Motor rotating at normal speed and then prevented from rotating

[Guillem_dlc]

Homework Statement

A generator of electromotive force $100\, \textrm{V}$ and internal resistance $1\, \Omega$ is connected in series to a motor. A voltmeter is connected between the terminals of the generator. When the motor is rotating at normal speed the voltmeter reads $95\, \textrm{V}$ and when the motor is prevented from rotating it reads $85\, \textrm{V}$. Calculate:
a) the resistance of the motor (R: $5,67\, \Omega$).
b) the counter-electromotive force of the motor (R: $66,7\, \textrm{V}$).
c) the useful power of the motor (R: $334\, \textrm{W}$).

Relevant Equations

Ohm's law

The current of the circuit would be:

$$I=\dfrac{\varepsilon - \varepsilon'}{r+r'}=\dfrac{100-\varepsilon'}{1+r'}$$

But I do not understand the two methods of operation of the engine that the statement talks about: it rotates in normal regime and the engine is prevented from turning. What conclusions could I draw from this?

 

[Steve4Physics]

When the motor is prevented from rotating, it acts as a simple resistor.

When the motor is rotating, it acts as a simple resistor and an emf-source in series.

You might find it useful to draw the two circuits.

 

[Guillem_dlc]

When the motor is prevented from rotating, it acts as a simple resistor.

When the motor is rotating, it acts as a simple resistor and an emf-source in series.

You might find it useful to draw the two circuits.

Thank you very much! With this hint I was able to do the exercise and I got what it says.