1 Answer

Answer :

Back emf: When the armature of DC motor rotates under the fixed magnetic field and cuts it, by Faraday's law an emf is induced in them. The induced emf acts in opposite direction to the applied voltage as per Lenz’s law. Hence known as back or counter emf Eb.

The significance of back emf: 

i) If the motor is at standstill or rest Eb is zero. This causes large current flow through armature, which produce high starting torque. ii) When motor takes speed, the back emf increases, causes armature current to decrease hence decrease in torque. iii)It follows therefore that back emf in DC motor regulates the flow of armature current i.e. it automatically changes the armature current to meet load requirements. 

Related questions

Description : Why back emf used for a dc motor? highlight its significance.

Last Answer : The induced emf developed when the rotating conductors of the armature between the poles of magnet, in a DC motor, cut the magnetic flux, opposes the current flowing through the conductor, when the ... speed of rotation of the armature conductors. In starting, the value of back emf is zero.

Description : A single-phase half-controlled rectifier is driving a separately excited dc motor. The dc motor has a back emf constant of 0.5 V/rpm. The armature current is 5 A without any ripple. The armature resistance is 2 Ohm. The ... of the motor will be  (A) 339 rpm (B) 359 rpm (C) 366 rpm (D) 386 rpm

Last Answer : 359

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Last Answer : Answer: A

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Last Answer : Answer: B

Description : What causes counter emf in a dc motor

Last Answer : Generator action.

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Last Answer : Ans: D Given N = 1500 rpm, Φ = 0.4 mWb, Z = 1000, P = 4, & A= 4 Therefore, Eb = NΦPZ / 60 A = 1500 x 0.4 x 4 x 1000 x 10^-3 / 60 x 4 = 60/6 = 10 volts

Description : Explain concept of back emf in DC machine. State how it governs armature current. 

Last Answer : Back emf: When the armature of DC machine rotates under the influence of driving torque, the armature conductors move in the magnetic field and cut it and hence emf is induced in them. The ... flow of armature current i.e. it automatically changes the armature current to meet load requirements.

Description : When a motor is just started, back emf is almost ______: a. Infinite b. zero c. minimum d. maximum

Last Answer : d. maximum

Description : If the motor is overloaded, then the magnitude of ‘back emf’” a. Increases b. Decreases c. Constant d. Becomes zero

Last Answer : Increases

Description : The back emf of synchronous motor depends on?

Last Answer : The back emf of synchronous motor depends on load angle.

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Last Answer : Answer: B

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Last Answer : A = number of parallel paths. That is for lap winding it is equal to Z and for wave winding it equal to 2. 

Description : The induced emf in the armature of a lap -wound four -pole dc machine having 100 armature conductors rotating at 600 rpm and with 1 Wb flux per pole is  (1) 1000V (2) 100V (3) 600V (4) 10,000V

Last Answer : The induced emf in the armature of a lap -wound four -pole dc machine having 100 armature conductors rotating at 600 rpm and with 1 Wb flux per pole is 1000V

Description : What is back emf? 

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Description : What is the back EMF?

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Last Answer : Answer: B

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Last Answer : Answer: A

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Last Answer : Speed.

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Last Answer : Answer: B

Description : A cumulative dc compound motor runs at 1500 rpm. on full load. If its series field is short-circuited, its speed  (1) becomes zero (2) remains unchanged (3) increases (4) decreases

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Description : Application of DC Series Motor

Last Answer : 1. Electric traction 2. Cranes, 3. Passenger elevators, 4. Continuous conveyors, 5. Grinders, 6. Polishers,

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Description : State any two applications of brushless DC motor. 

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Description : List the various losses on a DC Motor. 

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Description : A separately excited dc motor operating in first quadrant is fed from 3-phase semi -converter free -wheeling diode is open circuited, then the motor :  (A) can be operated in first quadrant (B) ... in first & second quadrants (C) can be operated in first & fourth quadrants (D) cannot be operated

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Description : In a differentially compounded dc motor (A) the speed is constant as the torque increases (B) the speed falls non-linearly as the torque increases (C) the speed increases non-linearly as the torque increases (D) the flux increases as the load increases

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Last Answer : principle

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Description : Self induced emf and Mutually induced emf

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