I attached a d.c. motor to my scope as shown in the attached files.
I'd like to know the following:

Did I accurately demonstrate that the current and voltage are in phase? If so, can I ignore reactance, and take the r.m.s. voltage that is shown across the 1 ohm resistor, and call it the current for a power calculation?
Thanks again, Rich

 

Hi,

It's a DC motor. You only have phase realtionships in the case of AC motors, where the inductance of the motor coils causes the current waveform to lag the applied voltage waveform. To see something like that in a DC motor, you would have to rectify a low voltage transformer output, but not filter it before applying it to the motor, producing pulsating DC. If the motor was big enough, you might be able to see the current lagging the voltage on your o'scope.

 

The fluctuations in the voltage drop across the motor and the 1 ohm resistor are not the result of inductance?

 

Hi,

They might be the result of going from one commutator segment to the next. Motor EMF will vary as the armature field reacts against the coil's field, and as the commutator switches. Quality of the brush contact can also cause variance in motor current.