Star/delta starter

Knobhead · Sep 25, 2010

A mate has asked me if I’d help him with a fault on a star/delta starter. It’s a beast of a thing 800HP closed transition change over. The starter even has it’s own 800KVA 11/.44KV transformer.

He’s sent a drawing but it’s that old it’s like trying to piece the dead sea scrolls back together. Could someone just confirm the sequence of these things for me, it’s been a long time since I had anything to do with them.

A/ main contactor closes
B/ star contactor closes (time delay)
C/ transition contactor closes (small time delay)
D/ star contactor opens (small time delay)
E/ delta contactor closes (small time delay)
F/ transition contactor opens

Apparently every so often the O/L’s trip opening the 11KV OCB (the O/L’s, P&B Goulds are wired to the OCB direct and not the contactors) at the transition point. All my mate says is there’s one hell of a flash and bang when it trips.

Any ideas would be welcome as well.

Sintra · Sep 25, 2010

A hell of a bang at the point of transiton could be down to a phase reversal on the delta contactor or at the motor. Does he know if anyone has been changing things.
Could also be down to interlock failing between contactors which could be checked with motor off load.

malcolmsanford · Sep 25, 2010

I was reading and thinking that perhaps the star to delta timer may be sticking sometimes espicially, if it's old, and therefore not changing over which would give you the overload situation.

The band and flash though would indicate a fault on something

Knobhead · Sep 25, 2010

I would have to be “He-Man” to change anything, from what I can make out on the drawing it’s wired in 400mm singles. I dread to think what the terminal box is going to be like with 6 of them going in!
Most of the time it starts OK it’s just the odd time it throws a wobbly. It seems the plant it’s on has changed operation so the motor starts 5 times a week now where it only used to start once a week. It’s got to the stage the plant operators have developed a nervous twitch when they go to start it.
Contactor interlocks are one of the first things on my agenda to check. I don’t know if there’s any sort mechanical interlock between them, I don’t even know what type they are yet.

Darkwood · Sep 25, 2010

Also consider the contactor points themselves with years of use they may have blasted away and now giving poor contact, with the sheer size of load the bang and flash may not be a short but just a poor contact point making under the high loading,,,, another thing to check is if the motor duty has changed and the so the motor is working harder this could be for many reasons, bearings belts blockage etc etc if this is the case then the star delta timings may be out and it may change over too fast which can give a massive surge, this can usually but not always be checked by listening to the motor reach its full speed which should be before the timer pulls in delta.

Knobhead · Sep 25, 2010

Well that was a fun few hours. I expected to be looking at the starter Sunday, but a knock on the door this morning said different.
It’s a beast of a thing! The contactors are old BHI things left over from building the ark. The contactors have 3 coils, 1 per phase to pull them in and opening coils fed by capacitors. It’s DC control just to add to the fun.
First we went through all the main contacts, not a mark on them except the star contactor. Each pair of contacts must weigh a ¼lb. All auxiliary contacts looked OK.
First test with the motor isolated the change over happened so fast we couldn’t keep a check on it so I increased the transition time. 2nd trial all seemed OK. Now came the fun, we’d got the panel door open and went for a start. Main & star closed OK and the motor ran up to speed. Came the transition the aux contactor closed which does cause a short circuit but the loading resistors control it. The delta contactor pulled in just as the star was opening. FIREWORKS! Dear god was it loud! I’m glad the O/L’s open the OCB and not the main contactor, I was about in need of a pair of pampers as it was!
Checked the star contactor to find a worn pivot bearing and a burnt connection on one of the opening coil capacitors. BHI panels always look neat, all the control wiring is run at the back of the equipment tray. Great until you’ve a duff connection to find. We also altered the star contactors delta interlock contact to give more clearance. In true BHI style even the aux contacts are adjustable.
Tried another 4 starts and all seems OK. The starter is limited to 2 starts per hour so to get around that we took the covers off the resistors and had a 24” fan blowing on them.

malcolmsanford · Sep 25, 2010

Sounds like proper electricary that ............wonderful

Knobhead · Sep 25, 2010

Not so much fun when you’re getting blamed for a 2000HP motor slip rings failing 6 times within 3 months. That turned out to be a batch of fault brushes, did anyone say sorry for having a go at me…… We even filmed the explosion, that was fun!