Mod 6 DC motor load and back emf (1 Viewer)

[NexusRich]

Using the diagram below, could anyone please explain why the current of the DC motor is very high and reaches its peak at the start ? Also, as it rotates faster, the back emf increases, so the net emf (voltage) decreases, but why the speed and the current of the motor eventually becomes steady (the graph flattens out). The textbook mentioned 'load torque' and 'motor torque' , but I could not understand these terms and what they have to do with the speed of the motor.

 

[fnsndgfg]

Back EMF is proportional to the change in flux that the rotor experiences as seen in the formula emf=change in flux/change in time. The rotor starts spinning from rest, this means the change in magnetic flux through the rotor is relatively low as the rotor needs time to reach full speed meaning low back emf and high current required. As the motor turns faster and faster, the back emf grows, always opposing the driving emf, and reduces the voltage across the coil and the amount of current. This explains why current peaks at the start as it takes time for the back emf to build and oppose the current.

As for the speed and current flattening out, you need to understand that a motor cannot endlessly increase in speed as it would break the law of conservation of energy. Once the motor has reached its maximum speed, the back emf created will become constant as the change of flux is constant. This means the current required is constant.

I'm only in year 12 now so if someone could check this that would be good. As for the terms I'm not really sure of their specific meaning.

 

[notme123]

itd be good if there were associated formulas that made direct connections between load torque, motor speed, back emf etc. The faradys law equation isnt sufficient imo

 

[fnsndgfg]

itd be good if there were associated formulas that made direct connections between load torque, motor speed, back emf etc. The faradys law equation isnt sufficient imo

Yes, I completely agree. I.e for back emf should I have stated voltage required = emf supplied - back emf? What other specific formulas should I have included? Maybe I could have included the magnetic flux formula then go to faraday's law?

 

[notme123]

Yes, I completely agree. I.e for back emf should I have stated voltage required = emf supplied - back emf? What other specific formulas should I have included? Maybe I could have included the magnetic flux formula then go to faraday's law?

ye there's that one but I mean like a direct one that links motor speed to everything like speed = 1/back emf or something if something like that ever exists. I think nesa just kinda just assumes us to know that torque of the motor declines with motor speed. but then again at equilibrium speed, there isn't any net force on the coils so I guess you can argue that there's no torque cuz no force.