High Leg 3 phase and vfd's

Never enough money to do it right the first time
but always enough money to do the right way the second time
funny how that works
 
The issue is not actually that it is a "high leg" delta, the problem is that it is a delta supply at all. VFDs are all designed for a consistent voltage reference to ground, as you would find in a grounded Wye power system. Most Asian drives simply give you a warning telling you not to connect them to anything OTHER than a solidly grounded Wye system, but they often use terms like "TT, IT or TN system", terms that mean nothing to the average user here (those are IEC terms for different grounding schemes). Delta power distribution is not used anywhere else in the world but here.

Drives that are designed for the North American market have to contend with Delta systems, so they provide ways to modify the internal ground references that exist on the line side MOVs for protection and any filters for Common Mode Noise and EMI/RFI. You must remove the references to ground when using the drives on Delta supplies. In the case of A-B, they provide simple jumpers to pull or bolts to remove (depending on size). ABB does something similar, Siemens makes you disassemble the front-end of the drive to cut a wire (on the old MicroMaster drives, they may have fixed that now on Sinamics, I don't know). The A-D drives are made by Delta Electronics in Taiwan, who despite their name, have the typical Asian disregard for the fact that we have Delta power systems here. But they do offer that feature on some versions, which probably explains the difference between GS3 and GS4 reported by OkiePC. It should be noted however that in removing the ground references in order to accommodate a Delta source, you are also removing the protections and filtering that these systems provided. So a better plan, as mentioned earlier, s to use a "Drive Isolation Transformer" that will be a 1:1 ratio, but give you a Wye secondary. So a typical DIT for 240V would be 240V Delta primary, 240/139 Wye secondary. Nobody uses the 130V for anything, it's just the resulting L-N voltage, but the fact that it is consistent for all 3 phases and solidly referenced to ground allows the VFD to get what it wants and needs without sacrificing some of the protections.


By the way, you cannot give a 480V rated PF755 an input of 240V and have it work, it requires at least 342V input (380V -10%), so if it powered up at all, it would be in a perpetual state of Under Voltage.
 
Last edited:
+1 jraef

Some drives will trip on OV on delta power if the filters are left in, simply due to the caps in the filters charging up too high and bleeding into the bus round-about through the line... been there, done that. Sometimes, you can rotate the incoming phases and get lucky - for a while.

Go with the DIT, relatively inexpensive, and safer system, regardless which drive you use.
 
Just a random thought, is there any reason you couldn't use a 240 Delta to 480V Y transformer, and hook it up backwards as a step-up transformer? Drives are usually priced by amperage instead of power, so 480V drives are usually cheaper, especially for larger horsepower.


Though, come to thank of it, most of the 480->240 transformers I've commonly seen are Delta-Delta, so it may not be as easy and readily available as i thought....
 
...Though, come to thank of it, most of the 480->240 transformers I've commonly seen are Delta-Delta, so it may not be as easy and readily available as i thought....
Yeah, that's the problem, plus now the transformer must be specifically marked or notated by the manufacturer to be suitable for reverse feed (2014 NEC rule). That, and having to have a motor that can be connected at 460V. Most smallish (10HP and under) motors can, but bigger ones become more likely to be single voltage.
 
Last edited:
Wow, glad I asked about this. Lots of great info. I think I just went from maybe 5% to nearly 15% understanding of Delta wye terminology. Waiting on quote from local distributor and will follow up with my results.
I wasn't sure about ignoring low voltage fault on the 755. Just remember something in the parameters related to that. Glad that was answered before I wasted the time trying.
Hope this works out. Kinda miss playing around with my old Haas. Not much else to do these days.
 
OkiePC
I checked the link
it must be very old document referring to SCR's in driver they are not used much anymore
everything now is IGBT's larger drives may still use Power Darlington Transistor, the only place that still use SCR's is DC drives and high voltage VFD's 2,300, 4,160 then they stack them to get the voltage and current rating they need.
if you just install a good drive you don't have these problems
 
So Hammond has a 30 day lead time. Found these guys below. 15kva couple hundred bucks more than Hammond 11kva. Is there something to be said for a "drive isolation transformer" vs a "general purpose transformer".

https://store.maddoxtransformer.com...ducts/240-delta-208-y-120?variant=45588273993



This one is 240 Delta to 208/120 wye. This should be fine for my drive.

Normally, a distribution transformer *could* be Delta-Delta, depending on the rating. With a 208Y120 secondary, it's typically a Delta-Wye.

As far as I know, a Drive Isolation Transformer is *always* Delta-Wye, which helps isolate some of the noise. DIT's also typically have some shielding and a K-Factor rating (service factor for non-sinusoidal currents).
 
So the delta wye transformer was exactly the cure for my situation. Works perfectly.

I am on a job in Florida where the plant has an isolation transformer in front of a machine that is getting a new 75 hp vfd. Delta Wye 460/460 transformer. The primary wires coming in include a ground wire that is connected to a ground rod outside of the cinder block building. This was connected only to the transformer chassis. The flat Cooper ribbons (from core I think) were also bonded to chassis. X0 was left floating which I know is wrong. The secondary was fed to the panel along with can ground that was tied to the chassis. I know this is not correct.

I believe correct course of action here is to drive a ground rod, connect XO to it and also connect the ground conductor for the panel to this as well.
That leaves the incoming ground and the ground ribbons which I think should go to the chassis. Question is, should the incoming, outgoing/XO ground be bonded together as well. This would allow ground fault to split between two different ground rods. Not sure that is right.
 
Here is what's probably going on.


As GaryS said, VFDs do NOT care about phase to ground or phase to neutral voltages, from an OPERATING standpoint. But where it DOES matter is in the protection scheme on the input, AND on the Common Mode Noise protection scheme. On the input side, there is usually a set of MOVs that are providing surge protection for the bridge rectifier. Those are almost always configured in a Wye with a reference to ground. Being connected to any Delta system, whether it is high leg or not, is potentially detrimental to those MOVs because if there is a ground fault ANYWHERE on the line side, the MOVs attempt, briefly, to be the ground reference point for the entire system, generally resulting in catastrophic failure and if they fail, the drive fails shortly thereafter. In 240V 4 wire Delta systems however, that would only apply to a GF on the High leg, so the risk is a little lower.


The issue that kamgenes mentions is not exactly as stated, but there often IS a ground reference on the DC bus THROUGH Common Mode Noise capacitors, that safely allow CM noise to connect to ground and avoid bleeding into everything else. For Delta power systems you should remove that ground reference to avoid having issues with potentially damaging the caps. Higher-end drives like Rockwell, ABB, Siemens etc. provide ways to do this (some easier than others). But may low-cost drives, especially those from Asia, do not have a way to do this, because the manufacturers don't acknowledge that Delta power systems exist (Delta is a North American thing). Some are explicit in telling you this, maybe by saying something like "This drive is suitable for connection to a TT or TN system", which means nothing to us in North America, but means it MUST be a Wye power system that is solidly referenced to ground. Others make no mention of it one way or another. So when you connect them to ANY delta system, you can have troubles like this, or worse.


Since you already own this drive, your only viable solution is to install what's called a "Drive Isolation Transformer" ahead of it; 240 in, 240/138 Wye output, and you ground the Wye point (X0 terminal). These are readily available from most of the transformer mfrs.
 
Last edited:
I probably should have started a new thread. The delta wye transformer definitely fixed my milling machine and I mostly under stand how and why. This last question was about proper grounding of the primary and newly derived secondary.
 
And by the way just for future reference, the 480V PF753 with a 240V input would not have worked, even if you disabled the Under Voltage trip (set to Ignore). The drive needs at least 300VAC on the input, otherwise the electronics don't allow the Pre-Charge to complete and the drive will not power up. Setting the UV to Ignore works to allow the drive to KEEP running if the voltage drops during operation, but without 300VAC minimum, it will not power up (I've tried...)
 

Similar Topics

Not really my problem, but my city has asked for help. They have two 15 hp 230V 3-phase motors pumping make up water into their water tower. The...
Replies
11
Views
9,989
What is the US standard color coding of the high leg and other phases in a delta circuit? TIA This is what I have:
Replies
5
Views
3,485
I work in a plant that has a High Leg Delta electric service, very old plant... we are having several VFD's tripping on bus overvoltage, across...
Replies
26
Views
7,031
I have a 240, 3 phase High-Leg power feeding a control panel. Since there are heaters without Ground Fault Equipment Protection, there is a need...
Replies
3
Views
3,493
Can anyone recommend a flexible 600-volt shielded ethernet patch cable? I have used them from Allen Bradley / Panduit before and they are too...
Replies
0
Views
60
Back
Top Bottom