single phasing

[Bman1463]

I was told that the utility was single phasing and the contactor inside the UPS which is connected to the utility side for control purposes, that the coil had burned up do to it was receiving direct current voltage caused by the alternate current was single phasing. Is this possible and how does it work in theory. All the other connectors on the HVAC equipment are fine and they also experienced the single phasing at the same time.

 

[ron]

Someone is suggesting that the utility was providing DC voltage while single phasing their normal AC source?

 

[templdl]

Someone is suggesting that the utility was providing DC voltage while single phasing their normal AC source?

Yes, something new. The only thing that I can think of that would ruin a relay would Bevan under voltage condition that we could weld the contacts because try he relay was bring "teased"by low voltagee which can result in welded contacts. But the OP was unclear about the mode of failure.

 

[SceneryDriver]

This was my post from several years ago on another site that talks about a single phasing event I had to troubleshoot:

http://www.plctalk.net/qanda/showthread.php?t=75605

We burned up an AC-powered contactor due to single phasing.



SceneryDriver

 

[Jraef]

Yes, something new. The only thing that I can think of that would ruin a relay would Bevan under voltage condition that we could weld the contacts because try he relay was bring "teased"by low voltagee which can result in welded contacts. But the OP was unclear about the mode of failure.

You posted that from your phone, didn't you?

 

[Jraef]

What happens is this:
Your HVAC equipment includes 3 phase motors. When the PoCo lost a phase, the motors that were STILL RUNNING will STAY running and act as a type of Rotary Phase Converter to generate a phantom voltage on the missing phase, but without the added capacitors to boost and stabilize it. So although the utility may have completely lost the phase, you had SOME local voltage generated from the spinning motors, but not enough for the total load on line at the time. So the voltage dropped just enough to keep the contactor from holding in, but as soon as it dropped out and the load on the UPS went away, the voltage increased, so the contactor pulled back in. As soon as it did, the added load dropped the voltage again and it dropped out. This happens very fast and it "chattered", which will smoke the contacts and coil very rapidly.

The trick to preventing this is to have Phase Loss protection in your system that will detect the phase loss from the utility and disable all of the controls for everything that is 3 phase. that event put a lot of stress on those running 3 phase motors too.

 

[dkidd]

You posted that from your phone, didn't you?

On a back dirt road.:happysad:

 

[iwire]

Whatever the power company event happened it strikes me odd that a UPS had an issue with it.

Their entire purpose is to deal with power issues.

 

[StarCat]

A few accounts

A few accounts

You can get into situations with HVACR gear in particular that display the following characteristics when you drop one phase of incoming power.
I have seen it happen mostly on 208-240V machines, but some on 480V as well.
The result is that the control coil will see roughly half of the rated voltage, and the coil will fail due to sustained undervoltage when a certain phase is lost.
I have walked by this in process and noted a control coil humming hard during a phase loss fault, and not pulling in. Meter readings reveal 1/2 rated voltage and its not ghost, and the coil sees it for sure
There was some discussion of this effect in the recent past between Kwired and myself on another thread.
If you have say a walkin cooler with 208-240V control circuit that interconnects with a fan bank having a 120V LLSV coil, these can be major offenders.
This can happen even on single phase machines and you can also get a situation where a compressor will also be subject to 1/2 voltage. The behavior is a type of feedback based on what I recall and it can get complex to determine the path. Phase protectors will stop the problem and single phase units can be gotten.

 

[iwire]

You can get into situations with HVACR gear in particular that display the following characteristics when you drop one phase of incoming power.

For sure, lighting contactors as well but a UPS should be designed to tolerate under voltage.

 

[templdl]

You posted that from your phone, didn't you?

You got it. Pain in the in the butt for typos and spell corrects. A mind of its own.

 

[just the cowboy]

I was told that the utility was single phasing and the contactor inside the UPS which is connected to the utility side for control purposes, that the coil had burned up do to it was receiving direct current voltage caused by the alternate current was single phasing. Is this possible and how does it work in theory. All the other connectors on the HVAC equipment are fine and they also experienced the single phasing at the same time.

To answer your question no this does not happen.

 

[Jraef]

To answer your question no this does not happen.

Correct, it does not happen. But where the confusion or misinterpretation may stem from is probably something like this.

When you apply single phase power to a 3 phase motor that is already spinning*, the regenerated voltage sine wave on the lost phase crosses it's own zero point, defined as the distance between peaks, at a different level; it is not at the same zero level as the other two phases. This is referred to as having a high "DC Offset".
Here is a graphic of the concept;


It doesn't really mean there is DC on the circuit, but having a high DC offset on one phase can cause problems with some equipment, especially anything that rectifies AC to DC, as many types of UPS would.

I don't think that was the issue that caused the contactor to burn up, I'm just postulating on where someone may have come up with the idea that "there was DC on the circuit".

*If you apply single phase power to a 3 phase motor that is stopped, it will not spin. If you REMOVE one phase from a motor that is ALREADY spinning, it can act like a Rotary Phase Converter to "fill in" voltage on the missing phase.

 

[jim dungar]

.... a UPS should be designed to tolerate under voltage.

All UPS's are not equal.
An expensive double conversion style UPS is very good at isolating the load from almost all anomalies on the supply.
A inexpensive UPS, typical in industrial control circuits, often only corrects for extremely low voltage (e.g. an outage) while ignoring mid range problems (e.g. brownouts).

 

[Jraef]

All UPS's are not equal.
An expensive double conversion style UPS is very good at isolating the load from almost all anomalies on the supply.
A inexpensive UPS, typical in industrial control circuits, often only corrects for extremely low voltage (e.g. an outage) while ignoring mid range problems (e.g. brownouts).

Nonetheless, this UPS apparently has a contactor in front of it (which I admit is odd), so whatever the UPS can or cannot handle is moot, it was the contactor coil that suffered the damage.