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.

 

[che12345]

Dear Mr. imannul98
Follow-up No. 5
My humble opinion for your consideration:
1. Reference Mitsubishi contactor MS-N series catalogue.
1.1 Manufacturer recommendation SD starter with the (following contactor size) for starting on [no load or light load application] only. Main = Delta size S-N50, Star* size S-N20. Attention: Start* contactor is wired in delta formation!. For heavy load starting, higher size would be needed.
2. Contactor size selection
2.1 The present contactor size (Main=Star=Delta) size [S-N95] is [adequate] for AC-3 (no plugging or inching) application.
3. The possible? root cause
3.1 The motor 45kW could be (correctly? kW sized and may? have sufficient torque) if it is started by [Direct on line] starting.
3.2 With SD starting, the (torque produced during Star) is only [1/3 of DOL or Delta].
3.2.1 (With 1/3 of DOL torque), it has [insufficient torque to reach full speed with "thick" paint]. This result to the motor unable to reach the full speed even prolong the starting time or the starting current failed to drop. (Transfer it to Delta while the speed is low or the current is high) would be [detrimental to the contacts].
4 Proposed solutions:
4.1 If the (system/local regulation/law permits), replace the present [SD to DOL starting]. Note: The present Main S-N95 is sufficiently (AC-3) sized.
4.2 (Increase the motor kW size), such that at [1/3 torque value during Star is higher than the load torque up the full speed].
4.3 (Start the motor and wait until it had changed over to Delta) [before opening the valve to fill the tank] with thick paint.
4.4 (Lift the mixing impeller above the thick liquid level during starting). [lower the mixing impeller after the motor had changed over to Delta].
4.5 (If the motor stops by any reasons with thick paint), [drain the thick paint to level lower than the impeller or lift the impeller above the thick paint level]; before starting.
Che Kuan Yau (Singapore)

 

[Marke]

Check the timing of the operation of the contactors.
There should be a delay between opening the the star contactor and closing the delta contactor.
If there is no delay, you could close the delta contactor while the star contactor is still arcing and the contactors will burn.

Mark Empson
Advanced Motor Control Ltd

 

[che12345]

Dear Mr Marke
Follow-up No. 6
1. Your kind advice "....There should be a delay between opening the the star contactor and closing the delta contactor. If there is no delay, you could close the delta contactor while the star contactor is still arcing and the contactors will burn."
It is clearly mentioned in Mr. imannul98's post dated 18th Jan at the end of the first para which stated **... 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)...**.
2. I refer to my Follow-up No.2 dated 20th Jan ***... C) and 2 ...***
I had touched on this (dwelling time) indicating that it shall [NOT be lower than 50ms....] It is unlikely? to be harmful even if increased by another 50ms (total dwelling time = 100ms) by an external timer. Time delay of 1.6s is far too long. However, dwelling time is unlikely? to be the culprit....
Che Kuan Yau (Singapore)

 

[Marke]

Hello Che

The arc across the contacts will/should extinguish at the next voltage zero crossing after they contacts open, so the arc should not last longer than 20mS, so a 50mS delay should be adequate.
I have seen people use no delay at all and had lots of damaged contacts. A genuine star/delta control timer will include the dwell delay.

Ideally, the motor will accelerate to full speed in star before you transition to delta. With a high inertia load, this may take a long time, but with a low inertia load, this will take a short time. I have seen special applications where the motor has been star connected for minutes rather than seconds, but this is not normal.
In star, the torque produced is one third of full voltage torque. If this torque is too low, the motor will only accelerate to part speed. A transition at part speed will result in a very high start current. It is important to set the timer so that it operates either when the motor reaches full speed, or stops accelerating at part speed.

Mark Empson
Advanced Motor Control Ltd

 

[che12345]

Dear Mr Marke
Follow-up No. 7
1. Your kind advice had been fully covered in my earlier Follow-ups 1...5.
2. I am "not sure" of your advice that **....The arc across the contacts will/should extinguish at the next voltage zero crossing after they contacts open, so the arc should not last longer than 20mS, so a 50mS delay should be adequate...**
2.1 I think you mean the time in xxms NOT xxmS. Time, in second (s); Electric conductance, in siemens (S).
2.2 Consider the case of an "arc flash" . It can re-ignite (failed to extinguish) in the next half cycle with the surrounding ionized plasma.
3. Please advise and comment on:
a) what could be the possible root cause?,
b) whether the contactors Main=Star=Delta =S-N95 are correctly sized, e.g. AC-3 is adequate or it shall be AC-4 only ?,
c) propose the remedial action(s),
d) etc....
Che Kuan Yau (Singapore)

 

[Marke]

Hello Che

I am very sorry, I have just failed your test!
I do not think that in this context, ms or mS is at all relevant. In the real world, both terms are commonly used to describe milli-seconds.

Also in the real world, we used to use star/delta starters with common abandon, but they were controlled by special star/delta timers.
In more recent times, people with academic training, but little field experience, have been known to try standard timers with an electrical interlock between the contactors. We commonly refer to this type of control as chaser contactors and this will nearly always result in burnt contacts, blown fuses and tripped breakers.
The introduction of a small time delay between the opening of the star contactor and closing the delta contactor corrects this problem by providing time for the arc to extinguish. From an academic stand point, one could argue that there will be sufficient ionized plasma remaining to re-ignite the arc, however a contactor that is designed for switching motors and inductive loads is designed for rapid arc extinguish because opening on an inductive load will always cause an arc.

Your points
1. Maybe, but that does not preclude others from providing input also.
2. ms or mS, the contextual meaning should be obvious.
2.2 you will note that I said 20ms (or should that be mS) which is two half cycles, so the second half cycle was already allowed for. (I assume 50 Hz, not 25Hz)
3a. Contacts will always burn when switching OFF inductive currents. If the motor is not at synchronous speed, or close to it, then 1) the current will be very low power factor (very inductive) and b) a very high current. (close to the locked rotor current of the motor when the transition occurs. LRC of modern high efficiency motors is commonly in the order of 850% - 950% of the rated current of the motor, so breaking a high inductive current when switching occurs at less than synchronous speed)
3b. The contactors are carrying the winding current, not the line current, so if the motor is rated at 93Amps, then the effective motor current through the main contactor and the delta contactor will be 54Amps. At full speed, the star current is 47Amps. That suggests to me that the contactors are plenty big enough provided that the number of operations is reasonable and within the contactor ratings. AC3 is the normal rating applied for star delta starters, but AC4 could be applied for frequent operation.
3c. Ensure that :
i) there is a small delay between the opening of the star contactor and the closing of the delta contactor.
ii) ensure that there is absolutely no way that both star and delta contactors can be pulsed closed at the same time. This can be a control circuit problem, timer issue etc.
iii) ensure that there is no other circuitry connected to the coil of each contactor when it is opened. The control circuit must disconnect the coil of each contactor and at this stage, there must be nothing else across the coil. Additional circuitry across the coil will slow the opening of the contactor and extend the arc time and burn the contacts.
iv) ensure that the contacts that open the coil circuit are able to disconnect the coil voltage without an extended arcing time. Try using a plug in slave relay, rather than the pcb mounted relay in the timer. The AC voltage rating of the contacts used in modern electronics is often far too low to directly control contactor coils. i.e.240VAC AC1, but only 32VAC for inductive loads.
v) ensure that the motor is as close to synchronous sped as possible before the star/delta transition occurs. This will reduce the switching current and improve the switching power factor and reduce the arc energy.
vi) control the frequency for the starts. If more than 10 starts per hour are required, the system will probably need to be engineered properly.

Please note, this is not an exam paper, it is the voice of experience.
have a look at

http://www.lmphotonics.com/star_delta.htm

Mark Empson
Advanced Motor Control Ltd

 

[LionelHutz]

It should be obvious, but I guess the obvious needs to be stated. Failures every 5 or so starts indicates there is something seriously wrong. It's either contactor sizing or switching timing. You can pull up every catalog you can find to prove the sizing is right, but if the damn thing keeps failing then the "catalog proof" means nothing.

Given the application and the 1.6s delay time, I would suspect with the heaviest loads applied that the motor first doesn't reach full speed in star and then it also loses a lot of speed in the 1.6ms delay before the delta contactor closes. It's simply the wrong application for a star-delta starter.

Also, the OP indicated they change speeds but has yet to elaborate on how the speed is being changed. What is this speed change and how is it implemented? Need a schematic or some pictures and an explanation.

 

[waross]

As part of the change from star to delta, there is a 30 degree shift in the phase angle.
The shift may be leading or lagging, depending on the connections.
Best performance is when the shift is lagging and the transition time equals the time for the motor to drop 30 degrees or less.
With variable viscosity loads, there is no way that a star/delta starter may be properly tuned.
You may be able to re-purpose the contactors for use with an auto-transformer starting scheme.

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

 

[imannul98]

Dear all,

Thank you for all of the feedbacks. I have one other question. So, I have done plenty of experiments using the same loads with the same machine and by using new contactors(but the same type). The motor was able to reach full speed in less than 4 seconds, so I guess nothing is wrong with the machine, but only the contactors. And I noticed that there were burn marks on the contacts of the contactor after a few experiments. I have also reduced the transition delay to 0.1 seconds. So, I guess nothing were faulty with the starting method also, but just the contactors were undersized. However, before I buy the bigger current capacity contactor, I would like to know, if there is anything I should consider when buying a contactor? The contactors that are currentlu being used is Mitsubishi SN-95. And I'm thinking on buying Mitsubishi SN-220.

 

[RRaghunath]

imannul,
It is your decision to buy the bigger size contactor.
But, do make sure you have enough space in the panel to mount them (as bigger contactors need more space).

 

[che12345]

Dear Mr imannul98
Follow-up No. 8
My humble opinion for your consideration:
Q 1. "... I have done plenty of experiments using the same loads with the same machine and by using new contactors(but the same type). The motor was able to reach full speed in less than 4 seconds, so I guess....."
A1. If you mean with [SD starting with thick paint] and the motor was able to reach full speed in less than 4s, then the (SD starter contactor selection, assembly and the motor size selection are in order).
Q2. "... if there is anything I should consider when buying a contactor? The contactors that are currentlu being used is Mitsubishi SN-95.
A2. Refer to (Mitsubishi catalogue Fig 1.4.2 (2), it shows that the [present contactors N-95 are adequately sized ]. with (> 1 million electrical operation on AC-3 operation, as defined per IEC Standards.
Note: very often, 1milliom electrical operation is taken as the minimum.
A2.1 For SD starting/stopping (without plugging and inching); it falls within AC-3 utilisation category per IEC. Therefore, selection based on AC-4 would be over-kill.
Q3. "...And I'm thinking on buying Mitsubishi SN-220...."
A3. See A2 above. Sizes higher than N-95 are N-125, 150, 180, (220)....
A3.1 Perhaps size N-125 which would have a higher numbers of electrical operation, would be more than enough.
A3.2 Proposal of N-220 would be far over-kill; more cost and room to house it.
Q4. "...I noticed that there were burn marks on the contacts of the contactor after a few experiments..."
A4. It is "normal" to have burn marks on the contact surfaces. It is important that it shall NOT be welded or (excessive heavy burning) which [creates cavities on the fixed and movable contact surfaces].
Che Kuan Yau (Singapore)

 

[LionelHutz]

The motor reached full speed in star mode, or only after the transition to delta?

 

[imannul98]

Dear Che,

My arguments for buying the SN-220 is because the peak current for both the star and delta contactors after I've measured it is about 165-200 amps. Thus, I figured that if I buy the SN 220, which has a rated current of 220 A, there would be no more welded contactors. The thing is, I thought that the reason for the contactors to get welded is caused by higher current than the rated current during starting and the damage caused by it accumulates over time, which indicated by the burn marks on the contacts. Thus I thought that if I change it with the contactor that has higher rated current than the starting current, there would be no burn marks, no damage that would accumulate over time. Is my assumption correct?

 

[imannul98]

Dear Lionel,

It reached full speed in star mode

 

[waross]

You are always going to see burn marks on the contacts.
You should not see "Grapes" on the contacts.
"Grapes": Small balls of contact alloy solidified around the edges of the contacts.

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

 

[imannul98]

Dear Bill,

I'm not sure I know what you mean with grape. But here is the burn marks that I was talkig about.

 

[Marke]

Hello imannul98

Switching motors will always result in some "burning" of the surface of the contacts. The motor is inductive and while the contacts are opening, there will be some arcing.
Provided that the arcing is not excessive, there will be no major damage done.
AC3 rated contactors are designed for starting motors and allow for a high start current.
Provided that the current is not unduly excessive, the length of start is not excessive and the frequency or operation is not excessive, then the contacts should last. They are designed to absorb the heat from the overload currents.
If you use an AC1 rating, then there is no overload capacity at all and you must select the sizing based on the maximum currents.

1. Based on the information previously given, there should not be any need to oversize the contactors. The current flowing though the main and delta contactors (once the motor is up to speed) is equal to 57% of the motor line current, so they would normally be selected for 93A * 0.57 = 54 Amps. The star contactor would be selected on one third of the motor rating, so 32A.
These contactors would be selected using the AC3 ratings, or in more extreme cases, using the AC4 rating.

2. The photo of the "burnt" contacts is quite normal and what would be expected. If you have a problem that causes severe over temperature of the contacts and the surface is badly melted, then the contact resistance will become excessively high and the contacts will then over heat even more.

There are a number of factors that influence the life of the contacts, but primarily the problem is thermal. The contacts have a "sealed" contact resistance which together with the steady state current, will cause a temperature rise of the contact material.
The contact material will have a maximum temperature at which the contact material will be degraded, (melt or oxidize) and this is dependent on the contact material. Older contacts were commonly silver, whereas more modern contacts are made of silver cadmium oxide.
When the contactor is opened, the motor is inductive and so there is an arc. This arc causes rapid heating in the surface of the contact material.
If the coil voltage on the contactor is lower than it should be, there will be a reduction in the pressure between the contacts and an increased contact resistance and the contacts will heat up more than they should.
If there is circuitry across the coil of the contactor when it is opening, the rate of opening of the contactor can be slowed down. This will prolong the arc and "burn" the contacts.
If there is a second contactor that closes across the contactor output before the arc has extinguished, then the contacts will be seriously damaged.

Mark Empson
Advanced Motor Control Ltd

 

[imannul98]

Dear Marke,

Thanks for the explanation. Based on that, I assume the problem is actually the arc when switching the contactor. If that's the case then, what do you think is the suitable solution to make sure that the arc has been extinguished before the other contactor closes?

 

[Marke]

Hello imannul98

A small delay between opening the star contactor and closing the delta contactor.
Transfer time on the Omron star-delta timer. "Select from 0.05, 0.1, 0.25, or 0.5 s."
If you allow a transfer time (delay) of grater than two whole cycles, you should be OK. It does depend on how quickly the arc is extinguished, but most contactors have arc shutes and mechanisms to limit arcs and shorten time.

Mark Empson
Advanced Motor Control Ltd

 

[che12345]

Dear Mr. imannul98 Feb 21 13:45
Follow-up No. 9
My humble opinion for your consideration:
Q1. "...My arguments for buying the SN-220 is because the peak current for both the star and delta contactors after I've measured it is about 165-200 amps. Thus, I figured that if I buy the SN 220, which has a rated current of 220 A, there would be no more welded contactors. The thing is, I thought that the reason for the contactors to get welded is caused by higher current than the rated current during starting and the damage caused by it accumulates over time, which indicated by the burn marks on the contacts...."
A1. Extracted from IEC 60947-4-1 , in brief:
a) utilisation AC-3 applies to squirrel cage motors with breaking during normal running of the motor. (On closing), the [contactor makes the starting current, which is about 5 to 7 times the rated motor current]. On (opening), it [breaks the rated current drawn by the motor]. Application e.g. pumps, mixers etc...,
b) Electrical durability is the [number of on-load operating cycles*] which the main pole can perform [without maintenance]. It depends on the utilisation category, the rated operational current and the rated operational voltage. Note: one operating cycle* = one make + one break operation.
A2. The contact burn shown is [extremely light], which is normal . It could be just after a few operation cycles*.
A3. The existing contactors S-N95 are adequately-sized. For longer durability up-size it to S-125 would be enough. Increasing it to N-220 is not "engineering".
Che Kuan Yau (Singapore)