Hi,
I am trying to understand what effect the induced EMF in a DC and AC motor has on rotation.
As an example, here is what I've understood about DC motor operation: If I assume a single loop of wire (equipped with commutator, etc.) and it is in between the N and S pole of two permanent magnets, and the ends of the wire are hooked up to a DC battery, I understand that the current through the wire and the magnetic field due to the magnets will result in a force on the wire and the loop will spin.
Once the loop is spinning though is where I get confused. So now, the flux through the loop is changing, and there is an induced emf in the loop. Based on Faraday's law:
∫E*dl = -V + IR = -Nd(phi)/dt
So the back-emf is equal to the battery voltage less the voltage drop due to the resistance fo the wire. If the resistance is very small, then the back-emf nearly equals the battery voltage, and the result would be very little current flowing in the cirucit, and thus very little torque. How does the motor get away with this?
Also, how can the rotor turn smoothly if it is constantly challenged by the back-emf?
Finally, in an AC motor, where the current is varying sinusoidally, how is the force on the rotor constant?
Thanks in advance for your help!
I am trying to understand what effect the induced EMF in a DC and AC motor has on rotation.
As an example, here is what I've understood about DC motor operation: If I assume a single loop of wire (equipped with commutator, etc.) and it is in between the N and S pole of two permanent magnets, and the ends of the wire are hooked up to a DC battery, I understand that the current through the wire and the magnetic field due to the magnets will result in a force on the wire and the loop will spin.
Once the loop is spinning though is where I get confused. So now, the flux through the loop is changing, and there is an induced emf in the loop. Based on Faraday's law:
∫E*dl = -V + IR = -Nd(phi)/dt
So the back-emf is equal to the battery voltage less the voltage drop due to the resistance fo the wire. If the resistance is very small, then the back-emf nearly equals the battery voltage, and the result would be very little current flowing in the cirucit, and thus very little torque. How does the motor get away with this?
Also, how can the rotor turn smoothly if it is constantly challenged by the back-emf?
Finally, in an AC motor, where the current is varying sinusoidally, how is the force on the rotor constant?
Thanks in advance for your help!