Star Point Connection 3

[kinglong]

I'm not familiar with the term "star point connection".

I need to find out when starting up a 208V/3P motor, if there is any effect on full amp loads if you don't have a neutral star point.

If someone could kindly help me out, that would be great.

Thanks,

 

[aolalde]

The "star-point" of a Polyphase circuit is a common connection point were all the phase current paths converge. In a balanced circuit the resultant current in that point is zero.

 

[kinglong]

So what effects occur during times of start up?

In an unbalanced load, the neutral will take all the excess current, but what happens when there is no neutral? KABOOM???

 

[aolalde]

If the star-point is not connected to the line neutral you don’t have necessarily a failure.
With a floating neutral (not connected to the neutral line) the phase currents will unbalance to compensate the lack of neutral return. The return will be through the same power lines. Looking to the three phases as phasors ( 3 rotating vectors 120°E apart) and the neutral current added will help to visualize the results.
If the star-point for the three phases is open (not connected) , nothing happen since the circuit is open.
If only two phases are connected to the star-point your system will become a single-phase. Now your motor will not start or will be overheated if the star-point lost one phase connection while the motor was rotating. The overcurrent protection on the three phases should trip the motor off line.

 

[jraef]

kinglong,
If I may be so bold as to interpret your question...

It sounds as if you have a 208Y120V system, and someone has connected the motor with only 3 wires, or the neutral of the incoming line power is not connected. While this would have an effect on any 120V loads you may have had on the same circuit, it will have no effect on the motor whatsoever. The Neutral (star-point as you call it) is not used by the motor.

There is a starting method called Star-Delta where the motor, if designed for this, is connected with 6 wires. 3 of them are connected to a special contactor to create a star point in the motor in order to reduce the effective voltage through the windings during start-up, then reconnected in a Delta pattern to give full voltage during run. This also has the effect of reducing torque and current, so it is refereed to as a "Reduced Voltage Starter". But if your motor does not have 6 wires going to it and a starter with 3 contactors, this does not apply to you.

JRaef.com
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[davidbeach]

Even if the windings of the motor are connected in wye and there is some unbalance in the currents in the three windings, the voltage of the wye-point will shift to balance the currents and voltages, Kirchhoff's laws will be obeyed, no electrons will leak out. No neutral connection is provided at the motor, nothing to connect a neutral conductor to. If the motor windings are connected in delta, there wouldn't even be any theoretical place to connect a neutral conductor. From the outside of the motor you couldn't even determine whether the windings are in delta or wye by making electrical measurements.

 

[RalphChristie]

A little off-track, but I just wanted to tell you what happened to me after I just qualified a few years ago. I was called out late one night (on standby, one of my first times) to a 250kW, 3.3kV slipringmotor on a conveyor belt. The dayshift has already changed the motor, my job was to connect the cables back again.

The motor had two terminal boxes for the stator, one on each side. The secret was to bridge the one side's terminals if you connect the cable to the other side. Well, I opened one of the stator-boxes, saw the busbar-bridge and yes (don't laugh) connected the cable onto it. Well, after finishing, pressed the start-button, siren went off and, nothing happened! I was totally stunned. I went back to the MCC and noted the blown fuses. Then I realized what I did, and went back to the motor.

This time I took the bridge-busbar off, but did not connect it to the other side! Again started the motor, siren went off, and again, nothing happened. Went for the second time back to the MCC, but this time, no fuse was blown. I was in a total flat spin. And remember, at that time all the mechanical guys wanted to go home, and was just waiting for me.

Then it strucked me like lightning what I have done! With no starpoint there was no way that the motor could start.

Well, to end a long story, I will never forget a starpoint on star connected motor again!

Regards
Ralph

[red]Failure seldom stops us, it is the fear for failure that stops us - Jack Lemmon[/red]

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