Star Delta Contactors Got Burnt When Starting the Motor but The Main Contactor Did Not 9

[imannul98]

Hello

I have a 45 kw induction motor with starting method using star delta method. The star and delta contactors of the motor have been for several occasions got burnt. However, I haven't figured out why the contactors got burnt. The FLA is 93 A, and the contactors have a rating current of 105 A, breaking current of 930 A, and making current of 1050 A. I figured that it was probably caused by the star and delta contactors being closed at the same time during the transition. Hence, I added 1.6 seconds after the star contactor opened for the delta contactor to close. However, it got burnt again.

And then I realized that because the current that flows through the star and delta contactor also flows through the main contactor, thus, if the star and delta contactors got welded, then there should be three contactors that got burnt, not just 2. Does anyone know what seems to be the problem that causes the star and delta contactors to get burnt but the main contactor doe not? Thank You

P. S.
The timer for the star contactor in the close position is 4 seconds.

 

[imannul98]

Dear Marke,

As I've previously mentioned, it had been done before when the transition delay was set at 1.6 seconds. However the contactors broke down in just about a little more than a month. And since the star contactor broke down first, I guess the problem is not just the arc, but overcurrent also?

I reset the timer eventually to 0.2 seconds because when I measured the starting current, the transient current was higher at the delta contactor if the transition delay was longer. However, it only contributed in reducing the transient at delta, and didn't help in figuring out why the star contactor failed to open...

 

[waross]

All contactors are not created equal.
We used to service a bridge crane that gave us a call-out to replace blown fuses after a contactor flashover.
The bigger cost was the down time and lost production waiting for our electrician to get there.
We replaced all of the contactors with Allen-Bradley contactors and never got another call-out.
A different brand of contactor may give better results, ratings notwithstanding.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Marke]

Hello imannul98

You have also stated that the motor gets up to full speed in about 4 seconds I believe.

Looking at the star contactor, if the motor is rated at 93A, the full speed current in star would be in the order of 32A and that would not burn the contacts.
This make me believe that there is something else happening that we are not aware of.

If the motor was continuously stalled, then the current in star would be in the order of 256A, but if it was continuously stalled, there is no point in using a star delta starter.

One question that I have to ask is : Is this motor designed for star delta starting, or is this something like a dual speed motor?

Mark Empson
Advanced Motor Control Ltd

 

[imannul98]

Dear marke,

It is indeed that the full speed current in Star is in the order of 30s Amps. However, I'm concerned with the starting current that could increase as high as 200 Amps, that the contactor would get damaged because its rating for continuous Amps is only 105 A. The high current doesn't lasts long though, less than a second.

As far as I know, the motor has never stalled and the starting current is pretty consistent.

And yes, it is designed for star delta starting.

 

[edison123]

There is lot of beating around the bush in this thread.

@OP - Many experts have stated the present contactor is sufficient to do the duty. If you still want to upgrade, just do it.

Muthu

http://www.edison.co.in

 

[Marke]

Hello imannul98

If you were using the AC1 rating of the contactor, then I would agree with you as the AC1 rating does not allow for any overload current. - resistive current only.

The AC3 rating is somewhat lower than the AC1 rating because it has to take into account the overload current associated with Motor Starting.
Generally, for Direct On Line starting, you would select a contactor based on the AC3 current rating.
Direct On Line starting would typically have an overload current of 650% - 850% current for perhaps ten seconds, so that contactor, AC3 rated at 105A could be expected to sustain start currents of around 700 Amps for greater than 10 seconds.
Have you spoken to your contactor supplier and asked for their recomendation?

Mark Empson
Advanced Motor Control Ltd

 

[che12345]

Dear Mr. imannul98 9 Feb 21 01:05
Follow-up No. 10
My humble opinion for your consideration:
Q1. "... As I've previously mentioned, it had been done before when the transition delay was set at 1.6 seconds. However the contactors broke down in just about a little more than a month. And since the star contactor broke down first, I guess the problem is not just the arc, but overcurrent also? ....I reset the timer eventually to 0.2 seconds because when I measured ..."
A1. I think you are giving us WRONG/misinterpretation information:
a) " ... transition delay was set at 1.6s ..." ; [s Wrong, unlikely to reach approx rated speed within 1.6s.
b) "... I reset the timer eventually to 0.2s..." is also Wrong. Grave mistake!,
c) Now you say that the time is "...4s to reach approx full-speed".
Che Kuan Yau (Singapore)

 

[davidbeach]

So, instead of fussing about the minutia, why not focus more on that being the wrong starting method for that type of load? Wye-delta is more suited to fans, pumps, or chillers, perhaps even compressors that start unloaded. Paint mixing sure sounds like something that doesn’t make for gradual loading of the motor.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[imannul98]

Dear Che,

What I meant with transition delay is the delay between opening of the star contactor and the closing of the delta contactor.

 

[Marke]

Hello imannul98

"What I meant with transition delay is the delay between opening of the star contactor and the closing of the delta contactor."

Yes, that is correct.
There are two important delays. The first, is the time the starter is configured in star connection and this delay can commonly be from a second or two, to twenty seconds or more. I have worked with Centrifuge starters where the motor has been held in star for up to ten minutes. (motor needs additional forced cooling and must be a special motor!).
The second delay, is the transition delay which is the time period between the star contactor opening and the delta contactor closing. During this period, the motor is not driven and so it will slow down. Setting this at more than half a second will, in many cases result in the motor speed dropping considerably and this in not desirable. The transition delay is necessary to prevent the delta contactor closing while the star contactor is still arcing and would typically be in the order of 50 milliseconds to one hundred milliseconds.
Re looking at the pictures of the contactors, I can not see any signs of arc shutes. I wonder if these contactors are appropriate for star/delta starters where the arc needs to extinguish rapidly.
There is something that we are not seeing or we would not have these issues. - Ask the contactor supplier for answers!!



Mark Empson
Advanced Motor Control Ltd

 

[waross]

Re looking at the pictures of the contactors, I can not see any signs of arc shutes. I wonder if these contactors are appropriate for star/delta starters where the arc needs to extinguish rapidly.

I wondered about that also.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[LionelHutz]

If you can fit the bigger contactors then just put them in. There is nothing wrong with ignoring the catalog telling you it's not necessary. Make sure you put a mechanical interlock in there too so they can't be closed at the same time.

I still question the application, but if it gets to full speed under every possible load before transitioning then it's OK. But, if it doesn't get to full speed before transitioning with heavy mixes then it's the wrong starting method.

 

[che12345]

Dear Mr. imannul98
Follow-up No. 11
My humble summery of opinion for your consideration:
1. (Contactor size selection) for [mixer] application, utility category based on IEC AC-3 is appropriate.
1.1. Present contactors Main=Star=Delta = S-N95 are adequately sized.
1.2. For higher number of operation cycles, up-grade them to S-N125 would be sufficient. Excessively over-sizing them ( use of S-N220) would be an over-kill.
2. (Reduce-voltage starting with 6-wire terminal motors) by SD method is the lowest in cost and in foot-print.
2.1. SD starting method can be used for mixer load with [motor correctly sized]. Observe that during starting i.e. in Star, the motor torque which is only 1/3 of DOL torque shall be higher than the load torque during the whole period of speeding up.
2.2 Auto-transformer starting connected to 60% tapping produces about the same torque as SD; but about 2 times the cost and foot-print.
3. (Dwelling time delay = time delay between Start opens to Delta closes ) >50ms up to 100ms (but not longer) shall be observed.
4. (Transition time = time between Star closes to Delta closes) when the motor had reached 0.95 of motor rated speed. Any premature or prolonged in time while the current remained at high level would be detrimental.
4.1. If the motor is unable to speed up to say 0.95 of motor rated speed or the current remain at high level, it would be an indication that the motor is under-sized..
5. (Mechanical inter-lock) is [preferred but not minatory] with proper circuitry design; observing the >50ms dwelling time delay.
6. The (arc chute) around the [fixed contacts] are visible. Another part covering the movable contacts is on the contact cover, which is not shown. They are in order.
Che Kuan Yau (Singapore)

 

[waross]

We had chronic flash-overs with some contactors with de-ion plates in place.
Opening under heavy load, the arcs would project through and out of the de-ion plates on the face of the contactor.
The arcs would join up on the line side and take out the fuses.
The contactors were properly sized as per the manufacturers rating but when replaced with another brand we had no more issues.
There is no spec on the length of time for a contactor to clear the arc after opening.
Not all contactors may clear the arc fast enough in applications such as star delta starting where arc clearing time is critical.
It has been many years since I was teaching industrial controls to students.
When covering 'circuit race', I seem to remember an almost ten to one ratio of opening times of different makes of contactors.
This was a hands on course and we had available contactors and relays from the major brands.
The students were able to wire the same circuit with two different brands and see the different operation under circuit race.
Is the contactor properly sized for circuit closing? Probably yes.
Is the contactor able to extinguish the arc in a suitable time? Probably no.
Consider that this may be a brand specific issue.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[waross]

There is another possible issue.
Phase shift.
When the motor transitions from star to delta there is an inherent phase shift of 30 degrees.
This may be an advancing shift or a retarding shift.
Best practice is for the starter to be connected for a retarded shift and the transition time between star clear and delta close to equal the time for the motor to drop back 30 electrical degrees.
This affects the delta starter.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[che12345]

Dear Mr. waross (Electrical)10 Feb 21 09:46
W. "...There is another possible issue. Phase shift...."
C. Agreed in full to your learned advice.
For the benefit of the readers, please illustrate with a wiring diagram of line conductors only; showing the "correct" phasing connection, for i) clock-wise and ii) counter-clock-wise direction of motor rotation; viewed from the driving-end shaft.
Note: IEC practice: L1, L2, L3 for power source. Motor windings with 6 wire termination are labelled U1-U2, V1-V2, W1-W2,
Thank you.
Che Kuan Yau (Singapore)

 

[che12345]

Dear Mr. waross (Electrical)10 Feb 21 09:46
1. History: W. "...There is another possible issue. Phase shift...."
C. Agreed in full to your learned advice.
For the benefit of the readers, please illustrate with a wiring diagram of line conductors only; showing the "correct" phasing connection, for i) clock-wise and ii) counter-clock-wise direction of motor rotation; viewed from the driving-end shaft.
Note: IEC practice: L1, L2, L3 for power source. Motor windings with 6 wire termination are labelled U1-U2, V1-V2, W1-W2,
Thank you.
Che Kuan Yau (Singapore)10 Feb 21 23:08

2. Repeated request: we are still looking forward to your learned advice for the past ten (10) days.
Kind regards
Che Kuan Yau (Singapore)

 

[waross]

These contacts have been badly overloaded.
In the event that the conditions were even worse, molten contact material may be expelled, and solidify into little balls around the edges of the contacts.
We used to call those little balls "Grapes".
One cause was closing into a shorted motor or feeder.
Another cause could be an operator repeatedly rapidly jogging a stalled motor.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[che12345]

Dear Mr. waross
1. W. "...There is another possible issue. Phase shift...."
C. Agreed in full to your learned advice.
For the benefit of the readers, please illustrate with a wiring diagram of line conductors only; showing the "correct" phasing connection, for i) clock-wise and ii) counter-clock-wise direction of motor rotation; viewed from the driving-end shaft.
Note: IEC practice: L1, L2, L3 for power source. Motor windings with 6 wire termination are labelled U1-U2, V1-V2, W1-W2,
2. Repeated request: we are still looking forward to your learned advice for the past ten (10) days.
3. W. "...These contacts have been badly overloaded...."
C. ... "...another possible issue, possible shift phase..." which causes very high transient current of a very short duration (1/2-1 cycle) is unlikely to cause badly over-loading .
4. Repeated request No3. : we are still looking forward to your learned advice (on the "correct" phasing wiring connection for the past ten (10) days.
Che Kuan Yau (Singapore)

 

[Marke]

Hello Che

Re your "repeated request".
When star-delta (wye-delta) starters were in common usage, there was a lot of knowledge around on how to use them. Those of us with grey hair have lived through this and generally have a good understanding of their usage and design.
Today, the number of new star/delta starters that I see is very small, less than one per year.

In terms of the "phase shift" argument, this was pushed hard by many of the star/delta suppliers as a means of reducing the "auto reclose" or "open transition" switching transient.

The theory is that when you have the motor connected in star and running at full synchronous speed, the rotor will be in direct synch with the supply.
At the point when you disconnect the motor from the supply (open the star contactor) it will generate a voltage in synch with the supply, but slightly lagging in phase, so when you reconnect the motor in delta, (closing the delta contactor) you can either connect the end of the windings onto either the leading phase, or the lagging phase relative the the phase that the start of the winding is connected to.
i.e. assuming that the phase sequence is L1 - L2 - L3, and we consider the winding connected to L2. We have the choice by the wiring to the delta contactor, to connect the end of the winding to either L1 or L3. As an totally unloaded motor will generate voltage with a small phase delay relative to the driving phase, connecting the end onto the following phase will result in a much lower transient current required to establish the flux in the stator.
The switching transient is very short and will not over heat the contacts.
Auto reclose and open transition switching on motors results in a very high "inrush" current to the motors. This exceeds the LRC by quite some margin, but is only present for a couple of cycles. Associated with this inrush current, there is a severe torque transient that is responsible for much mechanical damage, broken chains, broken couplings broken gearboxes etc.

Inrush current is not the same as starting current which can last for seconds.

Mark Empson
Advanced Motor Control Ltd