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What will happen when I change the excitation in synchronous motor or generator?

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What will happen when I change the excitation in synchronous motor or generator?

Synchronous Motor or Generator can run in both form.

  1. Over excitation or
  2. Under excitation.

When run as alternator alone:

Over and Under excitation will increase or decrease the terminal voltage.

When run as alternator connected with the grid or infinite BUS

  1. Over excitation will deliver additional inductive VAR to the grid and and will operate at lagging power factor
  2. Under excitation will absorb reactive power from the grid and will operate at leading power factor.

When run as Synchronous Motor and connected with the system:

  1. Over excitation will deliver additional leading VAR to the system and and will operate at leading power factor
  2. Under excitation will operate at lagging power factor and will increase inductive VAR of the system.

Source: quora.com

Why air gap length is small in induction motor than synchronous motor?

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Why air gap length is small in induction motor than synchronous motor?

An Induction Motors (IM) works on the principle of Electromagnetic Induction. The induced EMF in the rotor winding or rotor bars is due to mutually induction caused from the rotating magnetic field produced be the stator winding. Thus, an IM can be treated as a Rotating Transformer as the EMF induced in the rotor is by Mutual Induction. If the air gap is more the leakage flux will be more and the mutual flux gets reduced, reducing rotor EMF, current and torque. Also, power factor of motor will decrease if air gap is more due to increase in leakage current and noise also increases.

But, in a Synchronous Motor (SM) the Magnetic Flux is set up separately by the field winding.The EMF induced in the stator armature winding is not by mutual induction, but it’s a dynamically induced EMF due to relative motion between the field and conductors. Hence, air gap is not the consideration, particularly for salient pole machines, in the region between the poles. the air gap is much more. Thus, air gap is not a governing factor of machine’s power factor.

Hence air gap length should be smaller in IM but can be larger in a SM.

Source: quora.com

Why is the efficiency of the synchronous motor greater than the induction motor?

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Why is the efficiency of the synchronous motor greater than the induction motor?

Unlike induction motor,synchronous machines run on synchronous speed. Not only that, but these machines are able to maintain constant speed throughout the operation irrespective of load. As speed is synchronous, there is no difference of speed between air gap mmf and rotor which makes it efficient over an induction motor whose speed varies as per load condition and slip.

Source: quora.com

Do you know how to measure the earth resistance?

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Do you know how to measure the earth resistance?

Actually the Earth resistance is measured using “earth
resistance meter”. You might think about megger is correct answer but megger is used only for high resistance
measurement i.e. in Mega-ohms but when considering upon
earth resistance it should not exceed 1ohm so earth
resistance meter is required.

In this meter, there will be four terminals(E, P1, P2, P3)
in which E and P1 will be shorted and connected to the grid
or electrode and P2 and P3 are reference electrodes which
will be kept on the earth surface on two different
locations for taking the reference resistances of the earth.

The meter shows the resistance of the earth. But it is
necessary that it should be below 1 ohm.

Source: allinterview.com

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