Why you need back emf in dc motor?

Answer 1

A back emf is DC machines ability to protect itself from over speed. From Farads' Law you can recall that emf is induced into a conductor immersed in a magnetic field.

In dc machines there is a magnetic field flow from one pole to another. As the conductor cuts through this magnetic field emf is induced into it. The amount of emf induced into the conductor depends not only at the strength of the magnetic field but also the rotational speed at which the conductor cuts through magnetic field.

Hence higher the motor speed, higher the back emf.

The back emf can be described by the following equation:

E = (Flux * angular speed of motor * number of poles in machine * (N/a)) / pie

Where N/a = series turns in the armature winding.

Answer 2

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I am an engineering student and during the past few days i was searching on the net to clarify my doubts on "motors".So i came across an article and on reading one part of it a doubt arosed in my mind.

Q1) "To take a practical example, an electric motor running under no load uses very little power. If the motor were frictionless and superconducting, it would use no power. This is because the back emf opposes the imposed voltage." This was the part of the article.My doubt is that If the motor was frictionless and superconducting will it rotate and produce mechanical energy??

Q2)Can you tell me how the electrical work done by the applied voltage in overcoming and causing current flow against back emf is possible?? Practically i know that when someone is pushing me in one direction in order for me to oppose it i need to give an equal and opposite force.That is a push is required to oppose a push.. So using this idea in motors our applied voltage must do an equal amount of work against this back emf.Hence our applied voltage must provide an equal amount of electrical energy to oppose the electrical energy produced by back emf..So if this was the condition can you tell me how this back emf aids in converting electrical energy to mechanical energy???

Q3)What will happen if there was no back emf??

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A1) if the motor is frictionless and superconducting, it will rotate with the generated voltage equal to the supply voltage, with zero current flowing, in a steady state, with a fixed amount of kinetic energy. However, to reach that state it has to run up from stationary, and during that time it will absorb energy from the supply (a frictionless motor with no resistance will do this in zero time, taking an infinite current until it reaches the steady state).

The generated voltage or back emf is proportional to speed, and the current is equal to the supply voltage minues the generated voltage, divided by the armature resistance. The angular acceleration is proportional to the torque, and the torque is proportional to the current. That gives you all the information you need to work out what is going on.

A2) The supply is providing power by raising electric charge through a potential gradient, and the charge then flows down the potential gradient in the load, doing work (or generating heat) as it does so.

Imagine you are sitting on a trolley and someone is pushing you, and ignore friction. Force equals mass times acceleration, so your acceleration is equal to the force divided by the total mass of you plus the trolley. At the point of contact you are pushing back on the other person, but this force is balanced by your acceleration. The work done by the pusher, force times distance, equals the kinetic energy of you and the trolley 0.5 times mass time speed-squared. The impulse provided by the pusher, force times time, equals the momentum of you and the trolley, mass times speed.

A3) You can ensure there is no back-emf by locking the armature of the motor (put a screwdriver in to stop it rotating). In this condition the current equals the supply voltage diided by the armature resistance.

Answer 3

Depending on the context, this could refer to several specific things. When electricity flows through a conductor (conductor A), it creates magnetic fields, which then create electric fields, around it and propogating away from it. If another conductor is nearby (conductor B), the electric field produced by the current flowing in conductor A will induce a current in conductor B. The current in conductor B will produce an electric field that is in the opposite direction of the electric field that produced the current in conductor B, which will attempt to cause current to flow in the opposite direction of current flow in conductor A. This is the "back EMF", or back electromagnetic force.

Answer 4

An electrical motor is powered by electromagnets, when the motor is switched off, for a short time the rotating component may continue to spin this spinning induces a voltage in the circuit, kind of like a generator, I think this is what emf is.

Answer 5

back emf is required in dc machine because of back emf, dc motor become a regulating machine i.e motor adjust itself to draw the armature current just enough to satisfy the load demand .

Answer

You don't 'need' it; it's there whether you need it or not!

Answer 6

A DC motor is also a generator at the same time and the voltage it would produce under rotation when it's disconnected from the supply is the back emf, also called counter emf because it's in opposite polarity to the supply voltage and tends to cause outward current flow -- from machine to supply.

The significance is that it determines the operating state of a DC machine. A DC machine working as a motor will have its counter emf always less than the supply voltage. The difference would depend on amount of mechanical load tied to the motor. As a result, current would flow into the machine. When it's working as a generator the counter emf is more than supply voltage (0) and as a resullt current flows out of the machine. Similarly during regerative braking the counter emf is more than supply voltage and as a result current flows out of the machine.

Answer 7

Yes, a current carrying object rotating in a magnetic field causes back EMF. It causes the motor to also act a generator in a sense.