Autotransformer Motor Starter Trouble

[big john]

Have a 4160V RVAT motor starter where the autotransformer appears to have fried because it never switched out of start sequence.

This thing is good and old: It still use a Sylvania pneumatic relay to actuate the timing functions.

What surprises me is that where I would expect to see multiple control relays for the multiple power contactors, this thing is running with a single control relay and a single timing relay. Attached is a picture:

I want to double-check my understanding before tearing into this:


1) All RVAT starters equipped with "Main", "Start", and "Run" contactors are by default going to be closed transition? Correct?

2) If you're doing all this switching with a single timing relay, wouldn't that mean it had to be a make-before-break set of contacts?

3) Why would those two power resistors be necessary?

Thanks for any insight.

 

[templdl]

Have a 4160V RVAT motor starter where the autotransformer appears to have fried because it never switched out of start sequence.

This thing is good and old: It still use a Sylvania pneumatic relay to actuate the timing functions.

What surprises me is that where I would expect to see multiple control relays for the multiple power contactors, this thing is running with a single control relay and a single timing relay. Attached is a picture:

I want to double-check my understanding before tearing into this:
View attachment 12820

1) All RVAT starters equipped with "Main", "Start", and "Run" contactors are by default going to be closed transition? Correct?

2) If you're doing all this switching with a single timing relay, wouldn't that mean it had to be a make-before-break set of contacts?

3) Why would those two power resistors be necessary?

Thanks for any insight.

You must have a wiring schematic of the control as that would be very helpful to have in order to understand the starting sequence of your starter.

 

[big john]

You must have a wiring schematic of the control as that would be very helpful to have in order to understand the starting sequence of your starter.

Agreed, if I had that, I probably wouldn't be asking these questions.

Unless I can get one from Siemens there's no documentation here.

 

[Jraef]

Not all RVATs are closed transition, years ago they usually used open transition. Closed transition was a solution to the transition spike problems developed in Europe years ago. It's called the Korndorfer Method (look it up. Interesting story). But because it involved a little more complexity in the control scheme and a more expensive transformer, U.S. mfrs didn't start using it very often until more European equipment began arriving here with it and people liked it better.

But there was an issue with MV versions using the Korndorfer Method that resulted in people adding RC snubbers to the circuit, which would involve a resistor and capacitor. But your power resistors appear to be in the LV circuit? I can't tell. If so, that's odd for sure.

 

[big john]

Not all RVATs are closed transition, years ago they usually used open transition...

Would it still be open-transition with three contactors? I ask because the only OT starters I've seen were just two "start" and "run" contactors.

That said, it looks like it's possible this is OT because I found a mechanical interlock running between the start and run contactor cabinets. I have to confirm it's still operational.

...But your power resistors appear to be in the LV circuit? I can't tell. If so, that's odd for sure.

They are definitely on the LV side. You ever seen these with so little else in the way of controls? My first reaction is that a lot of stuff appears to be missing from this scheme.

 

[Jraef]

Would it still be open-transition with three contactors? I ask because the only OT starters I've seen were just two "start" and "run" contactors.

That said, it looks like it's possible this is OT because I found a mechanical interlock running between the start and run contactor cabinets. I have to confirm it's still operational.

They are definitely on the LV side. You ever seen these with so little else in the way of controls? My first reaction is that a lot of stuff appears to be missing from this scheme.

The Korndorfer method still uses 3 contactors, and two of them would be mechanically interlocked, but it is closed transition.

One possible way to tell would be to disconnect the motor leads and watch the operation. Also, the Korndorfer needs a three coil autotransformer, which is bigger than the cheaper two coil designs. Take a look at that and if it's a two coil, it is NOT a Korndorfer (however if it is a three coil, that doesn't mean it is).

It is hard to tell from that photo what the heck is going on there however, the resistor is a complete puzzlement. Maybe it was cobbled together by someone who didn't know what they were doing?

 

[Jraef]

Oh wait... maybe those are part of an economizer circuit for DC coils on the MV contactors! That's actually fairly common on large contactors. Most new vacuum contactors have that all built-in, but if it's old enough, maybe it didn't at that time.

 

[electric_cal]

I would concur with Jaref, and suggest that you disconnect the motor from the system and dry fire the unit to trace out the LV control. You may have to draw the control schematic yourself.
I would trace out the circuits invloved with the 2 resisters. This may help in determining what they are being used for. You can use any control schematic for an auto-transformer start sequence once you have determined what the various components and devices are being used for that you have installed in the cabinet.
I would also agree with you, that from your description, it would appear that the system did not come out of start and transition into run correctly. I would suggest installing a "Thermo-Disc" device on the replacement auto-transformer. This will help in preventing a future failure if the system hangs in start again. The "Thermo-Disk" will detect the temperature increase in the transformer and shut the system down before failure of the transformer occurs.

 

[templdl]

Agreed, if I had that, I probably wouldn't be asking these questions.

Unless I can get one from Siemens there's no documentation here.

Unless you cut open the bundles of wires as show in the picture and are able to trace the control circuit you may be fighting a loosing battle unless you follow through with a request to siemerns for a schematic.

 

[Jraef]

Unless you cut open the bundles of wires as show in the picture and are able to trace the control circuit you may be fighting a loosing battle unless you follow through with a request to siemerns for a schematic.

If it has a Sylvania timing relay, it's so old that I can guarantee there is nobody at Siemens who can help with that thing. :happysad:

 

[templdl]

If it has a Sylvania timing relay, it's so old that I can guarantee there is nobody at Siemens who can help with that thing. :happysad:

I agree. Then it is the task of following the wires and putting together a schematic. Otherwise it is replacing the starter's control all together with up to date components which may e the best thing to do.

 

[Tony S]

The pneumatic timer in the picture was marketed by SquareD in the UK. Its a few years since I worked on one but you could still get replacement diaphragms for them.

A standard YD timer would be a straight replacement.

If you suspect the timer is faulty, stripping and cleaning is fairly easy. The usual problem spots are the miniscule air ways and bleed valve being blocked.


A bit of useless trivia:
By turning the diaphragm block around they change from delay on to delay off.

 

[templdl]

The pneumatic timer in the picture was marketed by SquareD in the UK. Its a few years since I worked on one but you could still get replacement diaphragms for them.

A standard YD timer would be a straight replacement.

If you suspect the timer is faulty, stripping and cleaning is fairly easy. The usual problem spots are the miniscule air ways and bleed valve being blocked.


A bit of useless trivia:
By turning the diaphragm block around they change from delay on to delay off.

Thanks, we need some levaty from time(r) to time(r).

 

[big john]

I appreciate the replies. I agree, dry-firing the controls would be fantastic, but these controls are a pain: They have at least two totally separate 120V supplies coming into it, and I'm not sure how or where or if they ever interact at this point. One comes from a PLC which I believe is the run command for the whole system, the other comes from somewhere else and may interact with that four-terminal circuit-breaker in the photo (why it has four terminals or what they're doing it also a mystery).

I agree the resistors most likely appear to be part of the economizers. What makes that extra fun is there are three contactors, all three have economizer circuits, but only two resistors.

As far as the operation of this system, it appears to be a bastardized Korndorfer starter, in that they are keeping an inductance in series with the motor at all times until it goes DOL. But contrary to all the literature, they do it with an open-delta autotransformer and a two-pole contactor. I've never seen this configuration before.

For some reason I can't post a picture, but here's a schematic.

In case you can't tell, there are a number of new and mysterious elements in this stupid design I've never seen before.

 

[templdl]

I appreciate the replies. I agree, dry-firing the controls would be fantastic, but these controls are a pain: They have at least two totally separate 120V supplies coming into it, and I'm not sure how or where or if they ever interact at this point. One comes from a PLC which I believe is the run command for the whole system, the other comes from somewhere else and may interact with that four-terminal circuit-breaker in the photo (why it has four terminals or what they're doing it also a mystery).

I agree the resistors most likely appear to be part of the economizers. What makes that extra fun is there are three contactors, all three have economizer circuits, but only two resistors.

As far as the operation of this system, it appears to be a bastardized Korndorfer starter, in that they are keeping an inductance in series with the motor at all times until it goes DOL. But contrary to all the literature, they do it with an open-delta autotransformer and a two-pole contactor. I've never seen this configuration before.

For some reason I can't post a picture, but here's a schematic.

In case you can't tell, there are a number of new and mysterious elements in this stupid design I've never seen before.

This is the power circuit. May you have a control diagram that includes the coils for the contractors and the timing relay to control the concoilsr coils?
This does seem to be a basic starter.

 

[SG-1]

1S,2S & R are the medium voltage contactors.

1S & 2S close together, this puts the lowest voltage on the motor. Motor begins to spin.
1S opens & the motor is connected to the 65% tap. Motor spins faster.
R closes & bypasses the transformer completely. Full voltage across motor.

Three step motor starter.

 

[big john]

...1S opens & the motor is connected to the 65% tap. Motor spins faster...

If this is a modified Korndorfer starter, then having the reactor in series with the motor is only used during the very quick time it would take to open the start contactor and close the run contactor and is just used to prevent open-transition starting. It wouldn't be used as a step in accelerating the motor.

I can't say for certain, but I don't expect to find any time delay on the closing of the run contactor.

 

[Tony S]

These were done for UK students, you’ll have to adapt the control system. A standard “off the shelf” YD timer can replace the two I’ve shown. The transition period isn’t longer than a YD.

 

[templdl]

Take a look at this schedule!matic:
http://archive.jenkins.com/jenkins/transformers/images/39p50i04.jpg

 

[big john]

The control circuit breaker (the brown thing in the original photo) and the power resistors are still throwing me completely for a loop. It turns out neither resistor was for the economizers. One is being used as short out a current transformer (I guess a piece of #10 wire would be too cheap), and the other, see below:

This is the best diagram I've made so far of how this thing operates.

For the life of me I can't see why it's necessary to electrically connect the two halves of that control circuit through that 345 ohm ceramic power resistor.

I also still can't explain why the control circuit breaker has an 4 contacts with an internal resistance of 13 ohms. Is it some type of heater overload...?