# soft starters and delta motors

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Has anyone experienced conduit heating problems from zero-sequence 180 Hz (3rd harmonic) currents that circulate in the delta windings of induction motors when the soft-start SCR-SCR controlers are connected in series with each motor winding (one SCR controller per motor phase)? Starter controls are physically seperated from the motor termination box but connected via parallel runs of conduit and cable. Two sets of conduits: T1-T2-T3 (phase A-B-C) leads in one conduit system, T4-T5-T6 (phase A-B-C) leads in other condit system. Delta connection is made at the motor control soft-start. Explain to me what causes the 3rd harmonic current to circulate in the delta. I think it is likely caused by the winding pitch factor on the motor? Could it also be caused by the SCR-SCR control and is only present during starting?
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Hello jkp

Welcome to the forum.

There are harmonics produced during start due to the fact that the current waveform is being distorted. This occurs with both inside delta (6 wire) control and line control. I have not heard of it causing any issues before.

The harmonics are only produced during soft start and soft stop so are for a very short period of time except for units that have an energy saver function operating. If there is an energy saver feature, this will reduce the voltage when the motor is at reduced load and this will also cause harmonics to flow. This is then a continuous case and if there is a heating problem, it could become and issue. - harmonics create losses and sometimes the losses outweigh the savings!!

Best regards,

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Excessive neutral current, resulting in overheated neutrals. The currents of triplen

harmonics, especially the odd harmonics, (3rd, 9th, 15th,...) are actually additive in

the neutral of three phase wye circuits. This is because the harmonic number

multiplied by the 120 degree phase shift between the three phases is a integer

multiple of 360 degrees, or one complete cycle. This puts the harmonics from each of the

three phase conductors “inphase” with each other in the neutral.

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Hi, I agree except that there is no neutral connected to an inside delta connected motor!

Best regards,

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• 2 months later...

Hi!

I suppose that one tiristor from bridge of soft-starter was broken down and it is the reason of third harmonic...:cool:

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That's an interesting take on the issue. I wonder what would happen in one SCR failed to fire? Not a short, because that would be a different problem, but a failure of the firing circuit? The inclusion of the SCRs "inside the delta" presents some additional aspects that I haven't thought through.
"He's not dead, he's just pinin' for the fjords!"
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Hello!

Sorry me, my english is bad.

If you have big 3rd harmonic, it is problem of incorrect work, because we need a generator of 3rd harmonic. In usual work we can have different harmonics, but they will be very small. In this case the generators can be two motors and two soft-starters and may be a power supply (power line). I would start a check with line after that motors after that soft-stars.

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