Motor starter contactors getting welded

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STAR DELTA MOTOR STARTER BURNING

STAR DELTA MOTOR STARTER BURNING

topgone said:
The usual problem I had on wye-delta starters was when the delta contactor closes fast while the wye contacts have just broken/the contact arcs still not extinguished. The solutions I did were:
  1. choosing different-sized contactors. With a larger delta contactor (compared with the wye contactor), there will be a longer travel for the delta contactor shorting contacts to make from the instant of initiation, leaving the arcs at the wye contactor to die down first, or
  2. insert a small delay for the delta contactor closing.

You must allow the wye contacts to break clear before your delta contacts make.
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Sir,

This problem should also persist in forward ( upper motor starter) rotation of motor. then why only reverse motor starter is getting burned.


Thanks
Kapil_Mohan21
 
topgone said:
The usual problem I had on wye-delta starters was when the delta contactor closes fast while the wye contacts have just broken/the contact arcs still not extinguished. The solutions I did were:
  1. choosing different-sized contactors. With a larger delta contactor (compared with the wye contactor), there will be a longer travel for the delta contactor shorting contacts to make from the instant of initiation, leaving the arcs at the wye contactor to die down first, or
  2. insert a small delay for the delta contactor closing.

You must allow the wye contacts to break clear before your delta contacts make.
Click to expand...
But it works OK on one motor but not the other.
That doesn't strike me as a starter design flaw.
And, unless I missed it, it's hard to see the justification for a YD starter on a 3HP motor.
 
Besoeker said:
But it works OK on one motor but not the other.
That doesn't strike me as a starter design flaw.
And, unless I missed it, it's hard to see the justification for a YD starter on a 3HP motor.
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I questioned why we need a Y-D starter on a 3 hp motor in post 2.

What soft starting characteristics may be gained don't seem to outweigh the complications that may be gained. If anything an electronic soft starter seems more suitable for the application.
 
kwired said:
I questioned why we need a Y-D starter on a 3 hp motor in post 2.
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Indeed you did. I just bumped it up to see if it gets any responses.

kwired said:
What soft starting characteristics may be gained don't seem to outweigh the complications that may be gained. If anything an electronic soft starter seems more suitable for the application.
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I'm with you on that too. If it needs a soft start at all given that it runs direct on line.
 
kapil_mohan21` said:
Sir,

Star delta starters are less costly than soft starters.


Thank & Regards
Kapil_mohan21
Click to expand...
Only the first one... Now you have spent any potential savings many times over in down time, replacement costs, and your time in trying to solve a riddle that need not have become a riddle in the first place if it had been done right initially. To quote an old English aphorism; "Penny wise, pound foolish" meaning that in order to try to save a few pennies, you end up costing yourself a lot more money.

And the answer to your riddle by the way is that your motor that consistently welds the contacts has a different level of residual magnetism, so when it transitions from Star to Delta, the phase shift inherent in that transition is taking place while the motor field has not yet collapsed, so it is akin to connecting two generators out of synch. There is a huge voltage spike that maintains the arc in the contacts opening, then causes a huge current (and torque) spike when the other contactor closes. This is a well documented and know phenomenon with Star-Delta starting and is the reason why Closed Transition Star-Delta starters exist. But Closed Transition requires a 4th contactor and a set of current limiting resistors, so at that point becomes MORE expensive than solid state starters. That's why you rarely see it used any longer.
 
STAR DELTA MOTOR STARTER BURNING

STAR DELTA MOTOR STARTER BURNING

Jraef said:
Only the first one... Now you have spent any potential savings many times over in down time, replacement costs, and your time in trying to solve a riddle that need not have become a riddle in the first place if it had been done right initially. To quote an old English aphorism; "Penny wise, pound foolish" meaning that in order to try to save a few pennies, you end up costing yourself a lot more money.

And the answer to your riddle by the way is that your motor that consistently welds the contacts has a different level of residual magnetism, so when it transitions from Star to Delta, the phase shift inherent in that transition is taking place while the motor field has not yet collapsed, so it is akin to connecting two generators out of synch. There is a huge voltage spike that maintains the arc in the contacts opening, then causes a huge current (and torque) spike when the other contactor closes. This is a well documented and know phenomenon with Star-Delta starting and is the reason why Closed Transition Star-Delta starters exist. But Closed Transition requires a 4th contactor and a set of current limiting resistors, so at that point becomes MORE expensive than solid state starters. That's why you rarely see it used any longer.
Click to expand...



Thank you for the information sir,

I will request for the approval of soft starter in my company.
But please also tell me that :- is residual magnetism for forward & reverse rotation are different??


Thanks
Kapil_Mohan21
 
Jraef said:
Only the first one... Now you have spent any potential savings many times over in down time, replacement costs, and your time in trying to solve a riddle that need not have become a riddle in the first place if it had been done right initially. To quote an old English aphorism; "Penny wise, pound foolish" meaning that in order to try to save a few pennies, you end up costing yourself a lot more money.
Click to expand...
It's British but let that pass.
We are still no closer to understanding why the starter is OK on the forward motor but not so OK on the reverse motor. Or why a YD was used on a 3HP motor.
I think that needs to be resolved/understood before implementing a new "solution" for an underlying problem.
 
Besoeker said:
It's British but let that pass.
We are still no closer to understanding why the starter is OK on the forward motor but not so OK on the reverse motor. Or why a YD was used on a 3HP motor.
I think that needs to be resolved/understood before implementing a new "solution" for an underlying problem.
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They are different motors, different characteristics, including the residual magnetism I mentioned. As to the why question, there might be several reasons that the original designer used it, some of which may have been regulatory in nature. The OP is in India, I've learned over the years in dealing with customers over there or OEMs sending equipment there that they implement numerous regulations on 3 phase systems that the rest of the world finds ludicrous, but they have their reasons. There is a lot of aging infrastructure that has a difficult time keeping up with growth, demand and illegal tap offs.
 
Jraef said:
They are different motors, different characteristics,
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I don't recall Kapil saying that they are different motors. Obviously something IS different between the two but if they are of different ratings, it doesn't appeat to have been mentioned. In fact he mentioned that the loads are the same. So, to reiterate my point, it sounds like a problem with the bottom motor.

I'd have tried replacing it, but appears that he now has a fix. I hope it stays fixed.
 
Besoeker said:
I don't recall Kapil saying that they are different motors. Obviously something IS different between the two but if they are of different ratings, it doesn't appeat to have been mentioned. In fact he mentioned that the loads are the same. So, to reiterate my point, it sounds like a problem with the bottom motor.

I'd have tried replacing it, but appears that he now has a fix. I hope it stays fixed.
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Let me straighten this out for you, no finger pointing intended just the facts;)

Second sentence in post #1 "The machines has two motors i.e one for forward rotation ( upper motor ) while second for reverse rotation( bottom one) ( both have independent supply)"

We kind of presume they are both 3 HP motors, but don't know if they are identical motors, and that each has it's own controller. We don't know what interlocking methods may be used to prevent both forward and reverse motors from being energized at same time, or if the machine ever has to overcome forward inertia when switching to reverse motion or vice versa.
 
kwired said:
Let me straighten this out for you, no finger pointing intended just the facts;)

Second sentence in post #1 "The machines has two motors i.e one for forward rotation ( upper motor ) while second for reverse rotation( bottom one) ( both have independent supply)"

We kind of presume they are both 3 HP motors, but don't know if they are identical motors, and that each has it's own controller. We don't know what interlocking methods may be used to prevent both forward and reverse motors from being energized at same time, or if the machine ever has to overcome forward inertia when switching to reverse motion or vice versa.
Click to expand...

Posts #14 and #15 at least infer that they are of the same rating otherwise the question of different currents depending on direction of rotation wouldn't be relevant. And they do evidently run at the same time.
No difference in load characteristics , just upper one moving in forward rotation & simultaneously bottom one in reverse rotation.
Click to expand...
So we're back to the question of why one starter regularly fails and the other doesn't. Still inclines me to think that's a motor related issue.
 
Besoeker said:
Posts #14 and #15 at least infer that they are of the same rating otherwise the question of different currents depending on direction of rotation wouldn't be relevant. And they do evidently run at the same time.
No difference in load characteristics , just upper one moving in forward rotation & simultaneously bottom one in reverse rotation.
Click to expand...

So we're back to the question of why one starter regularly fails and the other doesn't. Still inclines me to think that's a motor related issue.
Click to expand...

I interpret that as both motors being firmly connected to the same machine shaft, so that they rotate simultaneously, but that they probably do not have power applied at the same time.

I am going with the working hypothesis, pending confirmation or refutation by the OP, that under normal operation the machine is driven in the forward direction, but that from time to time it is driven in reverse (to back out tools, etc.). The really relevant question is whether the machine is allowed to coast to a full stop before engaging the reverse motor.
I think it to be less likely that the machine would be switched from reverse to forward rotation without stopping, hence no problem with the forward motor starter.
Any interlock that keeps both motors from being powered a the same time would not necessarily also prohibit switching without stopping.
If operating rules require stopping, I would not be surprised if some operators tried to cut corners.
Using a VFD on one or both motors would allow for dynamic braking to minimize direction change wait time.
 
If the same size and connected to the same shaft, why 2 motors? Why not just reverse a single motor? I interpreted this as being some milling machine (as stated) that does a different task in one direction as the other, and the motors are not the same, as shown in the photo. I would guess that perhaps one motor is 2 pole, the other 4 pole, something like that, which is why they could not use a reversing starter on one motor. Different speeds, different torques.
 
We are getting deep into the realms of speculation.
It would good if Kapil, the OP, could confirm the ratings and speed of both motors and if they are mechanically coupled or independent.
 
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