The casiest way to observe the behavior of the motor is with a middle ground mea

Question

The casiest way to observe the behavior of the motor is with a middle ground measurement between a 1 Ohm resistor and the motor in series. This will provide the following graph on the oscilloscope screen, though it may take some effort to find the correct time scale. The percent of time that the motor is on is simply (t_1 + t_2)/T. Using the run/stop button to collect data from the oscilloscope is useful for this because the triggering will be very sensitive and the will continuously lose the signal. Because the DC power supply driving the motor is grounded, you can't use a middle ground oscilloscope configuration: In the previous picture, why is the voltage across the motor equal to the source voltage when the motor is off? You may want to explain with circuit diagrams.

Explanation / Answer

As the motors are capacitor start motors, they need to fully charge a capacitor which then provides charge in the form of high current momentarily to kick start the motor when required, that is when the p[ower switchis pressed.

so, in the graph we see that potential difference across the motor is +ve when the motor is switched off and it drops down when it is started. this happens
becuase everytime we switch off the motor, there is residual charge on the capacitor. this charge takes time to dissipate and hence we see that the DC mototr potential differnec in not zero even when it is switched off.

this voltage drops down when the capacitor is forcibly being charged or discharged by the current in the circuit from the battery

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