POLYPHASE SYNCHRONOUS MOTOR

  1. Compare the structure and function of a generator to an electric motor
  2. CONSTRUCTIONAL FEATURES OF SYNCHRONOUS MACHINES.
  3. Speak about constructional features of synchronous machines.
  4. Synchronous-motor-rotor pole
  5. The market leader in the motor car (computer) industry in our country, its market share and main competitors.

Since a dc shunt motor is electrically and magnetically identical with ashunt generator, the shunt dynamo may be operated either as a motor or a generator, with only minor changes in the voltage ratings. In the same sense we should expect that the alternator may also be operated as an ac motor when electrical rather than mechanical energy is supplied to it. If the mechanical power supplied to arotating alternator is removed while the dc field remains energized, and an ac supply is then connected across the armature terminals, torque will be developed and the alternator will continue to rotate at a speed determined by the frequency of the acsupply . Changes in mechanical load within the machine's rating will not cause achange in speed, and we thus have a synchronous motor.

Construction.Essentially, the construction of a polyphase synchronous motor is the same as that of a synchronous alternator. In large machines, the armature is on the stator and is wound for the same number of poles as the rotor. Some smaller polyphase motors have rotating armatures, but this is rare. As with an alternator, a dc generator may be mounted on the motor shaft to excite the field, although synchronous motors, both single and polyphase, which require no dc excitation, are also manufactured today. In the most common type of field construction, the poles are somewhat different from those of the alternator. In addition to the exciting winding, a synchronous motor often has a squirrel-cage winding in the rotor. The slots of this winding are in the pole faces and are parallel to the shaft. The ends of the copper bars embedded in the slots are short-circuited in the same manner as in a squirrel-cage induction motor. This short-circuited winding on the rotor is known as the cage winding, the damper winding, the amortisseur winding, or sometimes, to describe its function, the starting winding.

Since the armature is wound in the same manner as that for an alternator, it is, of course, identical with the stator of a polyphase induction motor as well. With three-phase voltage applied to the armature, a rotating field of constant magnitude is therefore produced. The speed of the field, which is synchronous speed, depends upon the frequency of the supply voltage and the number of poles for which the armature is wound. A typical polyphase-synchronous-motor-rotor is shown in Fig 1.




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