Failure Of Contactors In 110kw Star Delta Starters

[AhmedFaiz]

I have an issue with 110kW Star Delta Starters...the application is RAW SEWAGE WATER PUMPS installed in a Sewage Treatment Plant.

 

Motor Details are as follows:

Rating : 110kW

Voltage : 380 - 480V

3 Phase

Ampere : 205 - 190A

Frequency : 50Hz

RPM : 1476

 

 

 

We have used the following components in the Star-Delta Starter:

Main Circuit Breaker : 250A Frame / 220A rated Motor Protection MCCB

Main and Delta Contactors : 225A

Star Contactor : 150A

Over load relay range : 132A....to....220A

Static Capacitor rating : 32.5 kVAR

Contactor for Capacitor : 95A

Timer used is a normal ON-DELAY timer (not a Star-Delta timer)

 

The problems we face are:

1. At the time of Starting, there was a spark in the MCCB, the Star and Delta Contactors got welded and the over-load relay got damaged.

2. We have another complaint that there is only one healthy output in the 3-Pole MCCB.

3. In the same MCC, we also have lower rated Star-Delta starters...we have received complaints of the Contactors getting burnt during initial start-up.

 

Could you please advise the possible causes of these faults and what remedial action I can take. I am attaching the Power and Control wiring diagram of this Star-Delta Starter.Could it be due to use of a normal On-delay timer instead of a Star-Delta timer?

 

Is there anything to do with the connection of the Static capacitor...As per the wiring diagram the Capacitors are switched on even in Star mode...is this correct...or should we bring the Capacitors on line after switching to Delta mode.

 

Is the Wiring diagram correct? location of the overload relay,..etc.

 

Thanks a lot for your advise.

 

 

 

 

[yuri]

Hello.

 

Sorry, I dont see any diagrams applied to your post.

 

What are the actual voltage and frequency, by the way?

 

Contactors amperage seems for me good, however, I would prefer a slightly bigger range to be safe 100%.

 

I may be wrong, but the idea that the capacitors are on line with the motor on start seems not good at all (I consider them better only to be connected after the motor has successfully started and runs).

 

Are you sure there is no problem with the pump's motor or the pump itself, and also with the load on the pump? (if it is possible I would at first tried the motor disconnected from the pump, then with the pump without load, then with the loaded pump).

 

Best of all to use a variable speed driver - it is scarcely much more expensive than the cost of all the equipment now spoiled and which should be replaced by new. Maybe still not late for a VSD?

 

All the best to You.

[marke]

Hello AhmedFaiz

 

Welcome to the forum

 

Your problem is almost certainly due to a timing issue.

It is imperative that you have a small delay between the star contactor opening and the delta contactor closing.

As you are not using a proper star delta timer, you will have a situation where the delta contactor closing before the arcing on the star contactor has extinguished.

You must have a clearly defined open state where the star contactor is open and the delta contactor is open.

Preferably, this open period is many cycles to reduce the generated voltage from the spinning rotor, but if you start a low inertia load such as a pump, you need to minimise this time to prevent the motor stopping during the open transition.

Add a 100ms turn on delay in series with the coil to the delta contactor and the problem will be sorted.

 

Best regards

Mark.