2 Pole Starters on 3 Phase Motors 1

[BJC]

We have got some HVAC units (Trane) with 2 pole starters (contactors) for compressors and fans in the units. All the motors are downstream from the main disconnect.
I know it will work but:
1.) I don't like it.
2.) Is it illeagle in some jurisdictions?
3.) Opening the contactor leaves the motor and the downstream side of the contactors hot.

Maby they have been doing this for years adn I have just missed it.
Is it OK?
Thank You.

 

[waross]

That would be covered under the UL listing of the unit.

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

 

[dpc]

I have never seen it, and I don't like it either. But as Bill says, if it is UL listed as a package system, it may be legal.

Most commercial HVAC equipment is pretty junky compared with typical industrial control systems.



David Castor

http://www.cvoes.com

 

[waross]

Did I say that I didn't like it either?

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

 

[DickDV]

There are SCR based softstarters that modify only two phases also. These leave the motor hot unless there is a three pole contactor open ahead of it.

 

[jraef]

Perfectly legal, understandably scary and unfortunately done all the time in the cheap-ass HVAC business where the $2.31 extra it costs to use a 3 pole contactor means a smaller bonus check for some executive somewhere, so they sacrifice long-term safety and security to keep their pockets lined just a little more deeply.

Whenever I come across it I insist they be changed out. I have had HVAC technicians try to argue with me that this is somehow necessary for the operation of the motor, otherwise the mfr wouldn't do it that way (naive). They pick the wrong guy to mess with, I don't back down. For a while I worked for one of the manufacturers who provides "2-phase" soft starters as DickDV mentioned, I refused to specify them. My feeling is that they are a disaster waiting to happen.

My chief argument against this kind of control method, be it soft starters or contactors, is this: In a 3 phase motor there is always a risk of single phasing because of welded contacts or shorted SCRs. But to make the circuit complete and run that risk, you must have at least 2 of the 3 phases conducting; the very same principal by which this design concept is viable. However when you already have one phase permanently connected, now all you need is either of the other two to weld/short and you lose the motor. Not worth that $2.31 in my opinion.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[itsmoked]

Most refer guys would be very surprised to find 3 pole contactors in packaged units.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[roydm]

I don't agree with it either but it's also very common to control just two phases of a three phase heater. Being a package unit you have to open the three phase disconnect before working on any part as the motors don/t have individual isolators.

 

[LionelHutz]

It's OK, but it's cheap and it's not as good as a 3-pole contactor. But then, there are a lot of new 3-pole components that are cheaper and lighter duty than the older equipment they replace too.

But still, the "motor is still hot" arguement has no validity. There is no excuse for not opening the main disconnect and locking it out before doing any service work.

Any working on the equipment live arguements boil down to meaning the process is more important than the lives of the workers, which is a hard arguement to win.

 

[waross]

"Any working on the equipment live arguements boil down to meaning the process is more important than the lives of the workers, which is a hard arguement to win."
When in this argument, more and more I find that I am arguing with a pointy headed boss who doesn't know enough theory to understand what you are saying.
I tried to explain to one outfit the possible issues that may arise from their flawed lockout procedure.
That was on a Saturday. When the office opened on Monday, I was informed that the client had instructed them to reduce the work force and I was gone. It was a short term job that was paying about $5000 per week.
I was a little bitter. About two monthes ago the same supervision team had two incidents on one shift that both came to the clients attention. The whole group from the supervisor down were removed from the site the next day.
I just got an E-mail from a friend who lost his job on that one. He is now back at the same job working for the replacement contractor.
Feels good.
I tell the young electricians;
"Don't trust these pr_cks. Lock it out but work it like it's hot."

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

 

[dpc]

But still, the "motor is still hot" argument has no validity.

I'm going to disagree on this. Of course the unit has a main disconnect switch - that's how they get by with this cheap trick. But if someone is troubleshooting a control problem, that disconnect will likely be closed. So the motor will be hot. I agree that is proper procedures are followed, it should be safe, but in the confusion of the moment sometimes proper procedures don't get followed, and this type of installation invites accidents rather than helping to prevent them.



David Castor

http://www.cvoes.com

 

[itsmoked]

Another contact? Another three failure points.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jraef]

Keith, I hope your sarcasm wasn't a little too subtle... someone might think you are serious.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[BJC]

It turns out that NEC artical 430.84 says that the controller is allso the diconnecting means interrupting two wires is OK.
They are assuming that the main disconnect is all you need to isolate the motor.

 

[LionelHutz]

But if someone is troubleshooting a control problem, that disconnect will likely be closed.

Well, they are just as likely to accidentally kill themselves on the hot power wires before the contactor...