Sharing neutrals in lighting circuits

Status
Not open for further replies.

roger

Moderator
Staff member
Location
Fl
Occupation
Retired Electrician
Jon, it's nice to have you aboard with the other engineers here.

You engineer people sure can type. :D

(BTW, that was in admiration not sarcasm)

Roger
 

iwire

Moderator
Staff member
Location
Massachusetts
I should probably start another thread however I will say here that I went to the meeting and did not learn anything.

They are still testing, we are going to do a weekend shutdown and perform a number of changes take measurements and then restore it back to original.

Some of the ideas for a fix from one of the engineering firms are downright scary and violated the NEC.

Rest assured they will be informed of any NEC violations. :p
 

iwire

Moderator
Staff member
Location
Massachusetts
Hi Jon, let me join Roger in welcoming you and your knowledge to this forum. 8)

winnie said:
Bob,

I rather doubt that the bearing problems are being caused by harmonics.

Yes I agree and am sorry if that is what it looked like I was suggesting.

I went way off this threads topic when Ramsey asked this;

ramsy said:
I wonder, how legitamite this concern of neutral harmonics, twice the phase value, is with industrial plants that involve VSD's, SCR's, & constantly changing configurations.

I just picked up on problems with VSDs and ran with it.
 

ramsy

Roger Ruhle dba NoFixNoPay
Location
LA basin, CA
Occupation
Service Electrician 2020 NEC
iwire said:
I just picked up on problems with VSDs and ran with it.
Bob, good to see you back, and thanks for showing us some manners, in welcoming new guests.

When the Faraday Shield reference identified two forms of PWM EMI emissions as differential (line to line) and common mode (zero sequence) [12]. (as) the dominant excitation source (for) coupling capacitances and bearing current discharge [5, 8-10] that meant harmonics were suspect as one source, not the only source for Electrical Discharge Machining (EDM) on bearing race to shaft.

And, not neccesarily 60Hz system harmonics, but a harmonic from high Hz fundamental, AKA noise. I believe the PWM EMI emission here is a high Hz fundamental before it was detected as a zero-sequence spectrum. Somehow that noise spectrum is said to create a potential or capacitive arcing across the weekest path to ground, often seen as EDM.

My question for the noise angle of EDM, is if the delta connected damping mechanism, known to reduce system harmonics, is not fequency sensitive, it may also Dampen high frequencies as well.

A study dated 2002 from Rockwell Automation, concludes more work is needed to find what factors this rare EDM event has in common. Across similar configurations some motors show fluting and some don't, they havn't conclusively pinned down why.

I am simply suggesting one common factor may be Wye connected motor loads, since Wye's don't dampen noise, and are the less common configuration for motors (rare).
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Roger,
since Wye's don't dampen noise, and are the less common configuration for motors (rare).
Are you saying the wye connected motors are rare? In my experience, I connect 40-50 wye motors for every delta.
Don
 

ramsy

Roger Ruhle dba NoFixNoPay
Location
LA basin, CA
Occupation
Service Electrician 2020 NEC
don_resqcapt19 said:
In my experience, I connect 40-50 wye motors for every delta.
I'm not refering to Wye supplied motors, fed by Wye transformers, but the internal motor load, Wye or Delta configuration.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Roger,
'm not refering to Wye supplied motors, fed by Wye transformers, but the internal motor load, Wye or Delta configuration.
That is what I am talking about. Motors with an internal delta connection are rare....at least in the area where I work. At the plant where I am assigned now there are about 1000 motors, and less than 10 are delta.
Don
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
Guys,

Thanks for the welcome.

I hope that I can do a good job of explaining theory as I understand it, and keep my mind open for the theory that I don't yet understand. For lots of practical stuff I'm even worse than an engineer, but I'm learning :)

ramsey,

My hunch is that connecting the VSD to a delta supply would make things worse for EMI, and induced currents on the motor side of things.

The voltage between the positive and negative rail is relatively constant, but the voltage to ground of either rail is 'bouncing around' with whichever phase happens to be most positive or most negative at any given instant. With a wye connected source, this is a small AC component superimposed on the DC rail voltages.

If instead your source were a corner grounded delta, then the entire DC bus would be bouncing around at full line voltage. Consider: when the most positive phase just happens to be the grounded phase, then the positive rail would get tied to ground via a rectifier diode. When the grounded phase becomes the most negative, then it is the _negative_ rail tied to ground via a rectifier diode. So at 60 Hz, first one rail and then the other would be at ground potential. The _DC_ voltage between the rails would be pretty constant, but the voltage between the wires and ground would see this mains voltage superimposed upon the switching voltages. This could easily double the maximum voltage seen by the insulation system.

If your system were ungrounded, then the entire system would jump around at high frequency because of capacitive coupling between the motor windings and ground at the switching frequency. With multiple inverters on the same system, your switching transients would be ugly as heck.

This is just my guess. I've not done extensive literature searches on the issue, but at one point I had a scope on the DC rail and on the supply transformer neutral while we were debugging some noise issues. The hash was ugly, and when it wasn't ugly there were beautiful clear spikes with clean resonant decay, with amplitudes that exceeded the supply voltage by a factor of two. With two VSDs on the same transformer, you could see the switching events slide into and out of phase, and when both switched at exactly the same time, the spike was even higher.

-Jon
 

ramsy

Roger Ruhle dba NoFixNoPay
Location
LA basin, CA
Occupation
Service Electrician 2020 NEC
don_resqcapt19 said:
That is what I am talking about. Motors with an internal delta ..At the plant where I am assigned now there are about 1000 motors, and less than 10 are delta.
You got me there. Do you know why? Never thought designers would prefer Delta's, since the higher current requires larger conductors, and more V-Drop losses, for the same power?
 

ramsy

Roger Ruhle dba NoFixNoPay
Location
LA basin, CA
Occupation
Service Electrician 2020 NEC
winnie said:
My hunch is that connecting the VSD to a delta supply would make things worse for EMI, and induced currents on the motor side of things.
You may be correct, and thanks for the insight. Your posts are elaborate and I think my short attention span can get most of your points, but checking this idea would require some testing.

Since none of the published fluting studies looked at this, that leaves iwire in a unique possition to checkt on it. I was hoping iwire could tell us if those motor loads are wye or delta configured. If they are already delta configured, that would satisfy my curiosity.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Roger,
You got me there. Do you know why? Never thought designers would prefer Delta's, since the higher current requires larger conductors, and more V-Drop losses, for the same power?
Horsepower is horsepower and the current required does not change based on the internal winding connections of the motor. The actual current in the windings of a delta motor will be that same as in a wye motor, assuming that both motors are designed to supply the same horsepower.
Don
 

ramsy

Roger Ruhle dba NoFixNoPay
Location
LA basin, CA
Occupation
Service Electrician 2020 NEC
don_resqcapt19 said:
..the current required does not change based on the internal winding connections of the motor.
Check me here, but don't loads and xfmrs vector power the same way.

For delta:
Vline-line = Vphase
Iline = Iphase x 1.732 (higher current)

For wye:
Vline-line = Vphase x 1.732 (higher voltage)
Iline = Iphase

Open Delta(V) and WYE behaves like series branch circuits, where line-line voltage sums for single phase or vector * Sqrt(phases). Closed Delta coils behaves like parallel branch circuits, where line-line current sums single ? or vectors * Sqrt(?).

I am not considering dynamic loads, such as Wye start, Delta run, or dual voltage connections, which can parallel Wye coils. I am only considering those Delta wired motors that closest resemble Delta xfmrs.
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
Two equivalent motors (same horsepower, efficiency, frequency, voltage, shaft speed, etc.) will draw the same current at the same voltage under the same load. It won't matter if the motors are internally connected star or delta.

Internally to the motor it will make a difference. Internally to the motor the delta connected motor will have more turns of wire electrically in series in a given phase, and the wire will carry lower current. The wire will be thinner, so the current density will end up the same, and the increased number of turns will mean that the net slot current is the same.

As a simpler analogy, consider two 30kW heaters. One is wye connected, using 3 10KW 277V elements. The other is delta connected, using 3 10KW 480V units. There is no neutral connection brought out. With respect to the external current and power being supplied to the heaters, the two are exactly the same. But clearly the resistances of the heating elements will be different, and the current flowing through the heating elements will be different.

-Jon
 

ramsy

Roger Ruhle dba NoFixNoPay
Location
LA basin, CA
Occupation
Service Electrician 2020 NEC
winnie said:
Two equivalent motors (same horsepower, efficiency, frequency, voltage, shaft speed, etc.) will draw the same current at the same voltage under the same load. It won't matter if the motors are internally connected star or delta.
Jon, bear with me a moment.

If switching motor connections between Wye & Delta draws the same current, what is happening to these single phase pool-pump motors when people switch them from 120vac to 240vac? Switching my 1HP pool-pump motor configuration, and changing it from 120 to 240vac cut the current draw from 13A to 6.5A.

With the same power delivering 1HP, the motor conductors drew half the current, the house lights stopped fadeing at start up, and V-drop / IR? loss was reduced. What did that internal motor switch do to cause this?

How does a configuration switch on the same motor, result in the same power output 1HP, with less current draw when more voltage is applied?
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
Roger,

You are exactly correct on the physics. The confusion here is one of terminology. You are considering a single motor operated in two difference configurations, Don and I are describing two different motor designs targeted for the same supply conditions.

If I take a particular motor, designed to operate at 240V, which is internally delta connected, and I change the connection to a wye connection, then to operate the machine properly I will need to supply higher voltage, but the machine will draw lower current.

But if I have two _different_ motors, both designed to operate at 240V, one which is internally delta connected, the other internally wye connected, but the motors were otherwise the same (same mechanical output, same efficiency), then the input current will be the same.

Again, please consider the heater example, because this lets us ignore the variability of the mechanical load and ignore questions of efficiency.

Say I want to buy a 30KW 480V three phase heater. The heater arrives as a box with three terminals. I connect the heater to my 480V supply, and 36A/phase flows into the box....For a short time, then the smoke starts pouring out and the ground fault relay trips, since I didn't connect the water :)

Internally, this heater could be built with 7.7 ohm resistive elements connected wye, or it could be built with 23 ohm resistive elements connected delta. The internal connection simply does not matter as far as the _external_ load is concerned. A resistance measurement between the terminals would give the same 15.3 ohms, either by two of the wye elements in series, or by one of the delta elements in parallel with the other two.

The two heaters would be operationally different if a heating element were to fail, but in the state of normal operation you couldn't tell them apart.

Similarly, I can design a 20HP 480V 2 pole motor, with internal connection that is either star or delta. The _external_ current flow will be essentially the same in either case. If the motor is _internally_ a delta, then the current flowing in a given phase winding will be lower, and the phase voltage higher, but the external voltages and currents, once all three phases were properly combined to the three terminals, would be exactly the same. If I took either of these motors and _reconnected_ them internally, then the _external_ terminal voltages and currents would change exactly as you describe.

-Jon
 

Dave58er

Senior Member
Location
Dearborn, MI
motor current

motor current

jon,

I believe your motor knowledge is more extensive than mine, so maybe you can help me understand this. You say current will be the same in a motor regardless of wiring in wye or delta. But isn't the point of a star-delta motor with a star-delta starter to reduce starting current? I just hooked up a 200 horsepower fire pump with this configuration and couldn't believe how small we were allowed to make the supply conductors so I did some resaerch. Let me plagerize my industrial motor control book:
"Assume a motor to be connected directly to a 480v line during the starting period. Each winding with an impedance of 0.4 during the starting period. If the windings are connected in delta, 480v will be connected directly across the phase windings, produceing a phase current of 1200A.

Iphase=Ephase/Zphase
1200= 480v/0.4
Since the line current supplying a delta connection is 1.732 x the phase current the line current will be 2078.4A (1.732x1200)

If the stator windings are connected in a wye configuration during start-up the inrush line current will be only one third the value of the delta.
with the windings connected in a wye the voltage across each phase winding will be less than the line voltage by a factor of 1.732 or 277volts. (480/1.732=277). This will produce a phase current of 692.8A when power is first applied.

Iphase= 277v/0.4=692.8
In a wye connected system the line current and phase current are the same. The line current has been reduced from 2078A to 692A."

I thought I had a pretty good handle on this untill I started reading this thread. Now I'm confused again. I think.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Dave,
You say current will be the same in a motor regardless of wiring in wye or delta.
Here we are talking about 2 different motors of the same horsepower and same voltage rating. The line current will be the same for both a delta and wye connected motor.
But isn't the point of a star-delta motor with a star-delta starter to reduce starting current?
Here we have a motor that is designed to be connected at its rated voltage in a delta configuration. If the design voltage is 480 volts the coils will see the full 480 volts. If you reconnect the motor in a wye for start, the coils will only see 277 volts. This is how the starting current is reduced for a wye start, delta run system.
Don
 

ramsy

Roger Ruhle dba NoFixNoPay
Location
LA basin, CA
Occupation
Service Electrician 2020 NEC
winnie said:
The confusion here is one of terminology. You are considering a single motor operated in two difference configurations, Don and I are describing two different motor designs targeted for the same supply conditions. .. If I took either of these motors and _reconnected_ them internally, then the _external_ terminal voltages and currents would change exactly as you describe.
Jon, Thank you for clearing up this confusion. I could not see where Don was going with this, without the design possibilities and load examples you provided. Many thanks.
 

ramsy

Roger Ruhle dba NoFixNoPay
Location
LA basin, CA
Occupation
Service Electrician 2020 NEC
Re: motor current

Re: motor current

Dave58er said:
I thought I had a pretty good handle on this untill I started reading this thread. Now I'm confused again. I think.
That formula you provided may be a nice way to match an unmarked motor's, terminal continuity with supply voltage. Thanks for showing it here.

The burning question that remains for me to ask, is if this design flexibility means Delta run motors are not as rare as I originally thought, what prevents operators from using line voltage or booster xfmrs to rewire them as Wye's anyway, taking advantage of even smaller conductors and installation costs?

If nothing prevents this switch, and efficiency or install costs remain an incentive to do so, shouldn't Delta run motors, not delta designs, be relatively more expensive to run than Wye's and therefore less common.
 
Status
Not open for further replies.
Top