# What is the back EMF?

What is the back EMF?

When the armature of a d.c. motor rotates under the influence of the driving torque, the armature conductors move through the magnetic field and hence e.m.f. is induced in them as in a generator. The induced e.m.f.acts in opposite direction to the applied voltage V (Lenz's law) and in known as backor counter e.m.f.(Eb)

The back emf Eb(= PΦZN/60 A) is always less than the applied voltage V, although this difference is small when the motor is running under normal conditions.

Back emf in dc motor

A shunt wound dc motor shown in figure below. When dc voltage V is applied across the motor terminals, the field magnets are excited and armature conductors are supplied with current. Therefore, driving torque acts on the armature which begins to rotate. As the armature rotates, back emf Eb is induced which opposes the applied voltage V.

The applied voltage V has to force current through the armature against the back emfEb. The electric work done in overcoming and causing the current to flow against Eb is converted into mechanical energy developed in the armature. It follows, therefore, that energy conversion in a d.c. motor is only possible due to the production of back emf.

## Related questions

Description : What is back emf?

Answer : Back EMF: When a current carrying conductor is placed in a magnetic field it experiences a force; when it moves due to the force it (conductor) cuts the magnetic field due to which an emf is ... (hence the applied armature voltage) is opposed by the induced emf which is therefore called as Back emf.

Description : What is back EMF of synchronous motor?

Description : What is back EMF in inductor?

Description : What is the back EMF in Transformers?

Description : What is the back EMF of a dc motor?

Description : What is the back EMF of the motor?

Description : Do AC induction motors have back EMF?

Description : The back emf of synchronous motor depends on?

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

Answer : The armature may burn if the back e.m.f. of a D.C. motor vanishes suddenly.

Answer : The mechanical power developed by a shunt motor will be maximum when the ratio of back e.m.f. to applied voltage is 0.5.

Answer : In case of D.C. shunt motors the speed is dependent on back e.m.f. only because flux is practically constant in D:C. shunt motors.

Description : What is Phase sequence? Draw waveforms of 3 phase emf.

Answer : Phase sequence: It is the order in which the three phases reach their peak or maximum values. It is shown in below fig. the phase sequence is A-B-C (or R-Y-B). In the following waveforms, it is ... 120º, B- phase voltage attains its positive maximum value. So the phase sequence is R-Y-B.

Description : Self induced emf and Mutually induced emf

Answer : (i) Self-induced e.m.f.: The e.m.f. induced in a coil due to the change of its own flux linked with it, is called self-induced e.m.f.  (ii) Mutually induced e.m.f.: The e.m.f. induced in a coil due to the changing current in the neighbouring coil, is called mutually induced e.m.f.

Description : Compare statically induced emf with dynamically induced emf on any four points.

Answer : Statically induced emf Dynamically induced emf Emf is induced without any relative motion between conductor and magnetic field. Emf is induced due to relative motion between conductor ... induced in transformer windings e.g emf induced in Generator, Alternator armature windings

Description : Explain the term self induced EMF with a neat diagram.

Description : Define: 1) statically induced EMF 2) dynamically induced EMF Write the names of equipments in which these two types of EMF get induced

Answer : Concept of statically induced emf: The emf induced in coil or conductors without any relative motion between it and magnetic field is called statically induced emf. When conductor is stationary and flux ... it. This emf is dynamically induced emf. e.g. Emf induced in generators, alternator.

Description : List the two types of induced EMF.

Answer : There are two types of EMF one is statically induced EMF and second is dynamically induced EMF.

Description : What is the EMF equation of the transformer?

Answer : E1 =2(3.142)fN1 (flux)1E2 =2(3.142)fN2(flux)2

Description : Symptoms of Back Pain Causes And Tips

Answer : Symptoms of Back Pain Causes And Tips Back pain is the most common complaint heard among the adults these days. It has been estimated that people do suffer from back pain at least once in their ... night sleep that will recover from all the bodily disorders and you wake up fresh the next morning.

Description : Explain dynamic and static induced emf with neat diagram.

Answer : Dynamically Induced EMF:-  In this case, either coil moves or magnet moves, so that there is relative motion between coil & magnetic field. Thus magnetic field is cut by coil & emf is induced in it. The emf ... The expression for induced EMF is e= -L(di/dt) or = -N(dϕ/dt).

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

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 : The fractional-pitch used for eliminating 7th harmonic from the emf wave of an alternator is (A) 2/3 (B) 5/6 (C) 7/8 (D) 6/7

Answer : The fractional-pitch used for eliminating 7th harmonic from the emf wave of an alternator is 6/7

Description : The counter e.m.f. of a d.c. motor?

Answer : The counter e.m.f. of a d.c. motor helps in energy conversion.

Answer : E.m.f. of a Weston cell is accurately measured by electrostatic voltmeter.

Answer : It is required to measure the true open circuit e.m.f. of a battery. The best device is D.C. potentiometer.

Answer : If B is the flux density, I the length of conductor and v the velocity of conductor, then induced e.m.f. is given by Blv.

Answer : When the armature of a D.C. motor rotates, e.m.f. induced is back e.m.f.

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 ... the flow of armature current i.e. it automatically changes the armature current to meet load requirements.

Description : If the back emf of a dc motor is doubled and its speed is also doubled, the torque developed in the machine will  (1) get doubled (2) become 4 times (3) get halved (4) remain unaltered

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

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 : A 200 V dc motor takes a field current of 2 A to generate a back emf of 180 V. If the field winding resistance is 80 Ω, the resistance of the shunt field regulator is (A) 10 Ω (B) 20 Ω (C) 100 Ω (D) 180 Ω

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

Description : What is called the EIectro-Motive Force (EMF) of a voltage source? (1) Terminal voltage when load is applied (2) Internal voltage when no load is applied (3) Product of internal resistance and load current (4) Electric pressure provided to the load

Answer : What is called the EIectro-Motive Force (EMF) of a voltage source? (1) Terminal voltage when load is applied (2) Internal voltage when no load is applied (3) Product of internal resistance and load current (4) Electric pressure provided to the load

Description : A battery is known to have an EMF of 5.0V but when a certain voltmeter is connected to it the reading is 4.9V. The battery can deliver a current of 0.40A when connected to a resistance of 12ohms. What is the resistance of the voltmeter?

Description : (a) Define permeability. (b) Define MMF. (c) Define form factor of an alternating quantity. (d) Define frequency and time period of an alternating quality. (e) Define transformation ratio of transformer. ... and state the meaning of each term used. (g) Give two applications of single phase motors.

Answer : Permeability : It is the ability of a magnetic/Non magnetic material to allow setting up of flux in it.  MMF : The magneto motive force (MMF) is the force required to setup flux in a magnetic / ... applications of single phase motors : Water pumps, ceiling fan, lathe machine

Description : Draw sinusoidal waveform of 3 phase emf and indicate the phase sequence.

Answer : Phase sequence is R-Y-B.

Description : Why the emf of a cell is always greater than its terminal voltage? Explain the same.

Answer : Every practical source offers some opposition to the current due to its internal parts or components. Such a resistance of internal parts of source is called internal resistance of source. When source delivers ... less than emf induced. So, emf of a cell is always greater than terminal voltage.

Description : An emf of 120 V causes a current of 3A to flow in a circuit how much energy is supplied by the source in 12 seconds. A. 4130 J B. 4310 J C. 4320 J D. None of these

Answer : An emf of 120 V causes a current of 3A to flow in a circuit how much energy is supplied by the source in 12 seconds.   4320 J

Description : If the supply frequency of a 3-phase induction motor is ‘f’, then frequency of rotor emf at motor slip ‘s’ will be   (a) sf (b) (1 – s)f (c) f/s (d) None of these

Description : When a conductor cuts magnetic flux, an emf is induced in the conductor. This is known as  (a) Joule's law (b) Faraday's law (c) Coulomb's law (d) Ampere’s law

Answer : When a conductor cuts magnetic flux, an emf is induced in the conductor. This is known as Faraday's law. There are two Faraday's law of electromagnetic induction. 1. Changing flux produces emf ... 2. Magnitude of induced e.m.f. is directly proportional to rate of change of flux linkage.

Description : Three equal resistors connected in series across a source of emf together dissipate 10 watts of power. What would be the power dissipated in the same resistors when they are connected in parallel across the same source of emf?

Answer : Three equal resistors connected in series across a source of emf together dissipate 10 watts of power. 90 watts would be the power dissipated in the same resistors when they are connected in parallel across the same source of emf.

Description : What is the back E.M.F in Transformer ?

Answer : Back emf in transformer is when apply voltage in primary side of transformer. This voltage produce flux according to Faraday law electro magnetic induction. This flux called mutual flux and this ... secondary flux oppos the applied voltage according to lenz law. its called back emf of transformer

Description :  If it takes a transmitted signal 1m sec to go up to the target and come back after reflection, how far is the transmitter from the target? A) 150 Km B) 150 m C) 15 Km D) 1500 m

Answer :  If it takes a transmitted signal 1m sec to go up to the target and come back after reflection, how far is the transmitter from the target? A) 150 Km B) 150 m C) 15 Km D) 1500 m

Description : A ______ Instruction at the end of interrupt service program takes the execution back to the interrupted program.   (a) Forward (b) Return (c) Data (d) Line

Answer : A Return Instruction at the end of interrupt service program takes the execution back to the interrupted program.

Description : In an isolated output dc-to-dc fly back converter, the high frequency transformer used works as an inductor (a) during turn -on period of the switching device (b) during turn off period of the switching device (c) during both (d) none of these

Description : The back e.m.f of a motor at the moment of starting : -  a) zero b) maximum c) minimum d) optimum

Answer : The back e.m.f of a motor at the moment of starting : - zero

Description : Explain statically induced EMF and Dynamically induced EMF with neat diagrams.

Answer : Statically Induced EMF:- When changing magnetic field links with a stationary coil an emf is induced. This emf is called statically induced emf. Statically induced EMF is of two types: Self-induced ... cuts the field and an emf is induced. This emf is called dynamically induced emf.

Description : Define EMF and state its unit.

Answer : Electromotive force(EMF): A potential difference generated by a source of electrical energy across its terminals which tends to produce an electric current in a circuit is called electromotive force. Unit of EMF is Volt

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