Star Delta

[SUNRAY]

Hi Guys

I rebuilt a couple of 21.5A star deltas today and now I'm confused.

I placed the O/L after the main but it tripped during the star cycle so moved it to the delta contactor.

As these take a long time to get to speed I set the timers to 12 seconds, when they changed over the 32A mcb tripped despite the fact that they would start in delta without the star cycle. After much messing about the time was reduced to 6 seconds and now they start ok.

What have I missed?

 

[DaftPunk]

How big is the motor? What tripping curve is the circuit breaker?

 

[ericmark]

In the old days the star/delta contractor was physically interlocked but today often they are only electrical interlocked if the contractor sticks then it will produce a short circuit I would guess the longer time means it sticks but short time does not. It may be a fault with the aux contacts.

 

[SUNRAY]

In the old days the star/delta contractor was physically interlocked but today often they are only electrical interlocked if the contractor sticks then it will produce a short circuit I would guess the longer time means it sticks but short time does not. It may be a fault with the aux contacts.

These are physically and electrically interlocked with all brand new ABB parts including dedicated ABB timers (spec of 20ms between changeover I think!). It feels like 1mm of travel between the points where the power contacts open and the aux closes on the contactors. I don't see any unexpected shorts with a buzzer

How big is the motor?

I rebuilt a couple of 21.5A star deltas

So I assume they are 10KW

What tripping curve is the circuit breaker?

32C, which I appreciate is a bit near the mark but they are original which was in service for several years AND they will start in delta mode without tripping.

To check the operation I tried a 1.5KW conveyor and a 4C mcb without it tripping on any time between 0 and 20 seconds.

 

[SUNRAY]

Forgot to mention there has been no doubt about rotation direction.

 

[DaftPunk]

How on earth can you assume the motor size? What does it actually say on the rating plate of the motor?

What made you think the starter needed rebuilding in the first place? Did it all work before you started tinkering?

 

[SUNRAY]

How on earth can you assume the motor size? What does it actually say on the rating plate of the motor?

What made you think the starter needed rebuilding in the first place? Did it all work before you started tinkering?

Daft questions again

My work involves this sort of thing on a daily basic and I don't go into it lightly.

The motors plates are buried deep in the machine and not visible, the machine manufacturers plate lists voltage 380-415V and current 22-20A but not power, this tallies exactly with the list on the workshop wall for a10KW motor and a 10KW motor on the same site so yes I on earth I can make the assumption.
Next stage, why on earth do I need to know the power rating when I am designing the system to the machine manufacturers spec and the measured current?

The machine and its control panel have just been moved and as part of the move the control gear was changed due to the customers policy of using all 24V for control. Sadly it was not I that removed the old gear and my first view of the panel saw 42 black wires (power) and a few red wires (control) hanging out of the containment. At that point the customers own electrician realised he was out of his depth.

Yes it did work before it was tinkered with and yes it works now running at 21.5A +- 0.1A per phase per motor and 12.5A +-0.1A per winding per motor, so I believe its running correctly.

 

[ericmark]

I would love to know the answer. In theroy I see no way a motor run on reduced voltage could draw more current than on run voltage so we must look at what it is driving.

I do remember problems with pumps in that swapping star/delta early did so before the pump had built up a head of water. Also refrigeration again very short time before the gas pressure had time to built up.

I also remember using a soft start with a hydraulic pump and had to alter the PLC program that opened a dump valve while the motor started. This lining up of mechanical and electrical can be a real pain had a harmonic arm and that really caused problems in the end I used a unit complete where motor, gearbox, brake, inverter drive, and ASii control were are combined in one unit it was only way to get brake to work at the correct time.

I would assume if there was a brake involved you would say however possible you don't even know it's there in a large machine.

On the travel for a crane there was a hydraulic drive designed to remove the start torque with these only option direct on line to add any electrical soft start except for full inverter would have defeated the whole idea of the hydraulic drive. (really a fluid flywheel)

So I would in light of new information be looking and a mechanical answer to your question.

 

[SUNRAY]

I would love to know the answer. In theroy I see no way a motor run on reduced voltage could draw more current than on run voltage so we must look at what it is driving.

I do remember problems with pumps in that swapping star/delta early did so before the pump had built up a head of water. Also refrigeration again very short time before the gas pressure had time to built up.

I also remember using a soft start with a hydraulic pump and had to alter the PLC program that opened a dump valve while the motor started. This lining up of mechanical and electrical can be a real pain had a harmonic arm and that really caused problems in the end I used a unit complete where motor, gearbox, brake, inverter drive, and ASii control were are combined in one unit it was only way to get brake to work at the correct time.

I would assume if there was a brake involved you would say however possible you don't even know it's there in a large machine.

On the travel for a crane there was a hydraulic drive designed to remove the start torque with these only option direct on line to add any electrical soft start except for full inverter would have defeated the whole idea of the hydraulic drive. (really a fluid flywheel)

So I would in light of new information be looking and a mechanical answer to your question.

Nothing complex in this machine, its just the fan motors for a dust extractor.
On a reduced voltage a motor tries to run slower and therefore not in sync with the mains, in which case it will draw more current. Think of it as the start up inrush has not yet dwindled down to the running current because it has not reached running speed. A lot over simplified but I hope it helps

 

[FDJ]

the 32A mcb tripped

Is this the problem?
Motor overloads have a time delay, MCB's are fast acting.

Frank

 

[SUNRAY]

the 32A mcb tripped

Is this the problem?
Motor overloads have a time delay, MCB's are fast acting.

Frank

It could be but I think not as the motors will start in delta mode ie removing the star point link. I have the option of changing the mcb's to 32D but I'm reluctant to make changes to it.

 

[SUNRAY]

Just bumped into this old thread and thought I'd give an update.

I have run into similar problems several times since this incident and ended up looking at a 55KW motor with a oscilloscope. What I found in the end was quite simple and obvious: when the star opens, if the motor is running near it's final speed, the back emf from the motor is 180' out of phase with the mains supply and reducing the changeover to under a second was needed to prevent too much emf being generated.

 

[ericmark]

I believe you, however I have worked with many star/delta, auto transformer, and resistor starters and never found that problem. However I have had some odd faults which did not seem to make sense. One case was a crushing plant used once every blue moon to crush rock to repair a road. The motor was tripping out, but tests on the motor showed no fault. However my test gear could not detect something like a short between wires in the same winding so there was a slight chance the motor was at fault, what I wanted was a way to measure the torque.

The mechanical engineer arrived on site and asked what the problem was, it was explained, and much to the dismay of the fitter he directed that the bearing cover should be removed and two hands full of grease removed and re-try and see if any better. Both myself and the fitter thought this was a waste of time, but he was the boss, so block and tattle was rigged the cover lifted and two hands full of grease removed. One getting everything bolted back down and re-trying it actually worked.

When the mechanical engineer returned we asked how he knew it was the problem, answer I have had it before with infrequent used machines, the operator always greases when he has finished but it does not run long enough to dispel that grease.

In your case with fans the load is unusual fans being free to run but air pressure likely to build up similar to how I found it with the water pumps, a quick change over happens before there is a back pressure, a slow change over and a back pressure has built up.

With large pumps we did not use star/delta, mainly resistor start was used, often 5 stages. So there were 4 large resistors, on each winding, on start all four are in circuit and then four contractors one by one short out the resistors, this means at no time is power removed, so back pressure can't cause the motor speed to suddenly change, I had never really thought about this being a reason for resistor start, to me it was just a more gentle start to star/delta but we often had to change resistors with crushing plants where it had stopped full and the operator had tried to restart it without empting it first.

Today electronic drives have replaced most of the star/delta starters and they are becoming museum pieces. Specially those with dash pot overloads.

 

[SUNRAY]

Eric
I have been building and commissioning control panels as my main source of work since 2002. Prior to that I had my fair share and if I had to give an estimate of the number of 'starters' I have worked on, I'd have to say thousands. With that, I have to agree with everything in your post.
Quite frankly prior to my OP I had not really had any problems of the type described and it got me scratching my head. Since then I have run into it several times, the 55KW fan was the one that gave me the most trouble and one I was able to keep visiting to play (final timer setting is well under 1 second). I wired it as a straight forward DOL with the star point connected to neutral. The most useful test was monitoring the voltage each side of one pole of the contactor and the moment it opened the motor side instantly changed polarity (sometimes with a massive spike well in excess of 2000V)
As such I found that the theory of how a motor works as a generator was absolutely correct in as much as it's generated output is out of phase compared to running as a motor. I have also looked at a couple of multi winding transformers and found that to be just as true.

I have always liked the way resister start works but interestingly any multi stage starters I have built or knowingly worked on have the resisters switched in parallel and more get switched in to reduce the resistance rather than shorting sections out. I have also even seen it done with three armoured cables of differing sizes between the starter and motor. Skinning cats comes into my mind here. But then how many ways have you seen of wiring the control side of a star/delta system?

One that really got my cogs whirring was a fault on a star/delta reversible fan that could run at 2 different speeds by taps on the windings. If I remember correctly, each fan had 8 or 9 contactors and took up a whole row in a floor standing panel.

Dash pot timers, eek , yuk. Never ever installed one, had many free issued as part of 'standard package' but they always went straight in the bin, terrible things.

 

[Nozzle]

A very interesting read chaps, nice work.

Nozzle