1. A dc motor с... the electrical energy back into mechanical energy. 2. There are various types of dc motors, d... o... the way the field coils are connected. 3. Each has characteristics that are a... under given load conditions.
35. /4 мин./ Составьте три предложения из следующих набо-. ров слов:
1. field, coil, parallel, armature, circuit, shunt, motor, have, connect. 2. this, develop, motor, type, variable, torque, constant, speed, essentially. 3. drive, machineshop, such, load, find.
36. /4 мин./Закончите следующие предложения:
1. Series motors have the field coils connected in series ... 2. Speed of series motors varies widely ... 3. Series motors are commonly-used to drive ... 4. A dc generator may be made to function as a motor by...'.
37. /10 мин./ а) Найдите в основном тексте фрагмент, в котором говорится о двигателе умешанного возбуждения, б) Изложите по-английски в письменной форме соображения, которыми следует руководствоваться при выборе двигателей этого типа для работы в судовых условиях.
,38. / 60 мин./ Внимательно прочтете следующий текст, озаглавьте каждый абзац и мысленно опешите принцип работы машины постоянного тока.
Operating Principle of Direct Current Machines
1. In a dc machine, as a matter of fact, the ac is flowing in the armature winding. However, in dc generators this current is rectified by the commutator & is supplied to the line. In dc motors the commutator perform the opposite conversion.
2. To make clear A.C. to D.C. conversion in the commutator machine, consider first the operation of an elementary a.c. machine. Assume that it works as a generator. The magnetic system of the machine comprises two poles N & S, stationary in space. They create a magnetic flux of constant magnitude. In the space between the N-S poles an armature is inserted, with a turn of wire bn its periphery in the diametrical plane. Let us bring the armature into rotation with constant speed. Suppose that along the entire active length of a conductor, i.e. that part which cuts the lines of the magnetic field, the magnetic flux density is of constant value. If v is the rotation speed of the conductor in relation to the magnetic field then, according to Faraday’s taw of electromagnetic induction, the instantaneous emf induced in the conductor with armature in motion is determined by the formula: E— BLV
3. For giver) conditions /l=cdtist, v=const/, the cHange in emf induced in the conductor with time is completely defined by the magnetic flux density distribution under the pole. The distribution of magnetic flux density /В/ under the poles of a workable machine is of a complex nature. But using the Fourier method of analysis & after expanding the periodic functions into a series of harmonic components it is possible to separate the first of fundamental harmonic. It may then be assumed that the magnetic flux density under the N & S poles is Sine-wave distributed. In this case the emf induced in the conductor varies with time also as a Sine-wave function. The direction of the induced emf may be easily determined by the so-called right-hand rule.
