PM motors

[gar]

190411-2359 EDT

These questions are for those not sure of the answers.

Given two nearly identical DC motors with permanent magnet fields that are wired in parallel.

If one motor shaft is rotated, then what does the other motor do?


Given an air cylinder closed at both ends. The port on one end is connected to a pressure gage. The port on the other end is open to atmosphere. Move the piston to the open port end, and allow the pressure gage to equalize.

Quickly force the piston to the closed port end, and hold the position.

How does the pressure gage reading vary with time, and why?

.

.

 

[GeorgeB]

Given an air cylinder closed at both ends. The port on one end is connected to a pressure gauge. The port on the other end is open to atmosphere. Move the piston to the open port end, and allow the pressure gauge to equalize.

Are we to assume the cylinder has a rod or 2 or some other piston attachment? Unless seals leak, removal of the force that moved the piston will result in the piston returning to its initial location ... ignoring friction and given time. If I hold it "extended", there will be a partial vacuum on the closed end.

Quickly force the piston to the closed port end, and hold the position.

How does the pressure gauge reading vary with time, and why?

You've not told us where the piston was to start. If it were at the closed port end initially, pressure would go from perhaps 1 psiA to 14.5 psiA and stay there. If it were in an intermediate position, volume in the gauge connection comes into play; assuming zero volume, pressure would THEORETICALLY approach infinity, get hot, and reduce to some lower order of infinity<g>.

Assuming some real volume (almost certainly true), pressure would rise based on the ideal gas law and due to compression increased temperature . As it cools, pressure will fall to that predicted by P1V1=P2V2.

A hydraulic-pneumatic accumulator is a real world example with the piston perhaps being a physical piston, a diaphragm, or a bladder. The liquid can push this "piston", but pulling it is somewhat more difficult.

What do you intend to be teaching us?

 

[LarryFine]

190411-2359 EDT

These questions are for those not sure of the answers.

Given two nearly identical DC motors with permanent magnet fields that are wired in parallel.

If one motor shaft is rotated, then what does the other motor do?

I'd say the other motor rotates, because motor A becomes a generator. This sounds like a servo system.

Given an air cylinder closed at both ends. The port on one end is connected to a pressure gage. The port on the other end is open to atmosphere. Move the piston to the open port end, and allow the pressure gage to equalize.

Quickly force the piston to the closed port end, and hold the position.

How does the pressure gage reading vary with time, and why?

With no leakage the gauge will show vacuum forever on the first stoke, and there will be no equalization.

With no leakage the gauge will show pressure forever on the second stoke, presuming prior equalization.

 

[gar]

190412-2130 EDT

Both of the questions I asked are something that allows simple experiments to illustrate the concepts. Useful to show young people how basic physics works. It appears I was not clear enough in my questions, but that is OK because it allows you to question what the question means and make assumptions to support your answer. This is far better than multiple choice answers to questions.

See https://www.engineeringtoolbox.com/compression-expansion-gases-d_605.html for a good discussion relative to my air pressure question.

GeorgeB:

Your answer was OK.

I should have clarified my question in the following way ---

Given an air cylinder closed at both ends. The port on one end is connected to a pressure gage. The port on the other end is open to atmosphere. There is at least one piston rod. A rod on both ends won't change the concept of the experiment.

Move the piston to the open port end, and before connecting the pressure gage to what will become the closed port end allow the pressure at that cylinder end to reach atmospheric pressure. Now connect the air pressure gage to the closed end port. The type of gage will determine whether the reading is 0 or atmospheric. Doesn't matter.

In the real world one does not want a flat faced piston to flat bottom at either end of the cylinder on flat faced cylinder heads for a gas type application, usually air. Either internally this should be prevented or external travel limited.

At this initial condition state the internal pressure in the closed port end is atmospheric. The other end is atmospheric, but does not matter to the experiment.

Very rapidly move the piston to compress the gas in the closed end.

Since in a real cylinder I don't want zero volume in the closed end the pressure will not approach infinity.

Force times distance is energy. Because the operation is quick that energy goes into both compressing the gas and heating it above ambient. A momentary high pressure is created. As time lapses heat energy added to the gas in terms of temperature rise over ambient is dissipated to ambient and the pressure drops some, but not to atmospheric.


LarryFine:

In a sense a servo system, yes, but not very good in terms of input angle to output angle. There are a lot of losses.

The experiment illustrates the reciprocity in function of a permanent magnetic DC motor as a generator or motor. Again a good experiment to show young people how things work.

I did not ask about direction of rotation of the device as a motor vs generator. This is another important item to consider.

Small DC PM motors are available surplus and can be very good teaching tools. There are some simple basic equations that help understand how these motors or generators work.

.

 

[LarryFine]

I grew up playing with electricity and electronics, and even built motors from kits as well as home-made. I also played with slot cars and other motorized toys and a few house-hold appliances. Drove my mom crazy.

I know real servos use AC and 3ph to control rotational direction and orientation.

What did you think of my air-cylinder (hydraulic) answers?

 

[gar]

190413-1000 EDT

LarryFine:

To answer your question on what do I think about tour answer on gas pressure.

It illustrates how poorly I composed the question.

Gas pressure results from a huge number of collisions of gas molecules with each other and surfaces. At the very lowest level this is a discrete process, but when huge numbers are concerned it becomes a statical process. The usual gas laws are an observed result with a large number of molecules, and probably fail at very low pressures (vacuum).

I started to research this.

A good discussion on the gas laws is at
http://www.indiana.edu/~geog109/topics/10_Forces&Winds/GasPressWeb/PressGasLaws.html
but this does not address very high vacuum.

This is all for now.

.

 

[GoldDigger]

In the average over time the ideal gas laws should apply even better the lower the pressure since the spacing of the molecules will be greater.
What will happen is the pressure as measured by force on a surface will become "noisy" as individual molecule impacts happen less frequently.

Sent from my XT1585 using Tapatalk