Print

Please login or register.
1 Hour 1 Day 1 Week 1 Month Forever
Login with username, password and session length

[Stephen Swaffer]

What was the metered voltage?

In my area, "460" almost always is supplied as 480 volts-and on weekends/evenings when the grid is lightly loaded it can creep up to 490.  If the 3 phase panel derived from a "460" service it could easily read 220-230 volts.  It could also be affected by how a transformer is "tapped"-most ditribution transformers have taps meant to adjust the voltage-obviously a place for human error-or "I like it this way."

If it is a full 240 then Mike may be correct-but 3 phase heat strips pulled off a high leg delta could be a red flag-depending on exactly how the high leg delta is derived.

The only time I have run into the need for buck/boost transformers is on tanning beds  (I am sure there are other critical applications. The tanning beds made me cringe-when you need a 60 amp 3 phase service to one tanning bed that can't be healthy! ).  In most cases, if you ask me to supply 220 you'll get anywhere from 208-240 depending on what is available unless someone specifically aks for something different.

[Mike Sokol]

If it is a full 240 then Mike may be correct-but 3 phase heat strips pulled off a high leg delta could be a red flag-depending on exactly how the high leg delta is derived.

The first time I encountered buck/boost transformers was in the mid-70's for industrial heat tunnels that needed 240 volts and I only had a 208-volt/3-phase Wye sub-panel in the area. Of course, you can wire the same buck/boost transformer with the output legs in buck-mode to drop the voltage from 240 volts down to 208 volts. And since these transformers aren't passing the entire power load through themselves, they're a lot smaller than a full isolation transformer passing the same KVA. You're really building an autoformer. 

[Nathan Riddle]

What was the metered voltage?

Exactly 120/240 with the typical ~1v fluctuations.

I've covered this before, but it's most likely a High-Leg Delta service panel. That's the only simple way I know to get a true 240-volts between phases of a 3-phase panel. Of course, you can also install a buck-boost transformer to add the extra 32 volts, but nobody does that unless they REALLY need 240-volts. 
https://en.wikipedia.org/wiki/High-leg_delta

That makes sense. I felt like I had read some of your material on High-Leg Delta service, I couldn't remember if this was the exact situation and without much internet couldn't really look it up.

IIRC the thread had differing opinions on whether High-Leg Delta was bad or not. I know loading can be an issue sometimes, along with one of the legs being at 208v instead of 120v sometimes.

My load was ~15A per leg so I'm pretty well under the limit, perhaps I can use my big 20A 240v strobes

Anyways, thanks for the help everyone!

Gig went well, didn't encounter any liability issues with tieing in power and connection/disconnect times were down 100 fold (~30s vs 30min) 

[Mike Sokol]

IIRC the thread had differing opinions on whether High-Leg Delta was bad or not. I know loading can be an issue sometimes, along with one of the legs being at 208v instead of 120v sometimes.

There's nothing inherently wrong with a High-Leg Delta panel, UNLESS you don't meter things properly and connect to the High-Leg expecting 120-volts AC. As you can see from the diagram, there will be 240-volts between any of the phases, with one of the transformer windings being center-tapped, which is how you can get 120-volts from the neutral to either of the legs of that particular winding. But Neutral to the "High", "Red" or "Wild" leg will get you 208 volts, which will likely blow up all sorts of things that don't have universal switching power supplies. I've covered this before on this forum since High-Leg Delta panels were fairly common in mid-century (think the 1960's) industrial buildings. You tied the warehouse into the 240-volt/3-phase connections for 3-phase motors and heaters, but the office would use the single-phase 120-volt center-tapped winding for general Edison outlet power and ceiling lighting. And yes, I've actually seen modern electricians tie 120-volt gear in the High-Leg and do all sorts of damage. Measure twice...

[Jonathan Johnson]

There's nothing inherently wrong with a High-Leg Delta panel, UNLESS you don't meter things properly and connect to the High-Leg expecting 120-volts AC.

On the other hand, if you have a wild leg, which is the same as high-leg delta but missing a transformer, it will meter the same voltages but, from what I understand, weirdness can happen once you load things down.

Others here with experience dealing with wild leg delta can explain more.

[Mike Sokol]

On the other hand, if you have a wild leg, which is the same as high-leg delta but missing a transformer, it will meter the same voltages but, from what I understand, weirdness can happen once you load things down.

Others here with experience dealing with wild leg delta can explain more.

Yup,  you don't want to pull a significant 120-volt load from an Open-Leg-Delta transformer since it will cause all kinds of voltage changes on the 3-phase side of things. IIRC you're not supposed to load the 120-volt tap to more than 10% of transformer capacity.

[Stephen Kirby]

I've never seen a 3 phase heater before.  I work a lot with SMT reflow ovens which are typically 480 three phase service, 100-200A.  They have multiple heating zones which are series resistance heaters typically run through Crydom solid state relays across one of the legs.  The various heaters distributed across the legs to balance the load in typical use.  PLCs manage the SSRs.  SMT ovens are a bit odd in that the heaters run at progressive temperatures throughout the tunnel rather than all at the same temp.

[Mike Sokol]

I've never seen a 3 phase heater before.  I work a lot with SMT reflow ovens which are typically 480 three phase service, 100-200A.  They have multiple heating zones which are series resistance heaters typically run through Crydom solid state relays across one of the legs.  The various heaters distributed across the legs to balance the load in typical use.  PLCs manage the SSRs.  SMT ovens are a bit odd in that the heaters run at progressive temperatures throughout the tunnel rather than all at the same temp.

A 3-phase heater was probably a misnomer. We had a few shrink film tunnels for packaging that used Chromolox heating elements that were, of course, single phase. But for the bigger tunnels there would be banks of these heater across all three legs. Some of the smaller tunnels had single-phase 240-volt heating elements and we only had 208 volts, so that's when we put in buck-boost transformers. 

[Stephen Swaffer]

I've never seen a 3 phase heater before.  I work a lot with SMT reflow ovens which are typically 480 three phase service, 100-200A.  They have multiple heating zones which are series resistance heaters typically run through Crydom solid state relays across one of the legs.  The various heaters distributed across the legs to balance the load in typical use.  PLCs manage the SSRs.  SMT ovens are a bit odd in that the heaters run at progressive temperatures throughout the tunnel rather than all at the same temp.

Three phase elements would be a misnomer-I work with blow molding machines that have 400 amp services to the control primarily to service resistance heat.  Much like the SMT ovens, the extruder and heads have numerous heat zones-the extruder in particular is odd because once running the extrusion process creates heat so those zones have heat elements and cooling fans to maintain temps.  Each heat element is, of course, single phase controlled by SSR's, but the equipment is seen by the service as a 3 phase load.  IME, most high leg delta services (this may apply just be my part of the country) are open leg delta-installed to allow 3 phase (typically AC/freezer/cooler compressors) to be used, but allowing the utility to skimp on a transformer.  In that case, adding a large, resistive "3 phase" load might not be a good idea-but I could see someone installing an electric furnace and not understanding the situation.

[Jonathan Johnson]

I've never seen a 3 phase heater before.

When it comes right down to it, almost every "3-phase" load is really three single phase loads. Resistive loads really don't care if the phases are in phase or not. Motor loads DO care, but even a 3-phase motor is just a motor with three single-phase winding sets offset by 120 degrees from each other.

On second thought, are there any 3-phase motors where the windings rather than connecting phase leg to phase leg connect phase leg to neutral?

How about 3-phase switching power supplies?

[ProSoundWeb Community]