Wye Delta starter breaker trip on Startup

[NNPackerfan]

I have a Wye-Delta starter (440VAC) with a 800hp motor and am having intermittent tripping during motor starting. Motor LRA test data (Delta-Run) is 5980 amps and motor manufacturer and various publications are estimating that starting current in Y should be approximately 1/3 of the delta value. The installed breaker has a 1000Y element installed and is tripping with instantaneous set at band 3 (4000 amps) and in some cases band 4 (5000 amps), the starter is closed transition.

Have measured starting current for this application and are getting spikes that are well in excess of what would be expected even taking into consideration motor magnetization. Any ideas what could cause high starting current in this sort of application?

 

[Marke]

Hello NNPackerfan

I would suggest that there are two issues here.
1. The start current in wye connection is one third of the delta connection, so the wye current will be arround 2000 amps. The Delat start current stays very high until the motor is almost at full speed. If the transition from wye to delta occurs at less than about 90% speed, there will be a step in the start current to over 5000 amps and this could operate the breaker. Unfortunately, the torque in wye is also one third of the torque in delta and in most installations, that will not accellerate the motor and driven load to full speed, and there is nothing you can do to improve this except modify the machine to reduce the starting torque requirements. It is posible that the motor can produce enough torque, but that the time is set too short. Chech that the motor has finished accelerating in wye before the change over to delta occurs. Maybe that you need to lengthen the time a little.
2. There is a severe transition between wye and delta due to the "auto reclose effect". This can be reduced by using a correctly designed closed transition wye delta starter. Many closed transitions wye delta starters employ the correct circuit and sequence, but the resistors employed are too small and too high in value to have any real effect. The current through the resistors should be in the same order of magnitude as the current in the wye connection. This requires large resistors. You may need to look at this. If the resistors are ineefective, then you can have a very high current at changeover and this will also trip the breaker.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[jraef]

NNPackerfan,
Nothing wrong with what Marke said in item 1, but in item 2 I think he didn't notice that you altready have a closed transition starter, so the transition spike should already be taken care of. The possibility that I would check into here then is that the transition resistor bank is still good. They take a lot of abuse and are frequently the victims of improper selection of replacements. What I mean is somewhere in the past someone may have replaced it with one of dubious quality or the wrong rating, leaving you with a shorter lifespan. Another posibility is that the transition contactor is not working or has bad contacts. These are some of the main reasons why Closed Transition Y-Delta starters are frequently replaced with solid state starters.



Quando Omni Flunkus Moritati

 

[GusD]

Hi NNPackerfan

A lot of good answers to your problem or problems.
Close transition does not guaranty a smooth switch over from Star to Delta.As Marke noted,that transition as to happen at close to Sync speed or at least 90% of that.If it happens much earlier in the cycle you might have the high currents that could trip you motor.Your problem might just be related to a change in load conditions.If that is a possibility than your accelerating times will have to allow for that.Circuit problems related to the resistors is a good onre to look for,as someone already mention, they take a lot of abuse.
You don't mention if this is a new installation or is it an old installation with a new problem?
If it operated well before and just now started misbehaving,the problem could be load related.

Take care


GusD

 

[GGOSS]

Hi all,

I'll be off line until the 28th, but before I go I thought I'd contribute something to this thread that may be worth considering.

If the breaker contains a thermal release as well as a magnetic release, determining which function is causing the trip condition will assist to address & resolve the issue.

If for example, the breaker has a magnetic release only, the most likely cause of nuisance tripping is the current transient that is experienced when shifting from star to delta. This will vary in magnitude between operations but can be as high as twice LRC. Needless to say this could also imply the resistors have not been selected/sized correctly, the resitor contactor is not operating or the resistors have simply failed open. Have seen all three before!

If the breaker also has a thermal release, one would need to review the operational duty cycle of the machine. The problem may well be a simple case of too many starts or borderline time vs current characteristics.

Regards,
GGOSS

 

[jbartos]

Suggestion to original posting: Please, would you describe the motor load and post the motor nameplate data.
Also, the motor may experience inter-turn shorts within the stator windings.

 

[jraef]

GGOSS has a good point, usefull for narrowing down the possibilities. Are you sure that the breaker is tripping on instantaneous or could it be thermal? In addition to jbartos' suggestion, add the breaker info if possible.

Quando Omni Flunkus Moritati

 

[aldighe]

GGOSS, Marke and NNPackerfan

I am facing a similar problem with motor tripping on starting on the ship where I work. Only difference here is is on a DOL - direct on line starter. Motor is of 440V / 175 kW driving a centrifugal pump. Main bus bar is 6 kV and stepped down to 440V. During satring the motor is drawing excessive current causing the overload relay to operate. Also bus voltage drops to 420 V from 440 V.

Checked for the most obvious reasons for high starting current of motor such as:

Motor health- insulation resistance, coil insulation & continuity, motor bearings checked.
Pump shaft free, easily rotated by hand.
No wobble etc in shaft.
Discharge valve shut & started pump.
Assuming disch valve holed, even shut the suction valve & attempted to start pump .
Current at each phase measuring same abnormally high current
Info: The overload relay is of in line & monitors actual current in the line.
The starter is of ‘direct on line type’.
Incidentally the coupling between motor & pump removed & the motor starts very easily.
Could it be problem with AVR of the power provider ( Main propulsion diesel engine driving a shaft generator )

Kindly advise

Atul Dighe

 

[Shortstub]

1) Is this a new installation or an old installation with a new problem?

2) Although you mentioned intermittent tripping, you didn't identify whether or not the same phase(s) are involved. Are they?

3) Although you mentioned starting current measurements, you didn't identify "when" the tripping occurs! Does motor accelerate? Does protective device operate at a time coincident with transfer from wye-to-delta transition? Before? After?

4) Do not take this to mean that I impute your statement about suction and discharge valve alignment, but are you sure they were both closed? And, finally, are you sure there is no recirculation loop between discharge and suction?

 

[dpc]

Is the starter tripping on transition or at the instant of starting. If it trips instantaneously, the problem is likely the dc offset (asymmetrical) current in the waveform. The amount of dc offset depends on the phase angle of the voltage when the contactor closes, so tripping is often intermittent.

New energy efficient motors can have a higher asymmetrical current and may require instantanteous trip set as high as 13 time FLA (for across-the-line starting).

If it's tripping on transition, I basically have no idea, other than perhaps the transition is occuring too early. A soft starter would probably solve the problem, but that's a big expense.

 

[aldighe]

shortstub : From aldighe

Thanks for yr reply. Answers to your queries

1) The installation is a 20 yr old one and the problem is new. Before motor would start up on the DOL starter drawing normal starting current. Now of late is tripping off on overload as starting current is abnormally high.

2) Seems like same phases are involved

3) During starting, current very high for 4-5 sec during which period overload relay trips

4) No recirculation loop between driven centrifugal pump suction and discharge when suction and discharge valves are closed.

Hope any of you can provide some insight to causes of motors drawing high starting current on DOL starter causing Voltage dip in bus bar supply and tripping off on overlaod.
Driven centrifugal pump seems to be OK and free to rotate.

Thanks
Atul Dighe

 

[GGOSS]

Hello aldighe,

Regarding your last post; if the motor is being started DOL, then it is not possible for the motor to be drawing more current from the supply now than ever before. I would therefore ask you to confirm whether the same piece of measuring equipment is being used.

With a 20 year old plus installation, I would certainly recommend you seek the services of a specialist to check the calibration of the breaker. You may find (as I have experienced previously) that it is simply tripping pre-maturely.

Regards,
GGOSS

 

[Marke]

Hello aldighe

I agree with GGOSS. The start current drawn by the motor when started Direct On Line is a function of the motor design and the terminal voltage It is independant of the connected load. The duration of the startis a function of the inertia of the load. Loads that have a low inertia can accelerate very quickly and appear to be drawing less current, but that is due to the sloe responce of the instrumentation.
If the current has increased, then I would be looking for a beakdown in the stator insulation. The motor may be near failure. If the rotor gets damaged, the start current will go down, not up.
I would suggest that you a) verify that the current has actually gone up and investigate the stator, or b) verify that the current has stayed the same and investigate the breaker, or c) check for a longer start time and investigate the load.
Best regards,

Mark Empson

http://www.lmphotonics.com

 

[chad44]

Several years ago I had this same problem with a large chiller motor. An oscilloscope trace showed that even though we were using a closed transition starter, there was a large spike during the transition. In this case, setting the instantaneous trip up a little took care of the problem. Since that time, I have come across that problem 3 or 4 times, so I don't think it is unusual. Sometimes even the short time trip may need a little tweeking also.

 

[GGOSS]

Just thinking a little bit further on this there is one more possibility of increased starting current and this would relate to the quality of the power supply.

If when the previous measurements were taken, the power supply was soft, any voltage drop on the system would be reflected in a lower starting current.

If the power supply has been recently stiffened, the voltage drop would have reduced, this acting to lift the current towards locked rotor levels.

Regards,
GGOSS

 

[jbartos]

Suggestion to chad44 (Electrical) Jul 28, 2003 marked ///\\Several years ago I had this same problem with a large chiller motor. An oscilloscope trace showed that even though we were using a closed transition starter, there was a large spike during the transition. In this case, setting the instantaneous trip up a little took care of the problem. Since that time, I have come across that problem 3 or 4 times, so I don't think it is unusual. Sometimes even the short time trip may need a little tweeking also.
///Alternately, an adjustment of resistors used for the closed transition may be applied to lower the spike.\\\

 

[aldighe]

Hello Marke and GGOSS

Marke : The health of the motor seems OK as insulation windings resistance taken with Megger shows a good Ohmic value. Continuity of stator coils also Ok as well as phase balance.

Regarding power supply being stiffened I dont think so anyone has done that to my knowledge as GGOSS suggested as could be cause of higher current drawn towards locked rotor state.

Current measurements were taken with the same ammeter prior to the high current problem and even now and cross checked in line ammeter readings by using a clamp meter.

As mentioned earlier the load - centrifugal pup is in good condition. This problem has us really baffled on ship.


Any more inputs welcome.

Atul Dighe

 

[Marke]

Hello aldighe

Just a long shot but is it possible that this installation has static power factor correction and that the power factor correction has failed or changed? It is not uncommon for power factor correction fuses to fail and also for the capacitors to fial over a period of time. It the previous measurements were made with correction in and now you effectively have correction out, this could make the difference.
Best regards,


Mark Empson

http://www.lmphotonics.com

 

[markyspark]

with respects to ians reply, softstarts are not generally desinged with this is in mind. similar problem to you, 500KW motor, installed a soft start to limit the current on start up. at first we had problems with harmonics and the capacitor banks feeding the board. then when all the prblems were ironed out, we use around 3-4 control boards and i thyristor stack every 6 months. ok if you got money to throw at it. this is a saftronics soft start

 

[GGOSS]

Hello Markyspark,

As we are digressing from the original post, I will keep the response to issues you have raised as short as possible.

With over 20 years experience in the field of electronic motor control I would suggest repeated failure of a soft starter strongly indicates missapplication and/or inappropriate selection.

On reviewing many threads within this forum I see statements by numerous members re the suitability or otherwise of soft starters for various industrial applications, examples including gen-set p/s applications, high start torque & highly inertial application, loads requiring frequent starting, marine applications etc etc.

Although operating in a relatively small market we would distribute in the order of 4000 soft starters per annum and do experience the problems raised by others. I believe there are 2 reasons for this;

1. Our experience with and knowledge of industrial applications.
2. We do not 'compromise' to take the order, that is, we will only ever offer products that are suited to the intended application.

It is unfortunate your experience with soft start technology has not been a positive one. The question is, were the right products selected/installed?

Regards,
GGOSS