Electrical Q

[sigma]

A three phase supply containing three conductors supplies
a three-phase motor.
There is a fuse in each conductor.
If one fuse blows out while motor is running, but remaining
two fuses hold and motor continues to run, it will operate
a) on the two remining phases
b) at reted power divided by the square root of 3
c) at 2/3 of rated power
d) on one phase

[bornriding]

Since the second fuse did not blow, then the load does not have major torgue - probably a fan motor. The motor will operate on one phase at reduced power and torque. When the device stops & tries to restart - the second fuse will blow.

[pecmsg]

You should never allow a 3 Phase motor to operate on single phase. Replace fuses with a Breaker or starter. If one leg has a problem all 3 drop out.

[sigma]

Pecmsg

This is a quiz question from the text book, not from real life.
So what answer would you chose?

[valdelocc]

I agree with bornriding,In a 3ph circuit,3 sequences harmonic exist,positive sequence harmonic creates a magnetic field in the direction of the rotation. The negative sequence harmonic creates a magnetic field in the oposite direction of the rotation and the zero sequence harmonic creates a single phase signal that doesnt create a rotating magnetic field. by removing any of the phases while the motor is up to speed , the motor will keep on running using the two phases left, untill the power is interrupted to it , after that it will not start again.

[seaboard]

What?? Won't it draw locked rotor and blow at least one of the other fuses?

[NormChris]

Originally posted by seaboard
What?? Won't it draw locked rotor and blow at least one of the other fuses?

Many three phase motors single phase and then continue to operate only to burn out rather than shut down. Many compressors fail that way. Adequate motor protection can prevent this. Fuses alone are not always enough. Phase protection devices provide better protection.

[NormChris]

Originally posted by sigma
Pecmsg

This is a quiz question from the text book, not from real life.
So what answer would you chose?


What textbook did this question come from? The question and the possible answers could be better constructed.

[snipe70e]

The motor will keep running. until stopped or it burns up and stops.

[sigma]

[i]Originally posted by NormChris
What textbook did this question come from? The question and the possible answers could be better constructed.

[/B]


This question is from:

“REFRIGERATION - Mechanical Equipment Service Manual”
For Steamfitter-Pipefitter, Journeyman and Apprentice.

First edition August 1971

[NormChris]

Originally posted by sigma

[i]Originally posted by NormChris
What textbook did this question come from? The question and the possible answers could be better constructed.




This question is from:

“REFRIGERATION - Mechanical Equipment Service Manual”
For Steamfitter-Pipefitter, Journeyman and Apprentice.

First edition August 1971
[/B]

What do you expect from a book written by the union! :) Of all the HVAC books in my vast collection the union books contain the most mistakes. :)

[sigma]

[QUOTE]Originally posted by NormChris

What do you expect from a book written by the union! :) Of all the HVAC books in my vast collection the union books contain the most mistakes. :)

Norm
I think that you’ve just opened a can of worms. :D

[NormChris]

Originally posted by sigma
[QUOTE]Originally posted by NormChris

What do you expect from a book written by the union! :) Of all the HVAC books in my vast collection the union books contain the most mistakes. :)

Norm
I think that you’ve just opened a can of worms. :D


Yup, the can was union made and packed. :) See the union label is right there on the can's wrapper.

[Wild Leg]

Originally posted by sigma
Pecmsg

This is a quiz question from the text book, not from real life.
So what answer would you chose?


Since it's just a question from the text book, & not from real life, I would choose the answer from "the text book".

Then I would ask the instructor to explain all the other choices.

BTW...Why would anybody be using a textbook from 1971?

[maxster]

in the real world a single fuse blown is considered a weak fuse,2 blown either it be a compressor or evap fan motor the motor is the problem...good question? is 230V single phase considered a lower voltage(in amperage terms)then 230V-3 phase

[NormChris]

Originally posted by maxster
in the real world a single fuse blown is considered a weak fuse,2 blown either it be a compressor or evap fan motor the motor is the problem...good question? is 230V single phase considered a lower voltage(in amperage terms)then 230V-3 phase

You can have a current imbalance which causes one phase to draw higher current and blow that fuse.

[sigma]

[QUOTE]Originally posted by bwal2

BTW...Why would anybody be using a textbook from 1971? QUOTE]



Because he bought it on Ebay and did not know how old it was. :)
It is old book and I agree but how much an operation and construction of three phase
motor have changed over 30 years?
I will take this book over “Doolin’s Trouble Shooters Bible” any time.

[NormChris]

Some pretty good stuff is in the older books. I have HVAC books that go back to the 1940s that contain some great material. Even learned some things from some union publications. :)

[Wild Leg]

Certainly, old textbooks have value.
And certainly, three phase motors & power are pretty long in the tooth.

I just can't see any reason for any school or organization to use a textbook that is older than the person reading it.

For an instructor to reference to one is a different thing from using it as a textbook.

I just hope they flesh out the old information with something a little more contemporary. :D

[beenthere]

Does that mean people shouldn't work on equipment that is older then them?

It'll run on single phase till it burns up, or has to attemp to restart.

[onetime]

I think the question was designed to see if the apprentice knows what single phasing is and in that context it makes sense. Then you can have a good conversation about single phasing, causes and affects.

[ct2]

Has anyone seen a piece of equipment intentionally connected single phased?

I connected a 125 amp spot welder using two legs of a 480 volt 3 phase circuit

[jtricor]

The answer is d, but not for very long if a load is present