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.

 

[waross]

(Because most of the time, the motor started at low speed, about 200 or 300 rpm from rated 1000 rpm, we change the speed by changing the motor tap).

Can you explain this please?

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

 

[imannul98]

Dear Che,

Here is the damage on the contactor. I'm not sure what causes it, overheating or short ciruit. What certain is that the star and delta contactor is both melted, making them unable to open. My best guess is that the star contactor got burnt when starting, causing it to fail to open. So when the delta contactor closes, both contactor short circuited.

 

[imannul98]

Dear Waross

We change the speed by adjusting its winding to change its voltage, hence, its speed.

 

[davidbeach]

Speed of a 3-phase induction motor is controlled by voltage only to the extent that undervoltage increases slip. If you truly need to control speed, get a VFD and get rid of the wye-delta starter.

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

 

[RRaghunath]

imannul98,
"We change the speed by adjusting its winding to change its voltage, hence, its speed." - do you mean the motor has two windings that you can switch to get to speeds??
Appreciate if you could elaborate on 'adjusting its winding to change its voltage'

 

[che12345]

Dear Mr. imannul98
Follow-up No. 3
My humble opinion for your consideration:
1. "....My best guess is that the star contactor got burnt when starting, causing it to fail to open. So when the delta contactor closes, both contactor short circuited....."
In a SD configuration, there is (no current flow) when the [first* Main closes]. With the Main closed, followed by the second** Star contactor closes. The (Star contactor) does the [switching]. When * and ** are closed, the Main contactor current = Star contactor current.
2. Usually, the Star contactor may use one size lower. Noted, that in your case, they are of the same rating as the Main.
3. What had caused the Star contactor to got (burned/welded) resulting to [failed to open?]
I guess that: a) the Star was (lightly welded) due [to prolong over-loading}. When the *** Delta closes, it switches on to a short-circuit between the Star and Delta contactors. Noted: There is NO (mechanical inter-lock) [between the Star and the Delta contactors]. It may? had prevented the damage if there is a mechanical inter-lock.
b) the motor is under-sized. The (motor torque in Star) is too low, unable to over-come the [load torque] while speeding up, during starting. It failed to reach 90-95% speed even with prolong time. If the speed failed to reach 90-95%. the current would remain high. The switching transient would be damaging.
Che Kuan Yau (Singapore)

 

[che12345]

Dear Mr. imannul98
Follow-up No. 4
My humble opinion for your consideration:
1. Per IEC 60947-1 in brief: AC-3 is suitable for motor starting, switching off... AC-4 for starting, plugging, inching... Any contactor can be used for AC-1 ...AC-4...,depending on the [utilisation category and expected number of operation] etc... As for your case, (assuming no plugging and inching); [AC-3 is adequate].
2. There is nothing "wrong" by using SD starting which is a standard used in the IEC world. They work perfectly well when properly designed with proper selection of the components, transfer time and dwelling time etc... There is no justification to replace it with a Soft-starter, Close-transition SD or VFD etc... , before ascertained the root cause of the problem.
3. A closed-transition SD required an additional (transition contactor and three power resistors). They add cost and room to house them. It helps only in [reducing the Star to Delta transient/transfer current peck] during (change-over from Star to Delta).
4. Please attach: a) the power wire connection, which would show whether properly phased.
b) the control circuitry, which would show whether properly designed.
5. Please attach the schematic what do you mean by "Change speed by changing its voltage"?
Che Kuan Yau (Singapore)

 

[LionelHutz]

The motor should reach the speed where the current drops to be below motor FLA before the transition happens. Otherwise, the current spikes to a level that is close to locked-rotor current when the transition occurs, which basically makes having the wye-delta starter pointless. If you're going to hit locked-rotor current anyways then you might as well just across the line start the motor.

I don't understand what you mean by changing speed, but you can't adjust speed by changing the applied voltage. Is this a 2-speed motor? Is this even a wye-delta starter because a 2-speed 1-winding motor control is very similar?


Closed transition.

https://www.google.com/search?hl=en...+starter&oq=closed+transition+y-delta+starter

AC3 is only rated for breaking motor running current (up to FLA). AC4 is rated for breaking motor starting current above FLA. Transitioning at <80% speed means the contactor is breaking current > motor FLA which requires AC4. When you size the contactors as skinny as possible don't expect that AC3 rated contactor to do extra duty it's not rated for.

 

[imannul98]

I think I was mistaken. It is not by changing the voltage to change the speed, but by adding resistance

 

[imannul98]

Dear Che,

Thanks for the insights and advice. and about the schematic. I'm afraid we don't have one. However, I would try to draw it from scratch.

 

[waross]

That peak that you see in the graph is when the star contactor is open and the star contactor closes.
The curve to the left of that peak is the star contactor current.

What is this motor driving???

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

 

[imannul98]

Dear Bill,

Apologize, I just realized the current on the delta side is larger. Okay I understand now. The motor is used to mix paint, a very high density and viscosity paint. When the machine is mixing lower viscosity and density paint, there were no problems with contactors. But when it mixes the high viscosity and density paint, one in about 5 trials, the contactors got burnt. And it always those two contactors.

However, since higher current flows through the delta contactor, how can the star contactor welded first? Why not the delta contactor welded first, which would resulted in failure to open when the machine is stopped?

And the damage always happen when delta contactors closes which suggests that the star and delta contactors short circuited because of the star contactor welded first.

 

[imannul98]

Dear lionel

Thanks for the advice. I do believe so, that the culprit is because of the contactor is undersized. However, I'm struggling to find the right size. So about the AC -4 contactor, what should the rated current be? should it be at least the same with the FLA of the motor? Thanks.

 

[itsmoked]

imannul98. You find a contactor listing or catalog that shows the various contactors for the various FLAs. Using the AC4 column you work down the table to find the AC4 contactor for your FLA case.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[itsmoked]

Metasol Tech Manual

The above manual is by Metasol but it covers every conceivable detail of contactor usage. It explains EVERYTHING. It explains which AC-x are for what, how to size for phase loss, operation rates, special construction details, breaking DC, selection, protection, protection coordination, etc, etc, pertinent details of all the standards, approvals, and directives.


Metasol Catalog

A lot of their contactors list AC-4 duty ratings:

Keith Cress
kcress -

http://www.flaminsystems.com

 

[RRaghunath]

For normal industrial drive applications pumps, fans, compressors etc. AC4 duty is not specified.
It is specified for Cranes/hoists etc. where inching plugging duty is involved.
For other applications, AC3 duty contactors are widely employed and AC4 will be a overkill.
I feel it is appropriate to avoid the situation wherein contactors are asked to interrupt the starting current repeatedly. This can be done by managing the transition from star-to-Delta properly.
As I understood from the posts, the failure is happening when the motor is made to start with heavy load and thus taking longer time to run up (than that set in the starter for transition).

 

[waross]

The motor is probably overloaded.
A load with a widely variable torque demand is probably the worst possible load to start with a star/delta starter.
Star/delta starting is most suited for fans and centrifugal pumps.
Star/delta starting works best with predictable torque demand loads such as centrifugal fans and pumps.

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

 

[LionelHutz]

Your comments about changing speeds and adding resistors make no sense. This needs to be explained first before looking at this starter further.

Star/delta starting is only suited for unloaded fan and centrifugal pump starting.

Fixed it for you.

 

[djs]

Is this a slip ring motor? Is the armature (rotating piece) have a winding? Are there three copper rings on the armature?

As far as I know slip ring motors should not be started via start/delta. They are started with high resistance and then the resistance is reduced.

 

[edison123]

Going by the cooked contacts and the motor unable to pull into full speed under certain load conditions, both the motor and the contactors have to be upgraded to meet your load/process requirements of both torque and power.

Muthu

http://www.edison.co.in