Electrical Machines - short answer questions from AMIE exams (Winter 2019)

Answer all the questions (2 x 10)

Explain what happens when a d.c. motor is connected across an a.c. supply?

If a DC series motor is connected to a ac supply same current will flow through the series field and armature field after being rectified by the commutator. So, the machine will run but with high sparking at brush, poor pf, low speed and low efficiency.

What is Scott connection in a transformer?

In this connection, single-phase transformers having 50 percent and 86.6 percent taps on the three-phase side are required. One transformer, called the MAIN transformer is connected between two of the three-phase wires; the other, called the TEASER is connected between the third phase wire and its 86.6 percent tap is connected to the 50 percent tap of the MAIN transformer.

What are the advantages of having rotating field system?

  • It is always better to protect high voltage winding from centrifugal forces caused due to the rotation. So high voltage armature is generally keep stationary.
  • It is easier to collect large currents at very high voltages from a stationary members.
  • The problem of sparking at the slip rings can be avoided by keeping field rotating and armature stationary.
  • As we know that the emf get induced in the armature so for stationary armature large space can be provided to accommodate large numbers of conductor and the insulation.
  • Rotating field make overall construction very simple.
  • The ventillation arrangement for high voltage side can be improved if it is kept stationary.

Why MMF method is called optimistic method?

All voltage drops are treated as mmf drops. Effect of saturation is not considered and MMF method gives result less than actual: hence it is called optimistic method.

How does change in excitation affect the synchronous motor power factor?

The power factor of the motor is therefore changed from lagging to leading. It is thus seen that if the synchronous motor is overexcited, it takes leading current. This property of synchronous motor is extremely useful as it enables to run the machine as power factor corrector or a phase advancer.

How the speed of an induction motor can be controlled by changing the Applied Frequency?

The synchronous speed of an induction motor is given by N, = 120 flP. Clearly, the synchronous speed (and hence the running speed) of an induction motor can be changed by changing the supply frequency "f".

How does the slip vary with load?

The greater the load, greater is the slip or slower is the rotor speed. Whsit modifiriitiont would nwvtcnrv if motor ic reonirrd t

What factors determine the direction of rotation of the motor?

The phase sequence of the supply mains and the order in which the supply lines are connected to the stator winding determine the direction of rotation of an induction motor.

What is a universal motor?

  • A universal motor is a special type of motor which is designed to run on either DC or single phase AC supply. These motors are generally series wound (armature and field winding are in series), and hence produce high starting torque.
  • Universal motors find their use in various home appliances like vacuum cleaners, drink and food mixers, domestic sewing machine etc.
  • The higher rating universal motors are used in portable drills, blenders etc.

In which direction does a shaded-pole motor run?

It turns from the unshaded to the shaded pole.

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