Y Start, Delta Run, No Starting Torque

Most likely, the motor connections for Y-Delta starting were incorrect. it's a very very common problem and one of the reasons I NEVER spec Y-Delta as a starting method. In IEC countries it is used all the time and everyone is used to it. But in the US, many electricians are never trained on how it works and have no clue what the diagrams mean in the motor connection box, so they end up miswiring it. A 12 lead motor makes it even more difficult because it is a dual voltage motor, so there is even more potential confusion there.

How I wish everyone would ask questions as clearly as this. If there was a GQ rating I would give you one.

Unfortunately no help from me, except to say that when I was an apprentice I had to work on many of these things and if they did not work it was always because I had made a mistake when wiring the control panel.

I agree, definitely deserves an award for the best described problem.

I am still perplexed why it ran fine for a long time THEN exhibited the low torque fault.

Since it ran on delta, the motor and power circuit must be right. Check again the star connectiions it must be wrong.

shivaram

G'day Livewyre,

The 12 motor leads makes me curious, normally we only come across 6 in most motors.

Anyway, it is interesting, if no one has 'touched' the motor wiring, and it was starting in star fine before this, you have to think the wiring is correct. And it is normally when it switches into Delta where motor wiring faults will show up.

Are you able to disconnect the load, or reduce it, and see if it starts? Maybe there is a load issue, and in star the motor torque is a fair bit lower anway. Also I would have thought that you should see more that 70 amps with a stalled rotor in star.

With the VFD on the motor, what does the starting current look like? High for a while? What is the ramp settings, and did you need voltage boost? Possibly the VFD is starting the motor with higher torque than the star/delta starter.

Cheers,

Trevor.

Reason for "No starting torque":

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The torque developed in the motor is proportional to the square of the applied voltage. In your case, the load being a high inertial one, the voltage applied across the windings when they are connected initially in Star is definitely not sufficient enough to produce the required starting torque of load.

Hence I conclude that the voltage required to produce enough starting torque is above the Star voltage.

Reason for tripping in Delta:

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The Delta voltage must be sufficient for delivering the required torque. But why did the motor tripped within some time after delta changeover? The answer is, as the motor had not accelerated enough in Star, the I2t would have already reached a higher valve and by the time it changes over to delta it would have reached the tripping limit.

Solution:

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A starting method which can give a voltage more than Star voltage at start will solve the problem. A soft starter ( Series reactor type or Thyristor type with higher voltage output at start) will be enough to solve the problem.

Since VFD also have function of soft starter, you problem has been solved.

Hope puzzles are solved.

R. Nagaraj,

India

Sounds like there may be a capaciter bank in the system mabe for the power correction that has gone bad.

The answer to this may not lie with the motor. How were the various contactors of the starter?

I read your original post over again and saw where you mentioned the inspection on the contactors.

What was the state of the power factor on the 3 phase supply to the reduced voltage starter? I seem to recall reading once upon a time that power factor issues can impact starting torque on Y start Delta run reduced voltage starters. What the issues were escape me, but I'll try and research it.

MY GUESS is the following, and I do mean guess, because it's from overhearing comments made by my father, the electrician:

The motor was wired up with the three phases out of sync with each other.

Now I'll check the other answers to see if that has been given. If not, I'll return and explain myself better.

Okay, I forgot that the motor had been working fine for awhile.

I'm still stuck on the idea that the three phase wiring somehow got out of sync with one another.

Perhaps an electrician moved one of the leads in the distribution box, while doing some other work?

As I said before, this is really out of my league.

Excellent description, but I have one more question.

Did the motor ever work in this application???

If so, following all your test results, it would indicate

An error in the delta wiring connections of the motor or the control panel

This motor has two sets of winding per phase because of dual voltage.

Depending on the working voltage the winding are connected in

series or parallel

If the motor was never used in this application and the wiring is OK,

it would be possible that motor starting torque is low.

But if it is OK with a VSD, should be OK with a star/delta system

One item i did not see mentioned is that your starting torque requirements may have gone up. The system may have changed so that there is more resistance to starting now than there was when is was first installed.

Yes indeed ... perfect explanation ... *s* ... but ... one diagram might say more than a thousand words ... *s*

And wasn't there more info on the motor's name plate ? Maybe we're talking here about a so called "Dahlander" motor. This motor has 2 windings for every phase. (to have two speeds) These windings can be put in series or parallel or used individually. The windings (in parallel or in series) can be in star or delta ... We talk about Y/Δ or YY/ΔΔ

The 12 leads and the 230/460 made me think of a Dahlander. Usually for standard motors we have 230/400, no? (factor SQR(3)). 460 = 230 x 2 ! (windings in series ?)

Anyhow : good luck !

Thank you to all who replied or submitted answers and opinions. I just wanted to update the information and provide some closure.

The problem was first noticed about 3 weeks prior to my post. I took phase-to-phase voltage readings, but I admit I didn't pay much attention to them other than seeing that they were all around 4XX Volts. I was convinced that the problem was with the motor rotor, and did not leave myself open to other conclusions.

Upon closer examination, after reading most of the posts, we brought in a 3-phase power analyzer and found that A-B = 487V, A-C = 478V, B-C = 463V. It was then that I discovered there was a phase voltage imbalance coming from the town's substation.

Since it is a relatively new plant, the customer was the only one on the feeder from the substation, so no one else could notice any problem. After calling out the town's electrical contractor, it turns out that the C phase (single-phase) voltage regulator motor was seized and would not operate. It was stuck on 1 tap above neutral. The other tap changers were operating at several taps above neutral, which left a phase imbalance of a little better than 23V on B-C phase.

This voltage imbalance was only affecting this particular motor (using Y-Start, Delta-Run) and some of the electronic soft-starters. All the other motors in the plant were normal across-the-line Delta contactor motors and VFD driven motors, which did not seem to be affected by the voltage imbalance.

Following the replacement of the faulty tap changer, the problem has since disappeared.