I am electronics student and have little knowledge (if any) about power engineering.

there is a simply question I will need some help with, I am pretty positive about b and c, but I am not sure about a, I am assuming at situation a, the motor will not go and result in overheat? the picture below shows my attempted answer:

 

A is right, in theory. However, there are are buggers in practice. for example, once I had a contactor weld 2 of it's contacts together, so next time the contactor dropped out, there were still 2 phases connected and the motor caught fire. Thermal tripped, but the contacts were welded together, so FAIL.

Your other answers look good too.

 

I always like a practical answer, there is only that much I can get from a text book, nothing can beat answer from a experienced person, Thanks strantor

 

The question needs to state whether the motor is running or not during the fault and whether it is a light/heavy/no load situation. Loss of phase on a motor running under light or no load will not usually be noticed until the motor is shut down and an attempt is made to start it again. (one of the questions I used to interview potential maintenance electricians at a major aircraft manufacturer)

 

The question needs to state whether the motor is running or not during the fault and whether it is a light/heavy/no load situation.

It is usually OK to declare more than one running (or not running) condition and answer for each scenario. The teacher will probably not call you a smart-a**. In this case the way the questions are worded leads me to believe the motor is running when the fault happens.

 

my attempted answer was assuming the motor was running, thanks for pointing that out

 

Phase failure protection has been around for years. A phase failure relay is normally fitted in any industrial/commercial installation. Basically it is either fitted to the supply board on the 3 phase circuits - after any take off points for single phase taps, or occasionally at the motor controller switchbox.

Phase failure relays operate in two ways.
1) If the line voltage on any one phase exceeds a set deviation from the other two.
2) If the current flow in any one phase deviates by a set amount from the other 2 using a current balance similar to RCD.

The older types were electromechanical but most newer types incorporate electronic control.

Some of the newer types will also trip in the event of inadvertent phase reversal

 

A phase failure relay is normally fitted in any industrial/commercial installation.

This may lend to your experience, but I have yet to see one. In the plant I work in, there are none. We have a mix of old and new machinery and not a phase failure relay to be found. I know this for a fact, because when the motor caught fire that I mentioned previously, I was tasked to take apart and inspect every motor control contactor in the plant and check if the contacts need replacing (a task I still have not completed) and come up with a solution to prevent a similar thing from happening again in the future. One solution I considered was to replace all the motor control contactors with solid state feature-rich contactors which would detect phase failure, phase reversal, low voltage, high voltage, etc (example). but I ordered one and discovered that the part that does the switching/disconnecting is actually still an electromechanical contactor, with physical contacts that could still weld together just as before. Another solution I considered was a phase monitor at the incoming power distribution, but that still would not prevent this specific failure.

 

Sucks to be you today!

 

Sucks to be you today!

Today? well, not so much. It happened almost a year ago. The contactor quest isn't even halfway complete and I'm leaving the company in 2 weeks. So, sucks to be my replacement .

 

This may lend to your experience, but I have yet to see one. In the plant I work in, there are none. We have a mix of old and new machinery and not a phase failure relay to be found. I know this for a fact, because when the motor caught fire that I mentioned previously, I was tasked to take apart and inspect every motor control contactor in the plant and check if the contacts need replacing (a task I still have not completed) and come up with a solution to prevent a similar thing from happening again in the future. One solution I considered was to replace all the motor control contactors with solid state feature-rich contactors which would detect phase failure, phase reversal, low voltage, high voltage, etc (example). but I ordered one and discovered that the part that does the switching/disconnecting is actually still an electromechanical contactor, with physical contacts that could still weld together just as before. Another solution I considered was a phase monitor at the incoming power distribution, but that still would not prevent this specific failure.

I don't doubt you for one minute but they have been part of the CENELEC (European body for electrical standards) since 1973.I would say that they are also most likely mandatory in the USA as well.
Now you know and I know that rules are one thing and sticking to them is quite a different matter.