Electrical Machines

 

Fig. 1: Autotransformer (Construction & Symbol).

The normal transformer has separate primary and secondary windings. But the autotransformer is a special transformer in which a part of winding is common for the primary and secondary windings. The construction of an autotransformer is as shown in Fig. 1. It consists of only one winding wound on a laminated magnetic core, with a rotary movable contact. Thus from the autotransformer three terminals are brought out for connection. The autotransformer can operate as a step down or a step up transformer.

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Connection diagram of DC Series Motor

Fig. 1: DC Series Motor.

In the DC series motor, the armature and field windings are connected in series with each other as shown in Fig. 1. The resistance of the series field winding (R_s) is much smaller as compared to that of the armature resistance (R_a).

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A double cage rotor motor gives excellent starting and running characteristics than a deep-bar cage rotor motor. It also provides a high starting torque with a low starting current. Therefore it is also known as low-starting current, high starting torque, low slip-motor. The rotor is well designed in order to have high resistance rotor circuit during starting and low resistance rotor circuit when operating at running conditions. In case of double squirrel cage motor, the starting torque ranges between 200 to 250 percent of full-load torque whereas starting current ranges in between 400 to 600 percent of full-load current.

Double Cage squirrel cage induction motor with a specialty of different types of rotor having not only one cage but two different independent cages of different resistances and cross-sectional areas (Fig. 1).

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Single phasing means, one of the three phases is disconnected from the 3-phase supply so that the motor’s two phases are only remaining electrically connected. This happens due to the burning out of one fuse out of three fuses.

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This Block rotor test of Induction motor test is analogues to the short circuit test on transformer. This test gives information about copper loss of motor. The circuit diagram for this test is shown in Fig. 1.

Fig. 1: Blocked rotor test of Induction motor

The rotor is held stationary or rotor is blocked i.e. it is not allowed to rotate. The stator supply voltage is gradually increased till motor carries rated or full load current. The corresponding readings are noted down. The voltage required to circulate rated current is very less (merely 5 to 8% of rated value) so main flux is considerably less because flux is proportional to voltage. Hence the iron loss occurring is very less and can be neglected safely. The stator and hence rotor carries rated current so copper loss occurs. Since no useful output is extracted from motor (rotor is blocked), the entire power input is equal to copper losses. So wattmeter reading represents copper losses.

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High energy permanent magnets made up of rare earth or samarium-cobalt used for exciting the magnetic field of permanent magnet synchronous motors, enable them to be significantly smaller in size and weight. They have better efficiency because of the absence of a rotor winding and the small size of the rotor.

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In spite of the presence of torque ripples and higher acoustic noise, Switched Reluctance Motors are used in numerous applications because of their high starting torque, high torque to inertia ratio, high reliability, wide speed range, fast response, ruggedness, and low cost.

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In this topic, you study Synchronous Reluctance Motor Working, Construction, Diagram & Applications.

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