Motor Voltage Problem.. it's 480, I've got 240

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mayanees

mayanees

Senior Member
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Westminster, MD
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Electrical Engineer and Master Electrician
To my forum colleagues, I present my electrical dilemma, requesting your particular expertise....

I have a Well-pump building powered at 240V, 3 phase delta. The building is equipped with a VFD/Across-the-line starter combo feeding a 40 HP 480V motor.
... long silly story.. new construction... but what I'm told is that there is no 240V 40 HP deep well pump with the required characteristics, so we either change the service and starters, or .... and that's where I could use some of the forums experience... :)

My thought is to install a padmount transformer at the motor, 240:480 (3) phase delta/delta, sized at 3-5 times the motor size, or 150-225 kVA. I'll run an SKM Transient Motor Starting exercise, and get comfortable with the size.. for an across-the-line start.
But I'm uncomfortable with the VFD. My concern is the magnetization of the transformer/motor starting current, and how the VFD will respond to that.
I figured I'd call Square D, and ask them. But there are likely more experienced folks here than I'll get at 888 SQUARE D.

Any thoughts are appreciated, about the xfmr approach, or any other suggestions.

Thanks,

John M
 
Put a dry type 240 Delta / 480 Delta transformer ahead of the VFD and be done with it. I also would not be worried about greatly oversizing the transformer as the VFD will limit the start up currents.
 
I like bobs idea but I will research the 40 hp submersible pump..I would assume money is not the issue..so can you divulge the type of application you are dealing with..
 
iwire said:
Put a dry type 240 Delta / 480 Delta transformer ahead of the VFD and be done with it.
Click to expand...
Agreed. The sooner you step up the voltage, the sooner you can use smaller conductors and components.
 
.. response to ?s so far

.. response to ?s so far

Tx for responses guys..

The system has to handle the across-the-line starter, in the event that the VFD fails. That's why I'm oversizing the xfmr.

I must put the xfmr after the VFD, because the VFD is already bought. The system was designed for 240V, but a 480V motor submittal was mistakenly approved. That's what drove me to the thought of putting the xfmr right at the point where the motor feeder turns to take its 200' plunge to the well pump motor.

Divulge the application??.. absolutely.. It's for a City of Dover, DE well pump that will contribute to the City's water system. There are multiple wells around the City that contribute to the water header/water storage system. This well is called Well 8R, so I assume there are perhaps 4-5 other wells - with 1, 2 & 3 dying out over the years... just assuming..

The only reason we ended up at 240V is that's what the existing building was equipped with, and this was a replacement or addition to that building. But at this point, everything is installed, with the slight problem that the motor is 480V! Wiring is large enough for the 240V motor.

The City of Dover Electric Department (COD) is ready, willing and able to install any service mods. They'll set a 480V service just for the application, but other things in the building would have to be re-wired, and a different VFD would need to be purchased. I worked for the COD Electric Department for a couple years, and you'll never find a more capable crew who can run you a service feeder in no time, from 240/120 to 69kV. The cool thing about them is that they're independent so to speak, and not encumbered by procedures like large Utility companies. And they have enough generation to cover their City's usage, on the order of 175 MW. I digress...!

TX,

John M
 
Danger Will Robinson!!

Danger Will Robinson!!

mayanees said:
<snip>
I must put the xfmr after the VFD, because the VFD is already bought.
Click to expand...


That is the absolute worst thing you can do..most VFD's are NOT designed to feed into a transformer, their output MUST be directly connected to the motor to provide the proper speed/torque control and most importantly, motor overload protection.

For that matter, are there any transformers that can hande to output of the vfd with it's 4-8khz or more waveform?

Check with Square D, there is a possibility that they make a VFD that accepts a 240 delta in and steps up to 480 out. (I say this based on the fact that there are VFD's available that accept 120v single phase in and output up to 240v three phase out, usually only for motors of no more than 1-2hp though.)

So as suggested in a previous post, either make the well guys provide a different motor, or your gonna have to at least replace the VFD and possibly add that 480 service.
 
Motor overload protection can always be added between the motor and the transformer.

A transformer is effectively a motor that does not turn, it is completely possible for a VFD to feed a transformer (in fact this is how many wind turbines are connected to the power grid).

You mentioned that you need full voltage starting in case the VFD needs to be bypassed. I find this often to be a fairly useless function. In most cases it makes more operational sense to provide a backup VFD instead of having to change the process control to handle a constant full capacity flow.

If your partner is Square D, you should involve their pumping application group.
 
TX Sir

TX Sir

Jim D,

You just changed my way of thinking about a backup starting means for water well systems.
It's much easier to grab today's latest drive offering and install two of them, than to upgrade the service to tolerate across-the-line starting, just as a backup. ....which doesn't even speak to your point about process control - which is totally sensible.

I'm not sure where this will end up, due to the construction status of the job. I believe they're ready to terminate motor leads, and have just discovered the problem.

I will likely mount a padmount for the 240:480.. and thinking as I type, the VFD OL may serve as secondary cable OC protection for a delta/delta xfmr.
..... with a disconnect for the motor.

Thanks for the advice.

John M
 
I come from a DC motor control background so I'm just thinking out loud. But isn't having the VFD before the xfmr and not between the xfmr and motor defeating the purpose of even having a VFD? Yes it would work but the VFD would always be operating at the frequency of the xfmr and not able to regulate it to the motor.

Being that you don't want upgrade service to tolerate across the line starting, In my mind the easiest/simplest option would be to step up to 480v with the xfmr and buy a 480v VFD. I have not seen a VFD that steps up the voltage, only ones that have AC input and DC output for a DC motor. Not saying they aren't out there, I just have never seen one :)
 
As Jim mentioned, the magnetic characteristics of a transformer are pretty similar to those of a motor. A transformer rated 240V to 480V 100A input 50A output at 60Hz will work just fine at 120V to 240V 100A input 50A output at 30Hz. Basically if you maintain a constant V/Hz ratio the transformer core magnetization will remain correct. If you use a delta-delta transformer between VFD and motor, the VFD overloads might even work correctly.

That said, I'd like to through a couple of other ideas into the pot. I claim neither as the 'way to go', but points to consider:

1) I would go back and question the initial premise: "...but what I'm told is that there is no 240V 40 HP deep well pump with the required characteristics, so we either change the service and starters, or ..." this statement cannot be strictly true. It may be that no such pump is available on the shelf, or no such pump is available with an acceptable lead time. But if a manufacturer can build a 480V motor, then they can build a 240V motor. Hell, they may even be dual voltage motors buried under all of the water tight sealing, but because of the sealing it is impossible to change the connections. I would strongly suggest finding out what the real constraints are on getting the correct 240V motor.

2) Contact the manufacturers of line regenerative VFDs, to see if any will directly take 240V in but supply a 480V motor. Line regen VFDs are essentially two VFDs 'back to back', where one VFD acts as an input rectifier and _boost converter_, giving a high enough DC voltage to permit the drive to supply power back to the AC line. In theory this approach should work with a 240V line and a 480V motor, but I don't know if any production VFDs actually do this.

-Jon
 
Don't over think this.

You don't want to put a transformer on the output of the VFD. It takes a specially designed transformer to do that, and they are extremely expensive (compared to the alternatives). Off-the-shelf transformers are not designed to handle the varying frequency and high carrier frequencies of the VFD. The imperfections in the core laminations loom much bigger than they would in a fixed frequency installation. In other words, your transformer could cause serious losses in the circuit, which the VFD tries to make up for because the motor response is not what it should be, which then causes more losses in the transformer etc. until everything ends up over-driven.

Better plan:
Put the transformer in FRONT of the VFD. What's the problem with that? If you are concerned that you have a 240V rated bypass contactor, don't be. That contactor is now TWICE the size it needs to be for a 480V motor. Just change the OL relay (or heater elements}. If the motor starter was part of the VFD package and is already sized for 480V, even more reason to put the transformer ahead.

It is also a good idea to put the OL relay separately mounted downstream of both devices (the VFD and the Motor Starter). That way if they have been running on the VFD and the VFD dies so they have to switch over to the FVNR bypass starter, the OL relay has already been seeing the motor thermal condition so t doesn't "think" the motor is cold. No need to wire the OL relay to the VFD, in fact you don't want to since most VFDs have much better OL protection, it's justso that someone doesn't smoke the motor by running in Bypass when t is already too hot.

Hopefully you also have an isolation contactor in the output of that VFD by the way, because in that scenario you must NEVER put AC line voltage on the output terminals of the drive. It lets the magic smoke out.

So back to your original issue, yes you need to size that transformer for FVNR starting. Making the kVA to be 2.5 - 3 times the motor HP is a good simple rule-of-thumb. So for a 40HP make the transformer at least 100kVA. You might be able to get away with a 75kVA since you may be able to suffer a little bit of voltage drop at the motor without adversely affecting the pump performance (remember it is the only thing on that circuit), but it totally depends on the well depth etc. If you drop too much and don't create enough torque, the water never reaches the top of the well and you are toast. I'm all for being safe rather than sorry. If you have SKM, use it for sure.
 
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And no, there are no VFDs that can create energy that is not already there. Small VFDs that take 120V in and put out 240V 3 phase do so with a device called a "voltage doubler" board. It essentially rectifies the 120V 1 phase to DC, then uses capacitors to boost the DC voltage to 320VDC, feeds that into the VFD DC bus, which uses it to create the 240V AC output. At about 1HP, the cost of that voltage doubler board gets astronomical, far far more than the cost of a transformer.

I suppose at first glance that a Line Regen VFD could be configured the same way, but a Line Regen VFD typically costs twice that of a std VFD, and to boot, your front end would have to be twice as big for the lower voltage. WAY more expensive than a transformer!
 
rivalshad said:
I come from a DC motor control background so I'm just thinking out loud. But isn't having the VFD before the xfmr and not between the xfmr and motor defeating the purpose of even having a VFD? Yes it would work but the VFD would always be operating at the frequency of the xfmr and not able to regulate it to the motor.
Click to expand...

A transformer reacts to its applied frequency, it does not fix a frequency. As long as it sees the proper relationship between volts and frequency (V/Hz) at its input (from the VFD) it will generate a corresponding output (to the motor).
 
jim dungar said:
A transformer reacts to its applied frequency, it does not fix a frequency. As long as it sees the proper relationship between volts and frequency (V/Hz) at its input (from the VFD) it will generate a corresponding output (to the motor).
Click to expand...


Agreed. Downsides to supplying a 480v primary with less than 480v on a regular basis?
 
True, but the point is...

True, but the point is...

jim dungar said:
A transformer reacts to its applied frequency, it does not fix a frequency. As long as it sees the proper relationship between volts and frequency (V/Hz) at its input (from the VFD) it will generate a corresponding output (to the motor).
Click to expand...

Jim, I can agree with that statment to a point. :)

But the VFD would then be reacting to the transformer, NOT the motor. How is the VFD going to be able to accurately control the speed/torque of the motor with the transformer "in the way"?

Every VFD I've ever seen or used explicitly states that there can be NO extraneous loads other than the motor on it's output side. That includes power factor caps and/or line reactors.

And the VFD's algorithm to provide motor overload protection monitors the relationships of voltage, current, reactance, etc. to determine when to trip on overload...something I imagine the VFD can't read with a transformer in the path.

IMHO, this idea of a transformer on the load side of the VFD is setting up a huge, expensive disaster.

Jraef said:
Better plan:
Put the transformer in FRONT of the VFD. What's the problem with that? If you are concerned that you have a 240V rated bypass contactor, don't be. That contactor is now TWICE the size it needs to be for a 480V motor. Just change the OL relay (or heater elements}. If the motor starter was part of the VFD package and is already sized for 480V, even more reason to put the transformer ahead.
Click to expand...

That would be the most reliable, least costly way to solve this problem.

Or get the well guys to own up and get the 240v motor. :)
 
mxslick said:
But the VFD would then be reacting to the transformer, NOT the motor. How is the VFD going to be able to accurately control the speed/torque of the motor with the transformer "in the way"?
Click to expand...

A vfd does not react to the motor in any way it could not through a transformer
 
I should have stated in my last post that 'in theory a transformer could be made to work between the VFD and the motor'. I really was trying to direct away from this approach, because IMHO it is possible to make this work, but I think that it would be something of a research project rather than a stable production approach.

Consider the overload protection requirements for transformers and the situations where primary overcurrent protection does not provide secondary overcurrent protection. Would the drive correctly see an overload on a _single_ motor phase (caused by some fault)? Will the flux vector techniques work correctly with both the transformer magnetizing current and the motor magnetizing current? Etc.

Straight V/Hz control, with overload relays downstream of the transformer signalling the VFD to shut down...I think that this would work fine. But anything fancier is IMHO a gamble.

I think that Jim suggested the best approach: step up transformer feeding two drives, no across the line starting, one drive serves as backup.

-Jon
 
In a flux vector control mode a VFD takes into account the magnetic circuit of the motor and there for can provide "tighter" control. Flux vector control would be difficult through a transformer.

In V/Hz mode the VFD does not consider the motor at all and therefore it will be able to work "through" a transformer. Many years ago this was a very common, if not the only, method to provide VFD control on 600V to 4160V motors.
 
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