Three Phase

learner · Feb 3, 2011

Hi there,

I need to upgrade my practical knowledge about the three phase side of the electricity. I would like to get some useful/practical knowledge regarding the following items:

How to size motors for applications
How to size startes and overloads for the motors
How to size wires for applications (Amps/volts)
how to size fuses for the applications
useful hints regarding arranging the components inside the electrical panels

could you please help me with these regards?

Always a learner

shoelesscraig · Feb 3, 2011

Sounds a lot like a homework assignment to me. There are plenty of books and websites that can help you learn those topics. A simple search on Google will answer most of them.

DickDV · Feb 3, 2011

As to sizing motors, you must first understand the torque-speed characteristic of the load the motor will drive. Then, you will size the motor so its torque-speed characteristic is always higher than the load's.

The other sizing issues you mention are all defined by the electrical code.

Arranging the components can be a matter of convenience and is general specific to a particular application.

More details? Since this does look like a homework assignment, take the post above and this one and dig in!

Good luck!

learner · Feb 3, 2011

Thanks for the hints. By the way, I finished school long time a go. This was not a homework it was a new task.

learner · Feb 3, 2011

Thanks for the hints. By the way, I finished school long time a go. This was not a homework it was a new task. Ya, I know there is search engine called google.

Steve@HSDCorp · Feb 3, 2011

Good questions I am also looking to get more into these things. Thanks for posting

DickDV · Feb 3, 2011

Well, we're always learning too, learner. So, if not homework, let us know what specifics you need help with and we'll try to help. Let us know beforehand just what research you've already done so we don't end up duplicating your efforts.

Clay B. · Feb 3, 2011

In your list one item feeds the other.

The Motor size is dictated by the work it needs to do.

The size of the motor starter is based on the Full Load Amps of the motor you picked.

The wires size and insulation is based on the amps they will carry and the voltage. In the motors case it is also based on the distance the motor leads need to be.

The fuse size is based on how much current the circuit/device can handle. In the motors case it will be based on the FLA.

When you build your panel the more you can keep your low voltage and high voltage seperated the better. Also as a Rule I try to put my heat generating devices above my other devices. Heat disapation in a panel is always needed to be considered when sizing the panel.

Tom@Pton · Feb 3, 2011

As stated you will need to know the starting and running characteristics of the load.

Here in the States Article 430 in the NEC will pretty much answer questions 2-4.

Minimum wire bending space at terminals can be found in NEC 312.

I don't know what your Canadian Code references would be.

VFDs and other items will have space limitations and requirements in the installation instructions.

brucechase · Feb 4, 2011

Learner,
When it comes to useful/practical knowledge of any type of electrical system, the first thing that comes to mind is SAFETY.

Understand that voltages as low as 120VAC will kill you (even though many people don’t believe it). You need to understand the distance away from the energized sources that will keep you safe. Along that same line, you need to understand Arc Flash energy and safety. There is a calculated amount of energy available at all electrical systems that MUST be known. This will indicate what PPE is necessary and at what distances you should stay back from the exposed energized sources. Google “arc flash safety” for some good reading and some good videos of what can happen.

The reasons I state that is because in my opinion, most engineers and “designers” do not know how to properly design an electrical system. If the selected components have interrupting rating below the available fault current, then the system could fail catastrophically while you are standing there and if you are lucky, you will only be seriously hurt.

To answer some of your questions:

Motors are sized based on the load that they drive. The driven system is what determines the horsepower of the motor.

Starters are sized based on the full load current of the motor. In addition, the SCCR rating of the starter must be higher than the available fault current. If that is not available, then a current limiting device must be used on the front end to limit the damage.

For some information, Google fault current fuse protection and look at some of the results like these:

http://www.littelfuse.com/data/en/Short_Circuit_Current_Ratings_Literature/PF992UsingFusestoIncreaseYourSCCR.pdf

and

http://www.bussmann.com/library/docs/EPR_Booklet.pdf

Overloads are sized based on motor nameplate running current. The older bimetallic style used 2 different metals with different coefficients of expansion to move the relay contacts. These had to be sized for the exact current that you were dealing with. Newer electronic style have adjustable settings and are a little more forgiving for the design (temperature and distance compensation).

Wires are chosen based on insulation material, voltage class, and cross sectional area. The voltage class of the wire must always be higher that the voltage running in the wires (and the wires installed in a common raceway). For example, a 480 VAC motor must have 600 V insulation, but a 240 VAC motor CAN have a 300 VAC class of wire. Most people choose the standard 600V class of wire for industrial power systems at 480 VAC or less.

In addition, you need to know the ratings of the cable and the insulation. You have to put TC rate cable in an open cable tray, but not in a panel or conduit. The NEC has some information on this but do some searches for a more detailed explanation.

Fuses (and their breaker counterpart) are sized for the protection of the wire and equipment. They must activate (trip) before the damage curve is reached. There are many components to sizing a fuse or breaker other than current rating. The AIC must be properly chosen along with a co-ordination of the protective devices for selective tripping. This can get involved without some of the specialty software that exists out there. Unfortunately, this is often overlooked when designing a panel due to the complexity, laziness, and lack of knowledge of many of the designers out there.

Finally, I will end with safety again. A panel and subsequent electrical system must be designed with the worker (production and maintenance) in mind. This will involve a risk assessment to determine the hazards of the equipment along with designs to mitigate those hazards. This will help determine if you need a Cat2 safety system versus a Cat0 safety system and how the safety components interact with your motor starters and controls.

Sorry for the long winded post, there are many things I left out that are probably just as important. I just wanted to show that there is a lot involved with designing an electrical system correctly and, in most of my field experiences, much of this gets ignored. The consequence of an improper design is someone getting hurt or killed and the designer being personally liable and responsible for that.

learner · Feb 4, 2011

Thank you so much for the info. It gives me a place to start. I appreciate your help. Please feel free to send me any info with this regard.

Regards

Always a learner

Dravik · Feb 4, 2011

You'll want to consult the CSA for specific requirements in Canada. The NEC70 and the CSA do differ in some regards.

I is a noob · Feb 4, 2011

Motor size and enviroment would dictate most of the other aspects of this project, would it not?

Three Phase

learner

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