AutoTransformer Motor Starting

[mull982]

I am using an AutoTransformer to start a 300hp 4160V motor. The FLA of the motor is 41A and the LRC of the motor is 256A. The AutoTransformer is tapped at 50% voltage and the time between switching from Autotransformer contacts to across the line contacts is 9 seconds.

I have recently had some trouble when trying to start this motor on several occasions. The motor relay keeps tripping on overload and I am having trouble understanding why.

The one thing that I have noticed is that when I start the motor, the current draw is 153 A for the first nine seconds while the motor is running across the autotransformer at 50%. However after nine seconds when the across the line contact kicks in the current draw goes up to 324A, and the motor is tripping out due to the fact that this is 9X FLA.

The thing that I dont understand is why we are seeing 324A on the start when the LRC of the motor is only 256A. Also I dont understand why we are seeing this current while we are reduce voltage starting the motor. The starting acceleration of the motor is about 13.5s. Any reason why we are seeing this high current when the across the line contact kicks in?

 

[weressl]

I am using an AutoTransformer to start a 300hp 4160V motor. The FLA of the motor is 41A and the LRC of the motor is 256A. The AutoTransformer is tapped at 50% voltage and the time between switching from Autotransformer contacts to across the line contacts is 9 seconds.

I have recently had some trouble when trying to start this motor on several occasions. The motor relay keeps tripping on overload and I am having trouble understanding why.

The one thing that I have noticed is that when I start the motor, the current draw is 153 A for the first nine seconds while the motor is running across the autotransformer at 50%. However after nine seconds when the across the line contact kicks in the current draw goes up to 324A, and the motor is tripping out due to the fact that this is 9X FLA.

The thing that I dont understand is why we are seeing 324A on the start when the LRC of the motor is only 256A. Also I dont understand why we are seeing this current while we are reduce voltage starting the motor. The starting acceleration of the motor is about 13.5s. Any reason why we are seeing this high current when the across the line contact kicks in?

Open transition could cause the motor slip out of phase.

This seems like such a small load on 4160V that normally would not any special starting method. You must have a very "weak" system.

 

[mull982]

Open transition could cause the motor slip out of phase.

We are using an Closed Transition arrangement

 

[weressl]

We are using an Closed Transition arrangement

If the change occured lately and the motor did not exhibit this behavior before then look for what changed. Make sure that your timer setting did not change and your controller indeed does perfomr the closed trasnition and you are not experiencing a contact "race".

 

[hdpeng]

Tap/Torque/Alignment

Tap/Torque/Alignment

Some things to consider:

1) Have you tried using a 65% tap instead of 50% (or the next one up from 50% as applies to you equipment)? Depending on the load, the shift from 50% to 100% may be too great. I am assuming you are only using 2 steps. Do you have the capability for an intermediate step between 50% & 100% in the controller?

2) Are you sure the mechanical load on the motor is not slightly oversized (could be due to torque characteristics of the motor, aging of motor bearings, degradation of the loads bearings, etc.) Try doing a thermography test on the motor and load bearings.

3) Is the motor shaft properly aligned with the load? What type of coupling do you have?

What type of motor is it, and how old?

 

[mull982]

If the change occured lately and the motor did not exhibit this behavior before then look for what changed. Make sure that your timer setting did not change and your controller indeed does perfomr the closed trasnition and you are not experiencing a contact "race".

weressl

I want to correct my previous statement. I belive the arrangement is an open transition arrangement b/c it is one switch that is switching between the autotransformer and run contacts. When the motor is first started the switch is in position to energize the autotransformer contactor and then after the relay times out the switch then switches over to energize the run contactor. I'm assuming the time that it takes the switch to move from one contactor to the other is the open transition that you are referring to? What is the best way to correct this situation?

This is a new installation, that I am involved with.

 

[kingpb]

Typically, when using an autotransformer for starting you switch over to across the line when the terminal (system) voltage recovers to about 90 - 95%, not based on some time sequence. I agree here with Laszlo that 300Hp at 4160V is pretty small for 4160V. Across the line starting should not be an issue at all.

You need to look at the motor torque vs. speed curves, and the load torque curves. Make sure you have enough motor torque capability to overcome the load, specifically at the point in which your are switching to full load. From what you describe I'd say you will probably need to increase the time before switching. Don't forget that at the moment you switch. you will see a very momentary dip in system voltage before the motor takes off to full speed.

Second issue: was the motor manufacturer aware that you were going to start using an auto-transformer?

Third - Check the motor damage curves. It could be that you are taking to long to accelerate the motor, and you are hitting the motor thermal damage curve. You may need to simply change the taps on the autotransformer to the 65% instead of the 50% tap. Also make sure the relay is set according to the damage curve. It may not have been set properly.

If you check these things, most surely you should resolve the issue.

 

[mull982]

Typically, when using an autotransformer for starting you switch over to across the line when the terminal (system) voltage recovers to about 90 - 95%, not based on some time sequence. I agree here with Laszlo that 300Hp at 4160V is pretty small for 4160V. Across the line starting should not be an issue at all.

I am not sure why AutoTransformer starting of this motor was chosen, it was engineered by others and I will have to ask them for the reason for this design choice.

You need to look at the motor torque vs. speed curves, and the load torque curves. Make sure you have enough motor torque capability to overcome the load, specifically at the point in which your are switching to full load. From what you describe I'd say you will probably need to increase the time before switching. Don't forget that at the moment you switch. you will see a very momentary dip in system voltage before the motor takes off to full speed.

I have these curves that you are referencing but am not sure what to look for on these curves to determine when there is enough motor torque capability to overcome the load as you referenced. What would increasing the time before switching do?

Second issue: was the motor manufacturer aware that you were going to start using an auto-transformer?

No I dont belive that the motor manufacturer was aware of this design decision.

Third - Check the motor damage curves. It could be that you are taking to long to accelerate the motor, and you are hitting the motor thermal damage curve. You may need to simply change the taps on the autotransformer to the 65% instead of the 50% tap. Also make sure the relay is set according to the damage curve. It may not have been set properly.

I have verified that the relay is set acoording to the damage curves. Does longer acceleration of the motor result in more thermal stress? Above you recommended increasing the acceleration time. What effect would increasing the voltage tap have?

If you check these things, most surely you should resolve the issue.[/QUOTE]

 

[weressl]

Some things to consider:

1) Have you tried using a 65% tap instead of 50% (or the next one up from 50% as applies to you equipment)? Depending on the load, the shift from 50% to 100% may be too great. I am assuming you are only using 2 steps. Do you have the capability for an intermediate step between 50% & 100% in the controller?

2) Are you sure the mechanical load on the motor is not slightly oversized (could be due to torque characteristics of the motor, aging of motor bearings, degradation of the loads bearings, etc.) Try doing a thermography test on the motor and load bearings.

3) Is the motor shaft properly aligned with the load? What type of coupling do you have?

What type of motor is it, and how old?

All good suggestions, but I would refer to Ockhams razor.

Rewire it to direct, accross the line start.

 

[wirenut1980]

All good suggestions, but I would refer to Ockhams razor.

Rewire it to direct, accross the line start.

Before I did that I would check to see why an autotransformer soft start was installed in the first place.

 

[hdpeng]

Careful

Careful

Before I did that I would check to see why an autotransformer soft start was installed in the first place.

I have to agree with you on this. I think most of us are assuming the autotransformer was warranted to begin with. Also, if you're getting overcurrent situations with the shaft already having some rotational momentum, in theory you should experience even higher currents trying to connect to full voltage from a dead start.

 

[frenchelectrican]

one thing it kinda bother me why 9 seconds delay between the autotransformer and swiched to the DOL [ direct on line ] connections but i have a hunch but not sure it may ring in here right.

with the long delay the motor speed do drop down a bit and get the phase connect out of sych. by time the DOL contractors get on line and it draw just much current as plain jane DOL set up.

i will just suggest try to closen up the gap time from 9 seconds to something like 5 seconds or so. the reason i suggest that because it will really reduce the amout of the time the motor loose the speed when switching over.

Merci,Marc

 

[mull982]

I have extended the AutoTransformer to run time in increments all the way up to 25s and still am seeing the same result each time. The motor is building up to about 97% thermal capacity each time it is started. The current and thermal capacity seem to be fine up until the point when the motor switches to across the line, when everything goes high. Does anyone know why this is even at 25s delay?

I am going to change the taps from 50% to 65% tomorrow and see if I get better results during starting. I have not been able to get a hold of the engineer yet to verify why he chose and autotransformer start.

 

[wirenut1980]

Wow, 25 seconds and still happens. I think we can rule out plain old starting current. :smile: Have you gotten the manufacturer of the autotransformer involved yet? Something is going on with the switching over, but I can't imagine what exactly is happening here. Do you have capability to do some monitoring and take a look at the current and voltage waveforms during the switchover?

 

[RHJohnson]

Do not know your application/load. But the last soft start I did was on a 300 hp /4160 VAC. It had a pretty good load starting up, and so it took 70 seconds before transfer to run! I too think the 65% tap may help.

 

[Jraef]

Autotransformer starting is about reducing the starting kVA; current is relevant only as a component of that. 300HP is still 300HP, and that is a lot of power. It is not at all unusual to see a requirement for reduced voltage starting on motors that size. It also reduces the torque (to some extent), so changing to Across-the-Line should not be done without careful investigation and consultation with the original design engineer.

That said, forget for the moment your starting transition time, current, thermal capacity etc. as a transition decision. There is only one valid consideration;
The motor MUST get to at least 80% speed (85% or more is better) before transition or you will experience this issue. Anything less and the flux penetration on the rotor is so low that a transition to Full Voltage will be essentially the same as a full Across-the-Line start, just of shorter duration because the motor is already spinning.

So with that in mind, IF you have the taps set to where the motor torque is so low that you cannot create enough torque to accelerate it to >80% speed, then the transition time becomes irrelevant, except in that you are consuming more and more of the thermal capacity. In other words it will always be a problem. You may dance around the edge of the OL curve and be successful once in a while, but as bearings age etc. and it gets harder to start, the nuisance tripping will get worse and worse.

Changing the taps to 65% may be the answer. But as others have mentioned, first you need to determine why the original design called for an RVAT with 50% taps. If it was a conscious well thought out decision based upon a set of known circumstances, then messing with that should not be taken lightly. For instance someone may have performed what is called a Transient Motor Starting (TMS) analysis which takes into account the available system capacity, motor torque capability, allowable voltage drop, load torque-speed curves etc. etc. etc. and determined that the only thing that will work is an RVAT on 50% taps. Changing to 65% may have other undesirable consequences.

If on the other hand, the decision to use the 50% taps was done willy-nilly because nobody knew one way or the other, then 65% or even 80% may be just dandy.

Read this paper on RVAT starting. In it, the author describes the difference between open and closed transition. From this you should be able to determine what your sequence is. If you have open transition, that is another factor interfering with your success and really should be corrected. It can be done in the field, but needs to be done by someone who knows what the issues are. It's a little too complex to be done via the internet.

http://www.lmphotonics.com/m_start.htm#AutoTransformer

 

[mull982]

Autotransformer starting is about reducing the starting kVA; current is relevant only as a component of that. 300HP is still 300HP, and that is a lot of power. It is not at all unusual to see a requirement for reduced voltage starting on motors that size. It also reduces the torque (to some extent), so changing to Across-the-Line should not be done without careful investigation and consultation with the original design engineer.

That said, forget for the moment your starting transition time, current, thermal capacity etc. as a transition decision. There is only one valid consideration;
The motor MUST get to at least 80% speed (85% or more is better) before transition or you will experience this issue. Anything less and the flux penetration on the rotor is so low that a transition to Full Voltage will be essentially the same as a full Across-the-Line start, just of shorter duration because the motor is already spinning.

So with that in mind, IF you have the taps set to where the motor torque is so low that you cannot create enough torque to accelerate it to >80% speed, then the transition time becomes irrelevant, except in that you are consuming more and more of the thermal capacity. In other words it will always be a problem. You may dance around the edge of the OL curve and be successful once in a while, but as bearings age etc. and it gets harder to start, the nuisance tripping will get worse and worse.

Changing the taps to 65% may be the answer. But as others have mentioned, first you need to determine why the original design called for an RVAT with 50% taps. If it was a conscious well thought out decision based upon a set of known circumstances, then messing with that should not be taken lightly. For instance someone may have performed what is called a Transient Motor Starting (TMS) analysis which takes into account the available system capacity, motor torque capability, allowable voltage drop, load torque-speed curves etc. etc. etc. and determined that the only thing that will work is an RVAT on 50% taps. Changing to 65% may have other undesirable consequences.

If on the other hand, the decision to use the 50% taps was done willy-nilly because nobody knew one way or the other, then 65% or even 80% may be just dandy.

Read this paper on RVAT starting. In it, the author describes the difference between open and closed transition. From this you should be able to determine what your sequence is. If you have open transition, that is another factor interfering with your success and really should be corrected. It can be done in the field, but needs to be done by someone who knows what the issues are. It's a little too complex to be done via the internet.

http://www.lmphotonics.com/m_start.htm#AutoTransformer

Jraef

You are correct with the 50% tap chosen willy-nilly. The engineering design called for an 80% tap max, but the unit came from the factory tapped at 50% and was just left at 50%. Therefore there are no design reasons holding us back from changing the taps to 65% or 80% and therefore I am going to change them to one of these taps. I will let you know how I make out.

Is there anywhere I can read up on some more information regarding the 80% speed issue that you spoke about above?

 

[RHJohnson]

So you are finding you did not follow through on your job. The auto starter is come from the factory at 50% tap - this is normal. Did you check the tap setting before trying to run the motor? And did you check specifications and find what the tap was suppose to be? You are making a mountain out of a mole hill for failing to do proper start-up procedures. It is necessary to check all this in a normal course before ever energizing anything.

 

[hdpeng]

So you are finding you did not follow through on your job. The auto starter is come from the factory at 50% tap - this is normal. Did you check the tap setting before trying to run the motor? And did you check specifications and find what the tap was suppose to be? You are making a mountain out of a mole hill for failing to do proper start-up procedures. It is necessary to check all this in a normal course before ever energizing anything.

Give him a break! If he had known all that, he wouldn't be using this website. Come on people, be constructive, not destructive.

 

[Jraef]

Snip...
Is there anywhere I can read up on some more information regarding the 80% speed issue that you spoke about above?

I don't know for sure. I learned that under the tutelage of an old Westinghouse large motor engineer years ago while doing research on solid state starting for medium voltage motors. It may be in one of the many IEEE books and papers on large motor starting, but I honestly haven't spent much time thinking about it.