Back Emf (1 Viewer)

[mtsmahia]

Hi guys,

I was reading some information. It was taking about how the current, emf and back emf varies as the DC motor is in different stages; ie - just statred, normal operating speed, and overload.
It stresses the fact that "if the motor is overloaded, it rotates too slowly. This reduces the back EMF to such a low point that the net EMF becomes large enough to produce high enough current to burn out thin wires and destroy the motor".

My question is, why does the back emf reduce if the motor is overloaded?

thanks!

 

[study-freak]

Hi guys,

I was reading some information. It was taking about how the current, emf and back emf varies as the DC motor is in different stages; ie - just statred, normal operating speed, and overload.
It stresses the fact that "if the motor is overloaded, it rotates too slowly. This reduces the back EMF to such a low point that the net EMF becomes large enough to produce high enough current to burn out thin wires and destroy the motor".

My question is, why does the back emf reduce if the motor is overloaded?

thanks!

Back emf is the induced emf in a motor due to Faraday's law in action in the rotor coil that is moving in an external magnetic field (stator magnet's field).
Hence when the rotor is overloaded, it slows down, meaning that the net relative motion between the conductor (rotor) and the magnetic field decreases.
Consequently, induced or back emf decreases.
Note that back emf=-n d(phi)/dt (d=delta)

 

[mtsmahia]

Back emf is the induced emf in a motor due to Faraday's law in action in the rotor coil that is moving in an external magnetic field (stator magnet's field).
Hence when the rotor is overloaded, it slows down, meaning that the net relative motion between the conductor (rotor) and the magnetic field decreases.
Consequently, induced or back emf decreases.
Note that back emf=-n d(phi)/dt (d=delta)

Right. But can u please explain why does the rotor slows down when it is overloaded? Is it something to do with the counter torque?

 

[study-freak]

Right. But can u please explain why does the rotor slows down when it is overloaded? Is it something to do with the counter torque?

Yep, when it is overloaded, there is a counter torque as you said.
So it would be Net torque=torque due to current-counter torque due to load

 

[mtsmahia]

Yep, when it is overloaded, there is a counter torque as you said.
So it would be Net torque=torque due to current-counter torque due to load

so for the motor to be overloaded, the counter torque would be greater than the motor torque ?

 

[study-freak]

so for the motor to be overloaded, the counter torque would be greater than the motor torque ?

not necessarily.
It depends on what you mean by 'overload.'
If it's to a degree that motor burns down lol, it wouldn't need to be like that.
Just to the degree that current through the rotor is large enough (due to reduced back emf) to cause heat damage.

 

[mtsmahia]

not necessarily.
It depends on what you mean by 'overload.'
If it's to a degree that motor burns down lol, it wouldn't need to be like that.
Just to the degree that current through the rotor is large enough (due to reduced back emf) to cause heat damage.

okay!! Thanks heaps!!

btw, best of luck for your HSC results !!

 

[study-freak]

okay!! Thanks heaps!!

btw, best of luck for your HSC results !!

No worries and thanks for the luck because I might need it!