# Electric generators & counter EMF (Back EMF)

##### Intros
###### Lessons
1. Introduction to motors and electric generators
2. ac generator
3. dc generator
4. voltage Vs. Time graph ( ac generator Vs. dc generator)
5. Counter emf (Back emf )
##### Examples
###### Lessons
1. A motor is connected to a 12V dc supply and draws 5.0A when it first starts up. What will be the back emf when the motor is operating at full speed and drawing 1.2 A?
1. 7.0 A
2. 7.8 A
3. 9.1 A
4. 10.8 V
1. A motor has an armature resistance of 3.5$\Omega$ and is connected to a 12.0V source. At full speed the current through the armature is 0.18A. what is the back emf at full speed?
1. 0 V
2. 0.63 V
3. 11.4 V
4. 12.0 V
1. A dc motor is connected to a 12.0V power supply. When the armature is rotating, the current through it is 0.78A and the back emf is 10.6 V. What is the resistance of the armature?
1. 1.4 $\Omega$
2. 1.8 $\Omega$
3. 14 $\Omega$
4. 15 $\Omega$
1. The load on an electric motor is gradually increased. Which of the following quantities decreases? (Input voltage remains constant.)
1. current
2. back emf
3. armature resistance
4. heat produced by armature
1. The graph below shows the emf produced by an ac generator Vs. time. At which point in time is the rate of flux change in the generator the greatest?

1. a
2. b
3. c
4. d