Blinking Lights 2

[markstg]

At my house I have a 5 ton, 2 stage scroll compressor condensing unit, and my house lights blink on the unit starting.
Specs are 118 LRA, 230V, 23 FLA.

Lamps are incandescent 130 V.
Service size is 200A, 120/240V, single phase
Measurements at the unit disconnect switch are:
Normal Voltage 246 V
Starting Voltage: 231 V
Starting Amps: 121 A
Running Amps: 10 A (Low Stage)
Starting time: 1 second.
Start is on Low stage.

This is on a 1 year old house and has done this since installed.
There is no issue when the compressor goes from low stage to high stage.

Neutrals have all been checked, Meter, Panelboard, and are all tight.

Power compnay service is: 50KVA transformer, 7 Houses on overhead lateral, my service drop from lateral tap to service is 80ft., lateral distance tap to transformer is 120ft. POCO conductors are aluminum don't know size. POCO not interested in my blinking lights with such small voltage drop.

Attached is the Power company recording of Voltage and Current at my service.

I was thinking placing a capacitor at my service may mitigate the dimming lights. Any thoughts.

 

[Skogsgurra]

That recording is just moot.

The samples are taken with minutes between them and cannot show anything about flicker.

Make the utility do a correct recording. Then you (and they) will see what is going on.

It is obious that the starting current makes the lights flicker. And there are usually limits as to how much flicker one shall tolerate. At least, there is in the EU.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[Desrod2]

When your compressor is starting, you are creating an additional 6% voltage drop possibly bringing your lamps supply voltage at 89% of their nominal voltage (although I am not sure if you measure the voltage at your supply or the compressor terminals). Incandescent lamps rated 130V are usually used as "long lasting" on 120 V system. Operating in the upper voltage range is known to shorten the live of 120V rated bulbs so installing 130 V bulbs is a proven solution for system operating in 120+V range. The inconvenience to such choice is that when operating in the lower voltage band, you will more easily perceive the change in lighting since the lamps are at 89% their nominal.

The utility is not likely to help unless your neighbors start complaining. Your compressor is likely meeting standards although with a 6% voltage drop you are technically limited to a few starts a day max. Have you thought replacing your bulbs with LED? The are less voltage sensitive.

 

[DRWeig]

I've found a few cases like yours in which the meter socket is the culprit - a not-tight-enough contact. Could also be a loose connection at the panel main.

Best to you,

Goober Dave

Haven't see the forum policies? Do so now: Forum Policies

 

[markstg]

The voltage measurements are at the compressor, so my service voltage drop is not as severe. I have measured 4V drop at the service under compressor start.

LED's are not an option since they will not work with my dimmer switches.

I changed a room of lights to 120V, and could not notice any difference in the dimming on compressor start from the other rooms that are 130V.

 

[markstg]

All connections, hots and neutrals, have been checked for tightness, meter pan and meter pan jaws, service panel, and taps at the pole (by power company).

 

[Mobius44]

Loose connections commonly cause this issue, but since you've checked that out then you might try using capacitance to reduce the starting VAR surge. However you don't want the capacitance at the service entrance, rather you want it at the compressor, and only while it starts. Have you tried talking to your local HVAC company which represents the installed equipment manufacturer? They will likely recommend a "hard start kit", which basically adds more start capacitance to the compressor. There are several companies making these kits so if you want more details just use your favorite search engine.

 

[markstg]

I don't think the hard start capacitance is large enough to make a difference.

 

[Mobius44]

Keep in mind the start capacitance isn't being used to directly provide the ~100 extra starting amps, rather it only affects the start winding current phase angle with respect to the run winding current phase angle. Thus although the start capacitance is small, it has a large effect on how quickly the motor spins up to normal speed, which in turn has a large effect on how long the motor draws the extra starting amps.

 

[markstg]

I obtained 2 - 100 ufd run capacitors and connected them in parallel to my service.

The blinking lights are not noticeable with the capacitors in the circuit when the compressor starts.
I did 10 test and unless you are looking directly at the lights the blinking is not perceivable.

 

[waross]

The 121/118 Amp LRA/starting current has a large reactive component. Capacitors will supply part of the reactive current and so reduce the starting current. There have been proposed reduced current motor starting schemes which use enough capacity to supply all of the reactive current when starting. The reactive current drops as the motor accelerates. The capacitors are disconnected in steps as the motor accelerates.
I say proposed because these starting schemes have not been widely implemented. (I have read the paper but I have never seen this scheme used in the field.)
Enough capacity to supply the locked rotor VARs is too much for continuous use and may lead to over voltage and other issues.
You seem to have found a sweet spot with enough capacity to mitigate the lamp dimming.
It would be interesting to know if your voltage rises when the capacitors are turned on, and the current drawn by the capacitors.
You may get better results if the capacitors are connected at the motor end of the motor feeder. Maybe not. There are some offsetting factors which are hard to estimate.
VARs supplied at the motor terminals will result in less voltage drop in the feeder and so more starting current, but a higher voltage will result in more VARs from the capacitors, tending to reduce the current. Then the retio of the cable impedances from the transformer to the service and the service to the motor will be a factor. Given the high reactive component of the starting current the cable reactances are also a factor.
If you are happy with the results, I would not move the capacitors.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[markstg]

With the capacitors in the circuit the voltage rises 2V. I, like you, am more concerned with the capacitors in the circuit when I don't need the capacitor KVARS and neither do my neighbors, that my voltage will rise to much, which will have bad effect if not short term, at least long term.

 

[waross]

You can probably all live with a 2 Volt rise. You may want to re-check the rise with everything but the capacitors turned off. That will be the worst case of voltage rise on the grid.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[squeeky]

It is fine to talk of the FLC but what about the conductor size and distance? I am presuming that your feed from your 50kVA transformer comes to a Distribution Board and from there you feed your lights and compressor. The combined voltage drop on this feed that is causing the voltdrop at the common point of connection. Look at your conductor cross sectional area of your feed cable. For 180A on a short distance it is 50mm Sq. As your load is not normally this high, you would size the cable to your load.You may have only a 16mm sq cable. This reduced size is also dangerous due to fault levels.

Then you state that you wish to use capacitors. Yes they will lift the voltage if you install them at the DB. This will help when starting but not enough. Yes the 2% rise is fine, you should not even see the difference and the shortening of life span won't be that great.

If you put the capictors on the motor terminals you really will have a problem. There will be an inrush due to the motor starting and charging of the caps. In my view the dimming will get more pronouced due to the even greater current draw.

What about star delta starting or a soft starter to limit the current draw.

Then finally it may be the voltdrop on your Utility's side or loose terminals on the transformer, LV and MV.

 

[davidbeach]

The motor inrush and the cap inrush subtract, not add. Basic AC circuit theory.

 

[Skogsgurra]

squeeky

As davidbeach says.

It is true that there is a capacitor charging when switching on, but that charging is over in a few hundred microseconds and does not make light flicker at all.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[squeeky]

Dear Guys, Practical experience. Capacitors require special contactors. On AC it sees it as a short circuit until it is charged, yes it is quick but when combined with the inrush of the motor current it is additive until the capacitors are charged, then one counter acts the other. I've done this. If you look at point of consumption connection pfc, there is a time delay before the cap comes in. The motor contactor comes in and the 1 second later you energize the cap contactor. In bulk pfc, delays and discharge times are built in. We need to define the blink (not flicker) that was the part of the queation. The blink is the voltage drop on the motor starting and then it returns to normal. Quick dimming and then back to normal. Flicker is different.

 

[Skogsgurra]

Sorry squeeky, you are in the wrong ballpark.

The switching of the capacitor is not a problem in a system where impedance is as high as it is here. It is not about switching huge PFC capacitors at the PCC, where Isc is huge. It is about connecting a motor/capacitor to a grid that is quite weak (with a substantial X in the transformer) and therefore limits the charging current. I see no reason to worry about the contactor.

I dare say that davidbeach has a lot of practical (and an immense theoretical) knowledge. And, if may say so, so have I.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[davidbeach]

Thanks for the vote of confidence, Gunnar.

 

[waross]

Back in the days before PF controllers became common, it was common practice to connect caps to the motor terminals to correct the power factor. There are probably millions of applications still running out there. Caps on the motor terminals reduce the starting current.

Bill
--------------------
"Why not the best?"
Jimmy Carter