "Cahiers Techniques" are often considered as helpful "tools" for training courses. They provide knowledge on new technical and technological developments in the electrotechnical field and electronics. They also provide better understanding of various phenomena observed in electrical installations, systems and equipments. Each "Cahier Technique" provides an in-depth study of a precise subject in the fields of electrical networks, protection devices, monitoring and control and industrial automation systems. The latest publications can be downloaded from the Schneider Electric internet web site. Code: http://www.schneider-electric.com Section: Press Please contact your Schneider Electric representative if you want either a "Cahier Technique" or the list of available titles. The "Cahiers Techniques" collection is part of the Schneider Electric’s "Collection technique". Foreword The author disclaims all responsibility subsequent to incorrect use of information or diagrams reproduced in this document, and cannot be held responsible for any errors or oversights, or for the consequences of using information and diagrams contained in this document. Reproduction of all or part of a "Cahier Technique" is authorised with the compulsory mention: "Extracted from Schneider Electric "Cahier Technique" no. ....." (please specify). no. 207 Electric motors ... and how to improve their control and protection Etienne Gaucheron Graduate electronics engineer by training. After a short period with Thomson he joined the VsD (Variable speed drives and starters) division of Telemecanique in 1970. He then completed his training at ENSAM (the national college for arts and vocational training) in Paris during his position with Telemecanique's after sales service. As a specialist in motor control, he was involved in the development of variable speed control for AC motors. He has gained a wide range of experience in various positions: systems platform engineer, product manager for machine tool drive products, product manager for drives for asynchronous motors (Altivar products) and manager of the VSD project marketing team. He is currently a motor control "applications" specialist within the advance development team for the Schneider Electric PCP (Power Control and Protection) division. ECT 207 first issue, October 2004 Cahier Technique Schneider Electric no. 207 / p.2 Electric motors ... and how to improve their control and protection Nowadays, apart from lighting devices, electric motors represent the largest loads in industry and commercial installations. Their function, to convert electrical energy into mechanical energy, means they are particularly significant in economic terms, and hence, they cannot be ignored by installation or machinery designers, installers or users. There are many types of motor in existence, but 3-phase asynchronous motors, and in particular squirrel cage motors, are the most commonly used in industry and in commercial buildings applications above a certain power level. Moreover, although they are ideal for many applications when controlled by contactor devices, the increasing use of electronic equipment is widening their field of application. This is the case for start/stop control with soft start/soft stop units, and when precise speed adjustment is also necessary with variable speed drives/regulators. However, slip-ring asynchronous motors are used for certain high power applications in industry, and single phase asynchronous motors remain suitable for limited power applications, mainly for buildings applications. The use of synchronous motors, known as brushless or permanent magnet motors, combined with converters is becoming increasingly common in applications requiring high performance levels, in particular in terms of dynamic torque (on starting or on a change of duty), precision and speed range. After presenting the various types of electric motor and their operating principles, this "Cahier Technique" describes the technical and operating features of asynchronous motors, covering in particular the main starting devices, speed control and braking methods. It provides a solid grounding for the reader to gain a good understanding of all the problems involved with motor control and protection. This "Cahier Technique" touches briefly on variable speed control of electric motors. However, this subject is covered more specifically in "Cahier Technique" CT 208 "Electronic starters and variable speed drives". Motor protection is the subject of another "Cahier Technique" currently in preparation. Contents 1 Three-phase asynchronous motors 1.1 Operating principle p. 4 1.2 Construction p. 6 1.3 The various types of rotor p. 6 2 Other types of electric motor 2.1 Single phase asynchronous motors p. 10 2.2 Synchronous motors p. 10 2.3 DC motors p. 14 3 Operation of asynchronous motors 3.1 Squirrel cage motors p. 17 3.2 Slip-ring motors p. 19 3.3 Other speed variation systems p. 20 4 Conclusion p. 22 Cahier Technique Schneider Electric no. 207 / p.3 1 Three-phase asynchronous motors This section covers 3-phase asynchronous Other types of motor are presented in section 2. motors, the most commonly used motors for Section 3 describes and compares the main driving machinery. The use of this type of motor starting devices, and speed control and braking has become the norm in a large number of methods associated with the motors. applications because of its numerous advantages: it is standardized, rugged, easy to maintain and use, and inexpensive. 1.1 Operating principle The operating principle of an asynchronous motor is based on the creation of an induced current in a conductor when the conductor cuts the lines of force of a magnetic field, hence the direction of name "induction motor". The combined action of magnetic field this induced current and the magnetic field creates a motive force on the motor rotor. Let us take the example of a turn with short circuit ABCD, located in magnetic field B, and rotating around an axis xy (see Fig. 1 ). direction of current If, for example, we rotate the magnetic field (positive to clockwise, the turn is subject to a variable flux negative) and becomes the source of an induced electromotive force which causes an induced current I (Faraday’s law). According to Lenz’s law, the direction of the current is such that it opposes the cause that produced it by its electromagnetic action. Both of movements of the wire Fig. 2: Fleming’s left hand rule to find the direction of the force A x the conductors are therefore subject to a Lorentz force F (also known as Laplace force), in the South opposite direction to its relative displacement in relation to the field coil field. D B The left hand rule (action of the field on a current, see Figure 2 ) helps demonstrate the direction of the force F applied to each conductor. I I The thumb points in the direction of the North movement field. The index finger indicates the B direction of the field. The middle finger points in F the direction of the induced current. The turn is therefore subject to a torque that causes it to rotate in the same direction as the coil field, F y called the rotating field. The turn therefore starts to rotate and the electromotive torque produced balances the resistive torque. C Creation of the rotating field ° Fig. 1: Creation of an induced current in a shorted turn Three windings, geometrically offset by 120 , are each supplied by one of the phases of a 3-phase Cahier Technique Schneider Electric no 207 / p.4 AC supply (see Fig. 3 ). The windings have AC The flux corresponding to phase 3 is negative. currents flowing through them that have the The field is thus directed in the opposite direction same electrical offset and that each produce a to the coil. sinusoidal AC magnetic field. This field, which If the three diagrams are superimposed, it can always follows the same axis, is at its maximum be seen that: when the current in the winding is at its v The three fields rotating anticlockwise are maximum. offset by 120° and cancel one another out. The field generated by each winding is the resultant of two fields rotating in opposite v The three fields rotating clockwise are directions, each field having a constant value of superimposed on one another. These fields are added together to form the rotating field with constant amplitude 3Hmax/2. It is a field with one pair of poles. This field performs one rotation during one period of the supply current. Its speed is H1 B1 dependent on the line supply frequency (f), and ˚ 0 the number of pairs of poles (p). It is called the 2 1 "synchronous speed". Slip Motor torque can only exist if there is an induced B3 B2 current flowing in the turn. This torque is determined by the current flowing in the turn and H3 H2 can only exist if there is flux variation in this turn. There must therefore be a difference in speed between the turn and the rotating field. This is why an electric motor operating according to the principle we have just described is called an Ph3Ph2 Ph1 "asynchronous motor". The difference between the synchronous speed (Ns) and that of the turn (N) is called the "slip" and is expressed as a Fig. 3: Principle of a 3-phase asynchronous motor percentage of the synchronous speed. slip = [(Ns - N) / Ns] x 100 half that of the maximum field. At any instant t1 During operation, the rotor current frequency is in the period (see Fig. 4 ), the fields produced by obtained by multiplying the supply frequency by each winding can be represented as follows: the slip.
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