# Star Delta Motor System



## Almost Retired (Sep 14, 2021)




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## just the cowboy (Sep 4, 2013)

leslieman said:


> Went to troubleshoot the above system using a 100 HP/460 volts motor not pulling it's full load .
> Megger winding reads good, voltage to motor terminals was balanced and amps was 2% off from phases.
> What type of metering device needed to display fault in the power system, regular voltage and amps did
> not pick up problem. A new motor was installed and the same condition existed. Finally I decided to check the
> ...


That still should of shown up on the current check.
Sounds like the wye got it started but phase C delta did not energize but since it was moving it had no torque . But at his point C would not have proper current, so something don't sound right.
If it was a bad contact you do a fall of potential test across the contact. If contacts are fully made the voltage should be real close to zero.

A bad habit people get into when troubleshooting motors is they take a voltage reading at the starter then check voltage at the motor. They see voltage appear on the meter and don't check to see if it is exactly the same as the starter. Voltage drop effects this but I have seen a few that just glance at meter and what they expect to see, not the real number.


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## SWDweller (Dec 9, 2020)

I would have to get out my Electical Enginneering Handbook, have not seen a star-delta in decades.


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## gpop (May 14, 2018)

A power quality recorder/meter would have shown the problems straight away and as a bonus you do not need to be live front on a piece of equipment you know is dodgy. 

You would have seen balanced power until it switched to delta then you would have seen a major imbalance based on load until the overloads tripped. (presuming this is a basic Y/delta that simply switches on a time delay)


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## micromind (Aug 11, 2007)

Voltage readings on motor starters are mostly useless except for all of potential.

If there's an open contact, the motor will generate the bad leg and voltage will often look normal.

Current and fall of potential are your best friends.


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## paulengr (Oct 8, 2017)

Bad contacts will show up as a current imbalance. Downstream of it you will see a voltage imbalance, too. Upstream only current.

Most electricians are so used to “close enough” and most clamp meters are so inaccurate they just ignore obvious trouble.

First let’s define imbalance. Take 3 readings. Average them. Take the largest difference between the average and that reading. Multiply by 100 and divide by the average. That’s percent imbalance done the NEMA way. IEC is different and not worth explaining. So say you have 100. 100, 130. So the average is 110. The difference is 20. The imbalance is 18%.

Ok so you can do voltage and current imbalance. NEMA says more than 1% voltage imbalance requires derating the motor and over 5% is unacceptable. Remember +/-10% is acceptable for voltage IF IT IS BALANCED. Every 1% voltage imbalance causes a 6-8% current imbalance.

So we often get errors. So I start with current imbalance. If I have say a 1% voltage imbalance then look at the current imbalance. If there really isn’t any then I know that it’s just measurement errors. But if see significant current imbalance (over 2-3%) but no voltage imbalance then I know it’s there but the issue is downstream.

Once I ran into this on a 350 HP motor but finding it was difficult. The motor side was of course 6 lead so no way to easily measure currents because they were mixed. But I was able yo see the voltage imbalance clearly. So finally I snaked a flexible current probe onto the load side of the breakers and I could clearly see it on the running contactor. I was assured the contact tips checked good but as soon as the cover was off it was obviously worn badly. After replacement, the imbalance went away.

Pay the extra for a flexible current probe. They are expensive but worth it.


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