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   Articles | How-Tos | Ask the Expert   

3-Phase Electric

Understanding Industrial Wiring

WARNING: This article is technical and intended only for folks who have a thorough knowledge of electricity, motors, and troubleshooting. You should know and understand safety and follow normal safety and OSHA guidelines when working with industrial electricity and your work must conform to local and national building codes and NFPA guidelines.

Why 3-Phase?

Normal 110V and 220V is like a big strong fellow who is driving in a tent pin. At some point, the amount of work that has to be done can be tougher than what the big fellow can do. With 3-phase, it's like having three guys with sledge hammers, working together, each hitting the tent pin in a rhythm. While each may not be doing as much work as the big fellow, together the three of them can drive in the tent faster because they are hitting it with three small blows for every one time the big guy is hitting the pin, and the total amount of work being accomplished is much greater when added up.

Advantages of 3-Phase

Often the wiring is smaller, the motor may last longer, and there's a good chance that a lighter motor will outperform and save more energy than a single-phase motor.

Types of 3-Phase Power

  • Common 3 Wire Type
  • Common 4 Wire Type
  • Special 4 Wire Type
  • 3 Wire w/Grounded Hot Leg

The type of 3-phase power normally doesn't dictate the operating voltage. Usually, a voltage meter is required to determine the actual voltages available, which will also give you some clues about the type of 3-phase. It is extremely important to perform all the tests, especially if you suspect 3 wire w/grounded hot leg and will be using only two legs or you may create a dead short with your connections.

For traditional reasons, 110V/220V are used, but the actual voltage may be 120V/240V or 125V/250V. It will make more sense, especially when discussing 3-phase power, as you will see later. If this bothers you, then I suggest you are a layman and probably shouldn't be reading this anyway.

Common 3 Wire 3-Phase

Actually, this is the same as 4 wire 3-phase, and is actually used to describe how the wiring is done. The same voltages and types of service are still available, and is normally only done when the lowest voltage is not required, or for high-voltage 3-phase at 440V and above. For clarity, it is mentioned here, but to understand how it works, see the diagram and details for 4 wire 3-phase.

Common 4 Wire 3-Phase

The leads marked L1, L2, and L3 are hot leads, or "line," and typically, diagrams and schematics will continue to use the numbers, but change the letters to "S" for them after a switch. Sometimes, "M" is used to identify motor leads. "N" is neutral, and is never switched for this type of 3-phase service. Which is which? Actually, it is impossible to really know which is L1, L2, and L3, so we just pick out the three hot leads and start from there. Later, I'll explain how to wire these to a motor so the motor works properly.

The magic starts here! Let's say you check and find that you have 220V 3-phase service. Now, you can get three separate, single-phase 110V circuits by using L1 & N, L2 & N, or L3 & N. Also, you have three separate, two-wire 220V circuits by using L1 & L2, L1 & L3, or L2 & L3. For 3-phase, 220V, you need to use all three leads, L1, L2, and L3. Remember that you can't just decide to use a particular type of power because you want to — the device must be designed to work the way you hook it up.

At higher voltages, such as 440V 3-phase, the neutral (N) lead may not be provided, or a separate leads may be supplied for providing a 110V circuit.

A special type of 4 wire 3-phase service for 208V is available, too. This is accomplished in a strange way, and is most often used in lighting circuits where 208V two-wire and 3-phase service is desired, along with 110V service. It is less common, but may be found in older buildings and for special applications.

3 Wire w/Grounded Hot Leg is an older type of service. Sometimes, this is provided with 4 wires, so that 110V service can also be provided. This type of service is especially hard to test for, since most folks are not familiar with it. Normally, you have to check for voltage to ground, and the grounded leg will not show any voltage when a meter is used to check from the ground leg to ground. However, this lead can only be used for 3-phase, or a short to ground may occur. Otherwise, this service works like other 3-phase service.

Connecting Motors to 3-Phase

With single-phase and two-wire 220V, either a shaded pole, CSIR, or CSCR motor is used so it starts in the correct direction. One of the advantages of 3-phase is that the timing of the phases automatically make the motor directional. Unfortunately, there is no way to know which way the motor will run ahead of time, so you need to "bump" the motor — turn on the power for part of a second — to see which way it is going to turn. Some pumps and most compressors don't care, and when a motor gets older and starts to it is common to reverse the direction to make the motor last longer. To switch the direction of rotation on a 3-phase motor, simply switch any two leads.

Safety Ground

Industrial wiring is commonly run inside metal conduits, raceway, or armored cable, and the safety ground accomplished through these metal enclosures. The neutral wire is never used to carry the safety ground. Refer to current NFPA guidelines for proper safety ground installation.

Commercial wiring may have different requirements in your local area. Be certain to check beforehand and do the work in accordance with local building codes. If in doubt, call your building commissioner.

Switches, Controls, and Overcurrent Protection

With the exception of 3 wire 3-phase w/grounded hot leg, all three leads that are hot need overcurrent protection and should be switched. Some switches incorporate a starter that has special circuitry to limit the current draw, or apply a higher voltage, when starting the motor.

Some switches and controls will use single-phase, two-wire 220V, or other combinations, which the actual motor or device that uses a lot of power uses 3-phase. For example, a refrigeration compressor that operates on 220V 3-phase may have a 220V 2-wire heater and contactor coil, and a 24V solenoid valve to control the refrigerant from a cold control. Due to this complexity, it is extremely important that you know and understand how to read wiring diagrams and schematics, and be able to determine the voltages and type of power that each control or device uses.

Reprinted with permission, Homedoctor.net



 


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