Motor Part-I

Q. Define electric current.

Ans: Electric current is defined as rate of flow of electric charge i=dq/dt amperes where q is the charge in coulombs. The unit of current is the amperes which is the current that flows when 1 coulomb of charge is transferred in one second.

Q. State ohms law.

Ans: When the temperature remain constant current flowing through a circuit is directly proportional to potential difference across the conductor. i.e V∞ I or V=IR, Where R is constant of proportionality.

Q. State Kirchoff’s laws.

Ans: Kirchoff’s current law: The sum of the currents flowing towards a junction is equal to the sum of the currents flowing away from it.

Kirchoff’s Voltage law: In a closed circuit the sum of the potential drops is equal to the sum of the potential rises.

Q.Define Form factor and Crest factor.

Ans: Form factor= RMS value/Average Value

Crest(peak) factor=Maximum Value/RMS value

Q.Define average value.

Ans: The average value of an alternating current is that value of steady direct current which transfers the same charge as the alternating current flowing for the same time.

Q.Define RMS value.

Ans: The effective value of an alternating current is that value of steady, direct current  which produces the same heat as that produced by the alternating current when passed which produces the same heat as that produced by the alternating current when passed through the same resistance for the same interval of time.

Q.  Write down the equation for frequency of emf induced in an Alternator.

Ans: Frequency  f is expressed in cycles per second or Hz, is given by the following equation

F = (PN)/120 Hz, Where P- Number of poles and N-Speed in rpm

Q.How are alternators classified?

Ans: According to type of field system

• Stationary field system type
• Rotating field system type

 Q.Name the types of Alternator based on their rotor construction.

Ans: Alternators can be classified into the following two types according to its rotor construction:

• Smooth cylindrical type alternator
• Salient pole alternator

Q. Why is the stator core of Alternator laminated?

Ans: The stator core of Alternator is laminated to reduce eddy current loss.

Q.State the principle of 3 phase Induction Motor?

Ans: While starting, rotor conductors are stationary and they cut the revolving magnetic field and so an emf is induced in them by electromagnetic induction. This induced emf produces a current as the circuit is closed. This current opposes the cause by Lenz’s law and hence the rotor starts revolving in the same direction as that of the revolving magnetic field.

Q. Induction motor can run at synchronous speed? Explain.

Ans: No, if the speed of induction motor is Ns then the relative speed between the rotating flux and the rotor will be zero and so no torque is produced.

Q.Define power factor.

Ans: The power factor is the cosine of the phase angle between voltage and current. In other words PF is the ratio of  Real Power/ Apparent Power.

Q.What is a prime mover?

Ans: The basic source of mechanical power, which drives the armature of the generator, is called prime mover.

Q. How is the direction of induced emf determined?

Ans: Direction of induced emf is determined by using :

(i)  Right Hand Rule.

(ii)  Cork Screw Rule.

Q. State Lenz’s law?

Ans: Any induced emf will circulate a current in such a direction as to oppose the cause which producing it.

e = -N df/dt