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Induction Motor Trips While Shifting Star to Delta Connection

02/05/2017 9:29 AM

hi,

today i had problem, The induction motor getting trip while switching from star to delta connection in on-load condition, we have tried in no-load condition its working fine, the motor capacity is 150KW, we have checked some parameter like amps, during no-load in star connection its taking amps is 300, 280 and 40 and go to delta connection taking of 40-50 Amps and its run normal, but while on-load during star connection its taking amps 300 to 280 and its not coming down and while it switching to delta connection the incomer breaker get trip. can anyone give me the answer why its tripping in on-load.

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#1

Re: Induction Motor Trips While Shifting Star to Delta Connection

02/05/2017 12:06 PM

Sounds like the motor is overloaded...What tests have you run so far? Have you meggered the windings? How old are the bearings, and what maintenance has been performed on this motor? Have you checked the voltage coming in? Do you have a delta leg, and is it present? Has any work been done on the power feeding this motor? Is the breaker holding to spec?

http://cr4.globalspec.com/search/cr4search?query=Motor+Trips+While+Shifting+Star+to+Delta&newSearch=new

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#2

Re: Induction Motor Trips While Shifting Star to Delta Connection

02/05/2017 12:24 PM

It's actually a very common issue with Star (Wye) -Delta motor starting and is the main reason I stopped using it 20 years ago. The short version is that there is a phase shift that takes place in the open transition and any residual magnetism inside of the motor means it is still in a state of regeneration when you reconnect, which is like connecting two generators that are out of sync; there will be a huge voltage and current spike that can damage a lot of connected equipment or if you are LUCKY, only trip a breaker or blow fuses. So to avoid that, the transition time must be lengthened to give the magnetic fields time to collapse. But because the motor is no longer connected, it slows down and in doing so, reconnecting in delta means you get the same current spike (or worse) as you would from just stating it DOL (Across the Line), albeit for a shorter duration. That too then can cause breakers to trip. Finding that perfect amount of transition time is very difficult, and it will change with loading on the motor.

The solution for decades was to go to using a closed transition starter, which adds a set of resistors in place during that transition moment, requiring a 4th contactor as well to control them, plus the resistors get very very hot, so must be designed with care.

20+ years ago the cost of a Solid State Starter went down below the cost of a closed transition Star-Delta starter and even comes close to that of open transition, so that has been the solution of choice for me.

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Anonymous Poster #1
#3

Re: Induction Motor Trips While Shifting Star to Delta Connection

02/06/2017 3:49 AM

Oh, your problem we faced with Italy Star Delta

motor starter in deep turbine pumps 26 years ago.

and the other one I solved in 2015. I found main

root causes are

1- design of circuit diagrms.

2- Voltage drop during transition to delta.

3- Quality of contactors to hold full magnetic load .

4- Other sensitive electronic relays tend to open the

control circuit at transition.

5- Quality of timer.

The world is using many varieties of star delta motor starters without having no Timing Diagrams. If we have , we can diagnoise which portion is going wroung . Yes, But It is very difficult or consumes much time to design out.

In my experience the answer is modifying the control circuits to suit local parameters.More comlicated circuit has more chance to offer problems for Electrical Engineers.In many cases modification or simplifying control circuits sometimes give us time saving , can get new ideas that enhancing innovations , I think.

It is very simple to match in sequence of rotor voltage and delta supply voltage in a star to delta transition in a fraction of second.

Why we face no problem using Star Delta Motor Starter with manual change mode?

Best wishes.

You will get soon , I dare say.

UKZM , Electrical Engineer , Myanmar.

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Anonymous Poster #1
#4

Re: Induction Motor Trips While Shifting Star to Delta Connection

02/06/2017 4:20 AM

Hi !

Your problem may be in Circuit design.

Matching of Rotor circuit voltage and delta

voltage in a fraction of a second in Star Delta transition is necessary.

Best Regards.

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#5

Re: Induction Motor Trips While Shifting Star to Delta Connection

02/06/2017 3:36 PM

It is because the controls upgrade timer has elapsed.

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#6

Re: Induction Motor Trips While Shifting Star to Delta Connection

02/06/2017 10:05 PM

Sudarshan,

Nameplate data on the motor, please.

Type of load.

History.

This type of information will help us to understand the problem. Impossible to do without it because we are not at your site. Mr. J-Raef has given good advice regarding the use of "soft starters" that ramp the voltage up over a programmed time interval.

--John M.

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#7

Re: Induction Motor Trips While Shifting Star to Delta Connection

02/07/2017 9:07 AM

Like the others have replied, it's always helpful if you can provide more detail: Line Voltage, motor nameplate, what the load is, how old is the system, and who is the manufacturer, were there any recent changes, etc. You also said "today" you had a problem. That tells me that the control was working ok before. The immediate question then, is "What changed?" My previous background was in heavy industrial manufacturing and maintenance. In one plant, we had a large compressor which had a wye (star)/delta starter. It was a huge panel with a mechanical arm. We would start the motor with the arm (handle) in the down position, start the motor and listen for the motor to rev up to speed, then quickly throw the switch (lever) to the up position for the motor to run continuously. This is analogous to an electrical wye/delta auto starter and from what you posted, it does sound like a timing issue. In another situation, we had a large punch press motor that was new and drawing so much current that the starter overloads were turning cherry red! Turns out the replacement motor had the wrong "slip" rating. Perhaps it's time to upgrade? I do agree that JRaef has a great post. Best regards, JPS

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#8

Re: Induction Motor Trips While Shifting Star to Delta Connection

02/08/2017 11:21 PM

Send us your following details: a) Main Breaker Current Capacity. b) Star to Delta Timer setting. c) What load does your motor connected to? d) it is direct connected to the load or by Pulley and Belts. e) Motor Name Plate details.

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#9
In reply to #8

Re: Induction Motor Trips While Shifting Star to Delta Connection

02/12/2017 6:53 AM

a) Main Breaker Current Capacity : 400 Amps

b) Star to Delta Timer setting. 20 to 30 Seconds

c) What load does your motor connected to: This is fire fighting system electrical pump its connected to pump

d) It is direct connected to the load or by Pulley and Belts: Directly connected to the load with coupling

Name plate details:

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#11
In reply to #9

Re: Induction Motor Trips While Shifting Star to Delta Connection

02/13/2017 9:58 PM

Now we have all the details and I can tell you what the actual problem is. I have faced this problem before with our Rolling Mill water supply pump as the project is not design by me.

First of all I will not use an electrical induction motor with 2970 or 3000 rpm to run my water pump. We know that this high speed induction motor can increase the water pressure and flow rate but the torque of the motor are reduce. For the any water pump, we prefer to use an induction motor that is running at 1500 rpm and not 2950 rpm.

This is what you have to do to start these high speed motor for water pump and also it apply to all the water pumps induction motor with 1500 rpm .

1) Fully open your inlet water valve.

2) Close the outlet water valve to about 90% leaving about 10% open. Or you can fully close the outlet valve. Your motor will start as NO Load starting.

3) Now start the water pump and try to hear the induction motor full speed sound. ( other method is that you can use a tachometer with laser to detect the full speed of the motor in Start Point)

4) See if the Star Delta timer setting time are correct or not. (20 to 30 second are seem to be too long time. You may need to adjust back the time to about 13 sec to 15 sec)

5) If the timer are too long then set back the timer to the correct time for the starter to change to delta.

6) When all the above are set and correct done then restart back water pump again as follows;

a) Fully open your inlet water valve.

b) Close the outlet water valve to about 90% leaving about 10% open. Or you can fully close the outlet valve. Your Induction motor will start as NO Load starting.

c) Start the induction motor and observe all the starting parameter and condition including the starting amp, water pressure, motor starting sound and e.t.c.

d) When the motor had change from Star to Delta then slow open the outlet water valve and observe the motor running current.

e) Ensure that the motor running current are not over the motor rated current.

f) Ensure that the motor running current are not over the main MCCB rated current

Starting Water pump with fully open outlet valve.

This phoneme of the main MCCB tripping is when your Star Delta Starter changes to Delta are due to the pressure and resistance inside your fire hydrant water pipe line. Your fire hydrant water pipe line are basically a close type system where, if any air that is trap inside the fire hydrant pipeline the air are release thru the air release valve located at the top of the pipe line.

Your fire hydrant water pipe line must maintain the pressure at about 8 to 9.5 bar at all time. If the fire hydrant water pipeline water pressure drop to 7 bar your water pump will start automatically without any problem. The remaining pressure inside the fire hydrant pipe line will act as back pressure to your outlet valve or in other words, it act as your outlet valve is close at 80%.

In other hand, if your fire hydrant water pipe line pressure drop below 5 bar or lower you will loss the back pressure to your outlet valve. This means that you are starting with a fully open outlet valve. This situation will cause your motor current to increase as the speed of the water pump decrease due to the losses in the running torque to push the water in to your fire hydrant pipeline and during this situation, when your Star Delta Starter changes to Delta Point your Main MCCB will trip.

It happen in here too. So I come out with this procedure for starting the water pump.

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#12
In reply to #9

Re: Induction Motor Trips While Shifting Star to Delta Connection

03/19/2017 10:28 PM

I have been away for a few weeks. In the USA you would have a serious problem with the size of your incoming main breaker and possibly with the size of your starter. For fire pumps the incoming main breaker and starter are supposed to be sized to carry the locked-rotor current of the motor for a VERY long time (indefinitely). This is because the risk of damage to a building's contents from a fire when the pump controller has malfunctioned is much greater than any real or potential damage to the fire pump due to its failure to start.

This appears to be a poor match between the type of starter and the motor, with the motor's torque output at the starter's wye voltage being too low to get the motor up to speed before the starter transitions to delta. I would recommend replacing the incoming breaker and starter with ones specifically designed and sized for this application.

--JMM

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#10

Re: Induction Motor Trips While Shifting Star to Delta Connection

02/13/2017 11:10 AM

In my opinion, at the first moment of energizing [ at star connection] the motor will draw about 700 A and has to get down to 70-90 A at no-load.

In delta connection on no-load the current eventually gets down to 50 A.

A short shock of 700-800 A in the transition time could be, but the breaker does not trip.

In delta connection, on full load, the line current has to be close to 280 A.[247*1.15??]

If the motor succeeds to run in delta connection for a while and trips after then the problem could be a large sudden load or supply voltage interruption [ or an intermittent short-circuit in stator windings or rotor bar].

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