I am a new apprentice student...my boss has asked me the following questions....I am studying the NEC and am lost at present...#1: if I replace a three phase motor with a 115V single phase motor-will it work without modifications? #2: What is the formula for a 30 degree offset? Help! My lunch today is on the line! Dodiho
3 phase motor questions
- Thread starter dodiho
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- Lockport, IL
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- Semi-Retired Electrical Engineer
Re: 3 phase motor questions
#1: No. You will not be able to connect three wires (from the power supply) to two terminals (on the motor), so some type of modification will be needed. The modification should be to run a new branch circuit, starting with a single pole breaker.
#2. The question does not make sense. There is no "formula" by that name. You will have to get your boss to restate the question.
By the way: Welcome to the Forum.
#1: No. You will not be able to connect three wires (from the power supply) to two terminals (on the motor), so some type of modification will be needed. The modification should be to run a new branch circuit, starting with a single pole breaker.
#2. The question does not make sense. There is no "formula" by that name. You will have to get your boss to restate the question.
By the way: Welcome to the Forum.
Re: 3 phase motor questions
There's a couple of things to consider.
1) Single phase motor requires power from only one L-L connection, A-B, B-C, or B-A. That provides single phase.
2) Now for the "modification." A L-L voltage was not specified so one would have to guess what that may be. But, 3ph voltage is either 208, 240, or 480v so one of those would be your L-L single phase voltage. That leaves you with the challenge of coming up with 115v, or 120v nominal. You would have to add a 1ph transformer of a given KVA to transform the supply voltage of 208, 240, or 480v down to 120v to power the 1ph motor. Hence, the "modification."
There's a couple of things to consider.
1) Single phase motor requires power from only one L-L connection, A-B, B-C, or B-A. That provides single phase.
2) Now for the "modification." A L-L voltage was not specified so one would have to guess what that may be. But, 3ph voltage is either 208, 240, or 480v so one of those would be your L-L single phase voltage. That leaves you with the challenge of coming up with 115v, or 120v nominal. You would have to add a 1ph transformer of a given KVA to transform the supply voltage of 208, 240, or 480v down to 120v to power the 1ph motor. Hence, the "modification."
- Location
- New Jersey
- Occupation
- Journeyman Electrician
Re: 3 phase motor questions
The formula for a 30 degree offset is 2 times the depth of the offset. A 15 degree offset would be 3.86 times the depth and a 45 degree offset would be 1.41 times the depth.
The formula for a 30 degree offset is 2 times the depth of the offset. A 15 degree offset would be 3.86 times the depth and a 45 degree offset would be 1.41 times the depth.
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- Illinois
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- retired electrician
Re: 3 phase motor questions
It is also the cosecant of the angle, or if your calculator doesn't have that it is the inverse of the sine of the angle.
Don
It is also the cosecant of the angle, or if your calculator doesn't have that it is the inverse of the sine of the angle.
Don
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- Massachusetts
Re: 3 phase motor questions
Don't worry, the above answers are not from electricians, they are engineers.
Infinity gave you some exact figures but I just use the following.
15 degrees 4 times the depth
30 degrees 2 times the depth
45 degrees 1.4 times the depth.
For most work this is close enough.
[ January 17, 2006, 07:38 PM: Message edited by: iwire ]
dodiho Welcome to the forum.
Don't worry, the above answers are not from electricians, they are engineers.
Infinity gave you some exact figures but I just use the following.
15 degrees 4 times the depth
30 degrees 2 times the depth
45 degrees 1.4 times the depth.
For most work this is close enough.
[ January 17, 2006, 07:38 PM: Message edited by: iwire ]
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- Massachusetts
Re: 3 phase motor questions
I usually run EMT so shrink is something I hardly ever worry about.
I would have to go to my Uglys book for that.
I usually run EMT so shrink is something I hardly ever worry about.
B
bthielen
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Re: 3 phase motor questions
That's an interesting point. When I first read the post I was a little unnerved but I wasn't sure why. Now I know. Why is the OP searching the NEC for information about how to use a single phase motor on a three phase system? The NEC isn't a how-to manual or a training manual. Are we sure this isn't a DIYer?
Bob
That's an interesting point. When I first read the post I was a little unnerved but I wasn't sure why. Now I know. Why is the OP searching the NEC for information about how to use a single phase motor on a three phase system? The NEC isn't a how-to manual or a training manual. Are we sure this isn't a DIYer?
Bob
Re: 3 phase motor questions
was just thinking ..you said before ..my lunch depends on this (I think) hhmmm ... don't hurt anyone ... especially your self ... M
yo guys ..not to be cynical (is this the rite term ??)but, if he's an "apprentice" he should be attending some type of "Schooling" .. should ask your teacher ...
was just thinking ..you said before ..my lunch depends on this (I think) hhmmm ... don't hurt anyone ... especially your self ... M
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