# Buck/Boost Transformers



## Captlugi (10 mo ago)

We use 3 Buck/boost transformers to achieve 480v to 400v, 3 phase, the diagram shows tying the "neutral line together from the panel and to the equipment. In our case the neutral from the panel is not connected and the voltage across legs are 400, with the leg to ground being 230 ish. When i measure from neutral to ground i get anything from 80-70 volts.
Question is;
1. Is this normal?
2. the single phase of this goes to a VFD and converts to 3 phase for a motor and keeps faulting because of Overcurrent
3. Suggestions?


----------



## paulengr (Oct 8, 2017)

Captlugi said:


> We use 3 Buck/boost transformers to achieve 480v to 400v, 3 phase, the diagram shows tying the "neutral line together from the panel and to the equipment. In our case the neutral from the panel is not connected and the voltage across legs are 400, with the leg to ground being 230 ish. When i measure from neutral to ground i get anything from 80-70 volts.
> Question is;
> 1. Is this normal?
> 2. the single phase of this goes to a VFD and converts to 3 phase for a motor and keeps faulting because of Overcurrent
> 3. Suggestions?


1. Buck/boost is not typical. If you have 480 just buy the 480 V drive! But it’s not unusual in European derived control panels because chances are there’s a lot more to deal with. Typically you’d see an auto transformer or standard transformer here. Bucking that much voltage is unusual. Typically 48 is the limit. Either way an ungrounded system can be any voltage to ground but is typically under 100 V to ground. It can’t go too far because it is capacitively grounded. By this I mean the ground and power system are normally insulated and the definition of a capacitor is two conductors separated by an insulator. This floating arrangement is fine until you have an arcing fault. Code requires you to have “phase lights” or an equivalent alarm because otherwise you can have a short to ground, a fully energized (230 V) panel, and no warning that it is dangerous to touch it.

2. Overcurrent is measured by the drive on the load side. Most of the time this is a mechanical issue. Disconnect motor and Megger it. If it passes out it back together and check currents. Start there. Chances are it’s something mechanical. If you still think it’s electrical measure all 3 phases on the load side with a true RMS meter with the drive set up for constant load, even if you have to uncouple the motor. If it’s good (under 1-2%) AND matches the drive AND the motor name plate values are correct in the drive parameters then last thing check voltages (again true RMS). They should be the same.

There are some other common causes to watch out for that the drive manual will mention such as flying start. If a motor is already turning and you just turn on a drive it will start out at say 1 Hz. But if the motor is already turning, say 10 Hz, what happens? The motor is suddenly being forced to slow to 1 Hz so it acts like a brake. You get a huge current spike and power flows in reverse (motor to drive). Operations needs to understand this is a bad practice. If you can’t avoid it drives have flying start algorithms that put out low voltage and sweep the output frequency looking for the motor speed but these don’t always work that well. Might be just a change of operators or operating practices is where your problem is.

3. Need more info. Is it instantaneous on startup or random? Is there a consistent load? Have you checked the fault/alarm logs? Don’t assume the data it gives you is accurate either. Is your voltage to ground consistent? Drives are connected to ground and NEED a good ground. Some current flows through ground (common mode). Review the drive manual. Some drives can work on an ungrounded system but you need to disable a filter circuit and the over voltage protection built in. There is usually a jumper for this. Review that part of the installation manual. This must be done right. Many machine builders get it wrong and since the drive was not built in the same country they made assumptions about the power system that may be different where you are at.

If the issue is intermittent and the drive has a special “IGBT test” run that test.

Check drive age (when was it made). Drives are designed with 100,000 hours average life. This is for commercial reasons and is nearly universal.


----------



## gpop (May 14, 2018)

2 types of over current.
Hardware and overload. Post the drive name, model and fault code.

P.s if its hardware and you keep resetting it don't be surprised when the drive goes boom.


----------



## Captlugi (10 mo ago)

The machine is a UK setup thus the reason for the buck/boost. There I'm told its just a straight connection of 3 phase 400, with N1 and Ground. So we have to use the XFRM to assist. When i dug into the panel back near the sub panel i found that the Neutral lines were all connected from the xfrm to the machine but no connection back to the panel, so it was a floating N1, to fix the issue with the single to 3 phase VFD, I changed the drive over to a 3 phase in/out Power Flex 4, and eliminated the need for the neutral to be used. My question now is will there be an issue with connecting the neutral back to the panel like the schematic shows?


----------



## Breakfasteatre (Sep 8, 2009)

Is the buck/boost transformer an isolation transformer? is the neutral grounded at the transformer? 

I ran into this recently in terms of the neutral requirement. customer purchased equipment which was built for 480v 3p + N. Inside the panel, the only thing that used that neutral was a control transformers that used 1 phase to neutral for the line side, 277V. There was no neutral source at the 480v panel so we changed out the transformer to a 480v to 120v.


----------



## CMP (Oct 30, 2019)

Sounds like your supplied side power is not properly bonded on the upstream side, or is connected to a isolated delta. Seems the problem in on your supply side and not on the load side.

Are your 3 buck transformers the same brand size and model? You need to determine if your 480V supply source issupplied by a delta system or wye system before we can guess why there is a difference between your neutral and ground.


----------



## wiz1997 (Mar 30, 2021)

Over current faults on a VFD can be caused by the acceleration or deceleration time settings.


----------



## Captlugi (10 mo ago)

CMP said:


> Sounds like your supplied side power is not properly bonded on the upstream side, or is connected to a isolated delta. Seems the problem in on your supply side and not on the load side.
> 
> Are your 3 buck transformers the same brand size and model? You need to determine if your 480V supply source issupplied by a delta system or wye system before we can guess why there is a difference between your neutral and ground.


Its a delta setup.


----------



## oldsparky52 (Feb 25, 2020)

By any chance are you able to post how the (3) BB transformers are wired now and how the drawings show it should be wired? 

You are talking about a neutral and a Delta setup, it's confusing to me (sorry).


----------



## CMP (Oct 30, 2019)

Well delta is what i thought was the reason why you have different readings. There are three basic type connections, ungrounded, corner grounded, and resistance or impedance grounded. My guess is that you have an ungrounded system by the differential voltage you are seeing. Not the ideal system for supplying a VFD, a VFD produces common mode currents that need to return back to the system ground (system common) which on an ungrounded system you lack.

is your supply for the VFD coming from the building service panel or is it being supplied from a local transformer?


----------

