Back EMF (1 Viewer)

[boz]

What is the go with back EMF. You see questions like a resistor is put in series with an electric motor to stop it from burning out?

This is the only topic im fuzzy on and i got no idea.
Anyone out there with some answers.

 

[utopia_parkway]

when an motor is running there is back emf becuase of the relative movement between the coils and the magnetic field. this means that the voltage running throught the motor when it is turning is =supply emf - back emf.

when the motor is fist turned on becuaes the rotor is not moving there is not back emf becuase there is no relative moment, this means that all the the emf is just the supply emf, thus the emf will be alot bigger then when it is moving, so to stop it burning out a resister has to be put in place to stop the large current that will flow though the coil when it is furst turned on due to the absence o back emf.


hope that helps

 

[helper]

when an motor is running there is back emf becuase of the relative movement between the coils and the magnetic field. this means that the voltage running throught the motor when it is turning is =supply emf - back emf.

when the motor is fist turned on becuaes the rotor is not moving there is not back emf becuase there is no relative moment, this means that all the the emf is just the supply emf, thus the emf will be alot bigger then when it is moving, so to stop it burning out a resister has to be put in place to stop the large current that will flow though the coil when it is furst turned on due to the absence o back emf.


hope that helps

Be careful in your wording. Voltage doesn't run. The voltage across the motor or potential difference across the motor would be what they are looking for.

Also talk about why it burns out. A larger current woud be flowing, leading to a higher energy conversion to heat.

You know what your talking about.

 

[boz]

thanks thats top stuff, textbooks are crap.

ah thats why when the supply EMF = the back emf the motor stops accelerating and reaches an equilibrium got it

 

[wind]

Back emf is the emf induced in the coils of a motor as they spin in the magnetic field of the stator.

By Lenz's Law, the direction of that induced emf opposes the emf causing the motion of the armature. The current generated in the motor is an eddy current. The direction of the motor eddy current is such that it opposes the supply emf that produces the motion in the motor. The net emf applied to the coils equals the supply emf minus the back emf.

As the coil rotates faster, the back emf increases and the difference between the constant supplied emf and the back emf gets smaller. Clearly, this difference between the two emfs is equal to the potential difference across the motor coil and hence determines the actual current in the coil.

(This part's about the burning out bit)

When the motor is first turned on and the coil begins to rotate, the back emf is very small, since the rate of cutting flux is small. This means that the current passing through the the coil in the forward direction is very large and could possibly burn out the motor.

To ensure that this doesnt not happen, adjustable starting resistors in series with the motor are used, especially with large motors.

Once the motor has reached its normal speed, these starting resistors can be switched off, since by then the back emf has reached a maximum and has therefby minimised the current in the coil.

Also, if the load on the motor has increased, the motor will slow down reducing the back emf and allowing a larger current to flow in the coil. Since torque is proportional to current, an automatic increase in torque will follow an increase in load on the motor, possibly burning it out.

 

[speersy]

can we really consider the emf as eddy currents. I thought eddy currents were small oscillating circular currents.

 

[helper]

EMF is a potential drop. Eddy currents area flow of electrons.