RVAT starting question for large well motor 3

[bimr]

I have a new installation of a water well pump rated at 450 HP, 2300 V. We are having problems getting the pump going. The pump starter (RVAT) fails on pump acceleration and shuts down. The shutdown internal by the controller is set at 5 seconds.

Utility power is 12,000 Volts. The utility then has a 480 V KVA transformer to power the entire site. In the building, a 500 KVA step up transformer is used to step up the power to 2400 V for the well pump. This in turn is connected to an RVAT.

I also have a 750 KVA 480 V. generator that is not completely installed, but will be finished shortly.

Several questions:

1. Why won’t the pump start?
2. Are the transformers (s) too small?
3. Is the utility transformer too small?
4. The electrical engineer stated that the system gear that is used is 480V was because of the high cost for 2300 V gear. Is that as valid statement?
5. Is the pump starting time too short?

Thanks

 

[itsmoked]

Does lighting on the 460V system dim appreciably during a start attempt? This could show the 460V is already loaded up and your subsequently get two transformer drops on starting.

If there is no noticeable dimming/voltage drop in the 460V then that can be discounted and focus can be put onto the 2300V circuit only.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[itsmoked]

Do you know anything about the flow? Does any start?

Specifically an RVAT is a Reduced Voltage Argyle Toadstool?

We can't comment on the utility transformer as you haven't stated its KVA rating.

Have you measured the 2300V on a start attempt? This will tell you if the the pump transformer is an issue.

Warning: This motor will have mandatory start attempt rest periods you don't want to exceed!

Keith Cress
kcress -

http://www.flaminsystems.com

 

[electricpete]

Tough to tell from this end.

I think you will need to do some analysis or get with whoever did the design.

You have no less than 3 transformers between the 12kv and the motor... was the voltage drop of each of them accounted for?

One piece of info that will be helpful to anyone analysing the situation is to measure the actual voltage at the motor terminals during start. If can't safely measure this, can estimate this by measuring current and back-calculating voltage. While you're at it,check if your currents are balanced.

Once you know the voltage, get with the OEM or the designer and figure out how fast you expect the motor to accelerate with this voltage.... compare that to your trip settings etc.


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[dpc]

Tap setting of starter autotransformer?

I agree with electricpete, the most critical info needed is the voltage at the motor terminals. If this is not feasible, measure voltage at the output of the RVAT starter while starting.

If the only reason for the RVAT is to reduce voltage drop seen by the rest of the system, you could bump the tap up on the starter.

With all those transformers, you already have a built-in reduced voltage starter. I suspicious that the motor is just not getting enough voltage.

 

[bimr]

The well pump is brand new and was meggered to see if that was a possible problem. The pump seems to be fine.

The lights do not noticeably dim when the pump start is attempted. There are also no other electrical devices connected on the site.

The RVAT has an initial voltage line side of 2550V and then drops down to around 2000V when attempting to start the well motor.

The incoming line feed of the utiity transformer is 500V and then drops down to 443V when attempting to start the well motor.

 

[bimr]

The utility transformer is a 500 KVA oil filled transformer. The step up transformer is also 500 KVA, Dry Type.

I was told that it is easier to step up the voltage than to step down, so that the utility transformer would be more likely to be the limiting factor.

 

[dpc]

Step up, step down, the transformer doesn't really care - flux is flux.

If the voltage into the utility transformer goes for 500 V to 443 V on starting, that is, all by itself, a 13% voltage drop.

I'd make sure your RVAT is set on the 80% tap.

What is the basis for the 5 second timer that you say is tripping the motor?

When is the starter transitioning to full voltage?

 

[bimr]

The basis for the 5 second timer is a controller that is tripping. The controller has an error message indicating that the pump is not acceleratong and so it trips out after 5 secs. I understand that this controller is a multilin 465 proective relay.THe controller is installed in the RVAT.

The RVAT is on the 80% tap.

I would add that the controller reading does not appear to be showing that the amps are dropping significantly during the pump start.

There is never a tranistion to full voltage since the controller is tripping out.

 

[dpc]

Multilin 469

If the starter is using the 469 to make the transition to full voltage, you need to talk with whomever did the 469 settings. There is nothing sacred about 5 seconds.

The 469 has some options for the reduced voltage starting transition:

current
current or timer
current AND timer

It could be that the 469 is set to transition on current drop-off and this is not happening within the 5 second window that the relay is set for.

I would consider changing the 469 to allow transition on current OR timer. This will eliminate the Incomplete Sequence trip you are getting and at least let you get the motor started and figure out how long it is going to take to accelerate.

If this is a new motor, you should track down the motor data sheets and see what the expected acceleration time at 80% voltage is. If your actual voltage is less than 80%, which it probably is, acceleration will be slower.

Remember, the torque varies with the square of the voltage. So at 70% voltage, you will only have 49% of the normal torque.

Five seconds just may not be long enough in this particular situation.

You may want to consult with motor supplier and explain the situation and see if they have any guidance on how long you can safely drag out this acceleration before the motor gets too hot.

And keep in mind itsmoked's good advice on limiting the number of motor starts while you're getting this sorted out.

Good luck,

Dave

 

[dpc]

BTW, if this is a submersible pump, you really need to get the motor/pump supplier involved. NEMA motor standards do not apply to submersible motors.

 

[jraef]

I wholeheartedly agree. You need to get the motor control designer AND the pump supplier involved in this. One of the limitations of an RVAT starter is the transformer itself. The 5 seconds may be the thermal limit of what the transformer can take; I have never seen one that can take more than 15 seconds. So that 5 second "Incomplete Sequence Timer" in your protection scheme may be taking that into account. I would not mess with that until you know that for sure.

As dpc alluded to, the challenge here is that the current will not drop until the water gets to the top of the well. How long that takes is a function of the depth (head) of the well. It could be that your well is so deep that you cannot get the water to the top in 5 seconds, meaning that you may need to look at that time setting. But you still should make sure your RVAT starter has transitioned to Run in an amount of time that it can take (thermally) and no more. Transition on Current almost never works for a deep well pump; you will likely need a hard time value.


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[rockman7892]

I have recently struggled with an RVAT on a 300hp 4kV crusher motor. After experimenting with the different transition times to full voltage I have found that the quicker the transition takes place the less thermal capacity is used on the motor and the quicker the acceleration. Right now I have my transition set at 1.5s and this is the quickest acceleration and thermal capacity used thus far.

As dpc mentioned if the transition time to full voltage is too long of a time the motor will use up more of its thermal capacity which could cause tripping of the 469 relay as well. The longer the trasition time the longer the acceleration time, and the more thermal capacity is used on the start.

 

[bimr]

"If the voltage into the utility transformer goes for 500 V to 443 V on starting, that is, all by itself, a 13% voltage drop."

No, the voltage out of the utility transformer drops from 500V to 443V. The line side of the utility transformer is 12,000V.

"I wholeheartedly agree. You need to get the motor control designer AND the pump supplier involved in this. One of the limitations of an RVAT starter is the transformer itself. The 5 seconds may be the thermal limit of what the transformer can take; I have never seen one that can take more than 15 seconds. So that 5 second "Incomplete Sequence Timer" in your protection scheme may be taking that into account. I would not mess with that until you know that for sure."

The motor control designer AND the pump supplier are puzzled.

The multilin is timing out on 5 seconds because the current is not dropping in 5 seconds. The error message then reads acceleration timer.

"As dpc alluded to, the challenge here is that the current will not drop until the water gets to the top of the well. How long that takes is a function of the depth (head) of the well. It could be that your well is so deep that you cannot get the water to the top in 5 seconds, meaning that you may need to look at that time setting. But you still should make sure your RVAT starter has transitioned to Run in an amount of time that it can take (thermally) and no more. Transition on Current almost never works for a deep well pump; you will likely need a hard time value. "

This is a 1000 foot deep well. Why do you say that the current will not drop until the water gets to the top of the well? If water is moving at 5 ft/sec in the column pipe, that yould be 200 seconds.

 

[dpc]

You had 500 V, then you had 443 V, that means the voltage drop across the transformer increased, due to the increased current. The primary voltage is irrelevant.

The 469 can be configured to transition to full voltage based on current OR time. You do not have to wait until the current comes down. This of course should be reviewed by the motor and pump providers.

 

[burnt2x]

1000-foot deep well does not mean 1000 feet of travel for the well water! Deep wells are installed with "foot valves". The pump column is filled with water and therefore, water comes out immediately if the submersible pump runs below!

 

[bimr]

This well does not have a "foot valve". There are surge control valves that allow the water to backflow into the well at a slower rate.

"As dpc alluded to, the challenge here is that the current will not drop until the water gets to the top of the well."

What is the significance of the water getting to the top?

Another question for rockman7892.

The Multilin seems to be ending the pump start because the current is not dropping within the 5 seconds setpoint. Should the RVAT be set so that the transition to full voltage running occurs before the 5 seconds is over?

 

[rockman7892]

In my experiences with this autotransformer I have found that the quicker the transition to full voltage takes place the quicker the the motor will accelerate to full speed.

When accelerating up to full speed the current the current drawn by the motor is the starting current, and this current does not drop off signifigantly until the motor reaches about 90% of synchronous speed. Your multilin may be set up to detect a certain current drop and you may not be even getting to this current drop due to the fact that your motor is not getting near the 90% speed range.

As others have mentioned, you should look at the motor data sheet to see how long this motor takes to accelerate at full voltage as well as reduced voltage. Lets say that the motor takes 10s to accelerate on an 80% voltage tap, then the current drop off at 90% speed would most likely not happen until around 8 or 9 seconds which is longer than the 5s timer on the multilin and would therefore kill the start.

I would try to set the trasfer up for a set time and start with this time as low as possbible say 1s or so. From this low setting see if you are able to get the motor to start by lowering its acceleration time.

Another thing to try would be to set the transer time to a high value (higher than the published acceleration time) in order to let the motor start. Watch the current during this start and note when the current drops off during this start. You can then go back into the multilin and adjust you timer for the time you noted this current drop taking place.

 

[itsmoked]

I think 1 sec is seriously rushing a 450hp motor... I would stay out at the 4~5 second region to switch up. I don't picture a 450hp motor, with no load, reaching any kind of plateau in a single second, let alone pushing a pump rotor and a column of water.

But again you are trifling with big money here. One mistake can flush a huge wad of money and weeks of time down that well, and possibly your job with it. You need the motor starting time numbers from the pump and motor suppliers. If they don't have them then you need to sit on their desks in-person until they give you the numbers you need. They may have to ask other users what their settings are, but they need to provide this.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[dpc]

You haven't provided anywhere near enough information for anyone here to give you advice on changing the motor starting setup. You need to get some local expertise involved and the equipment suppliers so you can come up with a solution before this motor has a melt down.