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Why 3-Phase Motors Have 6 Terminals but Only 3 Supply Wires?

Zenthral Code

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Why does a three-phase motor have six terminals when the supply cable normally brings only three phase wires?

Open a motor terminal box and you will usually find six brass terminals in two rows, along with three metal links. Those links decide how the three stator windings are connected—and whether each winding receives the voltage it was designed for.

In this video, we explain why a three-phase motor has six terminals, what U1, V1, W1 and U2, V2, W2 mean, and how the same windings can be connected in Star (Wye) or Delta.

You will learn:

✓ Why three stator windings create six terminal ends
✓ What the metal links inside a motor terminal box actually do
✓ The difference between Star/Wye and Delta connections
✓ Why each winding sees a different voltage in Star and Delta
✓ What can happen when a motor is connected for the wrong supply voltage
✓ Why some larger motors use six leads between the motor and a Star-Delta starter
✓ Why Star and Delta contactors must never close together

Chapters:
00:00 - Introduction & The 6-Terminal Mystery
01:08 - Stator, Rotor & The 3 Windings
02:22 - Terminal Markings (U, V, W vs. T-Leads)
03:13 - Star (Wye) Connection Explained
04:25 - Delta Connection Explained
05:10 - Dual-Voltage Nameplates & Terminal Links
06:30 - What Happens if Connected Incorrectly?
07:44 - The Purpose of Star-Delta Starting
09:14 - Why Some Motors Have 6 External Wires
10:49 - Conclusion & Wrap-Up

Sources and further reading

Star-Delta Starters Explained — The Engineering Mindset
https://theengineeringmindset.com/sta...

Star-Delta Starter Theory — Electrical4U
https://www.electrical4u.com/star-del...

NEMA Lead Markings and Dual-Voltage Three-Phase Motors — Industrial Monitor Direct
https://industrialmonitordirect.com/b...

#ThreePhaseMotor #StarDelta #WyeDelta #ElectricMotor #InductionMotor #MotorStarter #ElectricalEngineering #ThreePhasePower #IndustrialElectrical #ElectricalSafety

15 Comments

@ZenthralCode Creator 1 day ago Pinned

Which markings do you see most often on three-phase motors where you work: U1/V1/W1 and U2/V2/W2, or T1 through T6? And have you ever opened a terminal box and found the links arranged differently from the diagram?

@billmoran3812 1 day ago

Most dual voltage three phase motors have two windings per phase. They are wired in parallel for the low voltage and in series for the higher voltage. This is in addition to the WYE or Delta connectin.

@RussellBooth1977 8 hours ago

I know, I am an electrician whose also rewound a few 3 phase electric motors, the start of each phase winding is labelled U1,V1,W1 & the finish of each phase winding is labelled U2,V2,W2.

The motor can be connected in either Star.(Wye) or Delta, & innan industrial scenario they're connected using bridging links in either configuration externally inside of the terminal box by the use of bridging links.

Or they can be wired up externally to a star/delta starter or to a.set of contacts which are controlled by a PLC (programmable logic controller), the direction can also be controlled by a reversing contactor as well !

@olbluetundra881 1 day ago

That wiring on depends on voltage. High/low. Its pretty simple. Its gets a little more interesting when you 1-9. High voltage is 4/7 -5/8 - 6/9 and line is 1/2/3. Still pretty simple. Most new motor wiring covers have the wiring diagram inside the covers.

@genovaspark 1 day ago

Excellent 👌

@Batman-lives 9 hours ago

6connections are start and finish of 3 windings
Can be connected star or delta configuration

@annasoderlund5924 7 hours ago

A wery good video

@kabandajamir9844 1 day ago

So nice thanks

@MrJoeroux 1 day ago

I have wired 480v motors that don't have the terminal block with links, on these motors you get the wires T-1 to T-6 and you connect the to the leads using the diagram provided. Connect them wrong and you let the smoke out of the wires.

@billmoran3812 1 day ago

Wait until you see 12 wires.

@davidcastle7212 1 day ago

Box on motor called peckerhead where i worked.

@PuncleLeonard 1 day ago

This explanation is WAY over complicated for the answer required. I'm not an electrician but with reasonable certainty can tell that the metal strips between each set of lugs are jumpers. This tells me that I need to only concern myself with placing three wires in the proper place while ignoring the function of the other three. Since the terminals are labeled it pretty much makes this a no brainer, an 11 minute explanation makes more revenue for the presenter than a one minute explanation.

@billkersten7372 1 day ago

Some have 12

@nickdracos5123 1 day ago

I’m sorry but too much hype that doesn’t help the normal person connecting these motors. You have missed one of the most important thing if I can put it that way. Very briefly as I don’t have much time now, why are there six wires coming out if you only connect 3. First why confuse people with this U1 and U2 nonsense. Simply you have three shoe laces, each one has two ends. You number the top three ends U,V,W and the bottom ends X,Y,Z.
Reason for six wires as follows:
Example a motor in Country with 380 V three phase connected in star would subject the windings to 220 volts.
You can take the exact same motor, let’s say to America where they use 220v three phase, and connect the same motor in Delta for it to work so windings will still be subjected to 220 volts.
This way the same motor can be used in Different Countries without having to rewind for that Country, just swop the bridge pieces. Like I said this is very brief and exact. I could make drawings to explain easier but from what I have said anybody with a bit of studying my explanation Wil eventually figure it out.