Electrical troubleshooting sample question #5

Troubleshooting Problem #5:

Use the wiring diagram shown and the symptoms described below to answer the following questions. 

Note - the system is using a new rectifier type 24vdc power supply with common return properly grounded and has no fault indications and no significant ripple voltage. 

Also - note that for SW12, the left hand / CCW position is considered the Forward direction and the right hand / CW position is considered the Reverse direction for operations. 

Symptoms: M18 is not working correctly. It will not spin in reverse at all and if the Speed control rheostat is adjusted towards maximum it will blow a fuse as soon as CR1 is energized and SW12 is moved to right hand position.

M18 does seem to work properly in the forward position at any speed (SW12 in left hand position with CR2 de-energized). 

Question 1: Select each possible fault from the list below that could cause ALL of these symptoms. 

1) CR2 coil is shorted out. 

2) Wire between SW12 and TB38 is shorted to ground

3) Pin 3 of CR2 is shorted to ground

4) Wire between M18 and TB49 is shorted to ground

5) Pin 8 of CR2 is shorted to ground

Question 2: Which parts do you think a "part swapper" might hastily and improperly change out as part of this effort? 

Scroll down for answers and explanation.

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Solutions / Explanation: 

Note that the bit about it being a grounded power supply is important because of how a grounded DC power supply will behave if a ground fault occurs. This is the most common setup and a very typical reason for blown fuses or tripped circuit breakers. 

Answer 1: Let's discuss each listed scenario in order.

1) If CR2 was shorted out, it would could possibly cause the fuse to blow when SW12 is switched to right-hand position - but it would not depend on the speed control rheostat being at or near max setting. So this is not a correct answer. 

2) If the wire between SW12 and TB38 was shorted to ground, it would cause the fuse to blow when switched to right hand position - but it would not depend on the speed control rheostat being at or near max setting. So this is not a correct answer. 

3) If pin 3 of CR2 was shorted to ground, it would cause the motor to have ground on both sides all the time - so M18 would never move in either direction. Interestingly it would result in a near constant source voltage across the speed control rheostat which could potentially cause problems - but most rheostats are able to handle sizable current so it is likely to be ok. But it is possible the rheostat could be a bit warmer than normal due to higher than normal current flow due to effectively shorting out the motor. So this is not a correct answer. 

4) If the wire between M18 and TB49 is shorted to ground it would cause ALL of the listed symptoms. When CR2 is de-energized current would flow through rheostat through CR2-2b to M18 and to ground (not needing to flow through CR2-1b since the TB49 side of M18 is already grounded). But When CR2 is energized and CR2-1a contacts close, the current will flow directly through CR2-1a to the ground fault, thereby bypassing M18 - so M18 would not move in that condition. This is why it spins only when SW12 is in the left-hand position. Furthermore, if the rheostat was above a minimum resistance it may prevent the circuit protection of F1. This is why the symptoms only occur when rheostat is at or near max speed setting. So, this is the only correct listed answer to the question. 

5) If pin 8 of CR2 is shorted to ground it would do essentially the opposite of condition #4 above. This fault would occur only when SW12 was in the left-hand position though since the circuit would feed power directly to ground fault when CR2-2b contact is closed and would work normally when CR2 is energized via the path of CR2-1a through M18 to grounded pin 8. This is not the correct answer because it would cause the opposite problem of what was stated by operator. So this is not a correct answer. 

Answer 2: As to which parts a "part-swapper" might change out on this fault, the sky is the limit and it'd be likely that they'd change more than one. Likely time and materials expenditures include:

• Lots of fuses
• M18
• Speed control rheostat
• SW12
• CR1 
• CR2

This problem attempts to show why it is important to make use of good symptom analysis during troubleshooting and strives to also help give people practice thinking about circuits and electrical diagrams. 

We focus heavily on the actual logical approach during our Basic Electrical Troubleshooting courses. Sure - sometimes a quick W.A.G. (wild anonymous guess) might produce a lucky quick result - but over time, the 'WAGGERS' will not succeed like the logical troubleshooters and will often add new problems. 

If you would like to download and customize these forms for your specific needs, download them at this link. 

Orion - 7-step TS Forms 

These scenarios are meant to induce thought of logic and hopefully spur learning and improvements. Some of these samples are very similar to scenarios that students troubleshoot in our Electrical Troubleshooting classes.  

Reach out today if you'd like us to deliver a custom version of this course at your facility or see our upcoming schedule to register for a public offering of our 3-day Basic Electricity & Electrical Troubleshooting course.