# How many ways can I flip switch positions to make the motor stop

Discussion in 'Homework Help' started by Alice24, May 22, 2012.

1. ### Alice24 Thread Starter New Member

Apr 22, 2011
15
0
For this exercise:

I found six

That's it, right?

2. ### mlog Member

Feb 11, 2012
276
36

1. How many switch combinations are there? You have 4 switches with 2 possible states for each switch. It can help to make a table (or a spreadsheet).

2. Which diode pairs need to be ON to run the motor? For example, it would seem that one combo would be diode 2 ON and diode 4 ON. After you identify the diode pairs, you can find the necessary switch positions.

3. Are there any combinations that are incompatible? For example, can the motor turn both clockwise and counterclockwise at the same time?

4. Are there any pairs or combinations that would create a failure mode? For example, what if diode 2 and diode 3 were both turned on at the same time?

I did not get the same answer as you did.

3. ### WBahn Moderator

Mar 31, 2012
18,093
4,920
You can also come at it from the other direction. How many switch configurations are there that will make the motor turn?

I recommend making a table and listing each of the possible switch combinations into categories as follows:

1) Motor runs CW (just assume if the current flows left-to-right through the motor that it turns CW).

2) Motor runs CCW.

3) Motor does not run due to no power (but no failure mode).

4) Motor does not run due to no ground (but no failure mode).

5) Motor does not run due to power-ground short (failure mode).

As a bonus, answer the following: Someone comes up and places the switches in random positions. What is the probability that they will damange the system (i.e., place it into one of the failure modes)?

If these suggestions don't do it for you, post the six positions you came up with and we will ask some additional questions to guide you along.

4. ### mlog Member

Feb 11, 2012
276
36
I prepared a spreadsheet to find all apparent RUN states. After I found the RUN states, I scrubbed out the FAIL states and had remaining only 2 states that would run the motor. So my conclusion was that all of the non-RUN states would stop the motor.